Guidelines for Designing and Evaluating Surface Irrigation Systems

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Guidelines for designing and evaluating surface irrigation systems

Table of Contents FAO IRRIGATION AND DRAINAGE PAPER 45 by W.R. Walker Professor and Head Department of Agricultural and Irrigation Engineering Utah State University Logan, Utah, USA (Consultant to FAO) FAO FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS Rome, 1989 The designations employed and the presentation of material in this publication do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

M-56 ISBN 92-5-102879-6 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying or otherwise, without the prior permission of the copyright owner. Applications for such permission, with a statement of the purpose and extent of the reproduction, should be addressed to the Director, Publications Division, Food and Agriculture Organization of the United Nations, Via delle Terme di Caracalla, 00100 Rome, Italy. © FAO 1989

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Table of Contents

This electronic document has been scanned using optical character recognition (OCR) software and careful manual recorrection. Even if the quality of digitalisation is high, the FAO declines all responsibility for any discrepancies that may exist between the present document and its original printed version.

Table of Contents Preface Acknowledgements 1. The practice of irrigation 1.1 The perspective and objectives of irrigation 1.2 Irrigation methods and their selection 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6 1.2.7 1.2.8 1.2.9

Compatibility Economics Topographical characteristics Soils Water supply Crops Social influences External influences Summary

1.3 Advantages and disadvantages of surface irrigation 1.3.1 Advantages 1.3.2 Disadvantages 2. Surface irrigation systems 2.1 Introduction to surface irrigation 2.1.1 Definition 2.1.2 Scope of the guide 2.1.3 Evolution of the practice 2.2 Surface irrigation methods 2.2.1 2.2.2 2.2.3 2.2.4

Basin irrigation Border irrigation Furrow irrigation Uncontrolled flooding

2.3 Requirements for optimal performance 2.3.1 Inlet discharge control 2.3.2 Wastewater recovery and reuse 2.4 Surface irrigation structures 2.4.1 Diversion structures 2.4.2 Conveyance, distribution and management structures

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2.4.3 Field distribution systems 3. Field measurements 3.1 Field topography and configuration 3.2 Determining water requirements 3.2.1 3.2.2 3.2.3 3.2.4

Evapotranspiration and drainage requirements Soil moisture principles Soil moisture measurements An example problem on soil moisture

3.3 Infiltration 3.3.1 3.3.2 3.3.3 3.3.4

Infiltration functions Typical infiltration relationships Measuring infiltration An example infiltrometer test

3.4 Flow measurement 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5

Cutthroat flumes Example of cutthroat flume calibration Rectangular thin-plate weirs Example of rectangular sharp crested weir analysis V-notch weirs

3.5 Field evaluation 3.5.1 3.5.2 3.5.3 3.5.4

Advance phase Ponding phase or wetting Depletion phase Recession phase

4. Evaluation of field data 4.1 Objectives of evaluation 4.1.1 Field data 4.2 Performance measures 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6

Application uniformity Application efficiency Water requirement efficiency Deep percolation ratio Tailwater ratio Integration measures of performance

4.3 Intermediate analysis of field data 4.3.1 4.3.2 4.3.3 4.3.4

Inflow-outflow Advance and recession Flow geometry Field infiltration

4.4 System evaluation

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4.4.1 Furrow irrigation evaluation procedure 4.4.2 Border irrigation evaluation 4.4.3 Basin irrigation evaluation 4.5 General alternatives for improvement 4.6 An example furrow irrigation evaluation 4.6.1 Field infiltration characteristics 4.6.2 Evaluation of system performance 4.6.3 Measures to improve performance 5. Surface irrigation design 5.1 Objective and scope of design 5.2 The basic design process 5.2.1 Preliminary design 5.2.2 Detailed design 5.3 Computation of advance and intake opportunity time 5.3.1 Common design computations 5.4 Furrow irrigation flow rates, cutoff times, and field layouts 5.4.1 5.4.2 5.4.3 5.4.4

Furrow design procedure for systems without cutback or reuse Design procedure for furrow cutback systems Design of furrow systems with tailwater reuse Furrow irrigation design examples

5.5 Border irrigation design 5.5.1 5.5.2 5.5.3 5.5.4

Design of open-end border systems Design of blocked-end borders An open-end border design example A blocked-end border design example

5.6 Basin irrigation design 5.6.1 An example of basin design 5.7 Summary 6. Land levelling 6.1 The importance of land preparations 6.2 Small-scale land levelling 6.3 Traditional engineering approach 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5

Initial considerations Engineering phase Adjusting for the cut/fill ratio Some practical problems An example problem

6.4 Laser land levelling 7. Future developments

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7.1 Background 7.2 Surge flow 7.2.1 Effects of surging on infiltration 7.2.2 Effects of surging on surface flow hydraulics 7.2.3 Surge flow systems 7.3 Cablegation 7.4 Adaptive control systems 7.5 Water supply management References Appendix I - Fortran 77 surface irrigation design program FAO irrigation and drainage papers

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Preface

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Preface This guide is intended to serve the needs of the irrigation technician for the evaluation of surface irrigation systems. The scope is focussed at the farm level. A limited series of graphical and tabular aids is given to relieve the user of some burden of computation. Unfortunately, the number of variables associated with surface irrigation prevents this from being completely practical. There are also two matters of philosophical nature that have led to the approach presented herein. First, the irrigation technician and engineer must understand the fundamental interactions characterizing surface flow in order to evaluate, improve, design and manage effectively. This suggests a mathematical presentation which briefly and concisely portrays these interrelationships. This guide omits nearly all theoretical development and presents the most basic mathematical description. Nevertheless, the complexity of the problem still requires an extensive mathematical analysis, even at this basic level. The expertise required of the technician is that of at least a secondary education and the engineer whose training needs to be at approximately the BSc level. The second philosophical aspect is the belief that irrigation engineering practices are moving steadily toward a computerized methodology. The interactions referred to above require large enough computational commitments that they are only feasibly evaluated with hand-held programmable calculators or microcomputers. As a result, the procedures outlined herein have been presented so they can be applied directly via computer. A diskette copy of this program source and executable codes for IBM PC and compatible microcomputers is available from FAO. Some of the material used to develop this paper is included in more theoretical texts of the writer's. Occasionally, direct quotes and figures have been extracted without citation in order to minimize the diversions encountered by the reader. When the work of others has been used, more careful attention to the detail of the citation has been given. Surface irrigation is a complex subject which many have investigated and written about. The purpose of this guide was not to review the technical literature exhaustively and many valuable works are not cited, but it is hoped that the essence of surface irrigation evaluation and design practice has been captured.

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Acknowledgements

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Acknowledgements This work has been Undertaken under the supervision of Dr. Abdullah Arar, Senior Regional Officer, Land and Water Development Division, FAO. His continual support and careful attention to the details involved in producing a document such as this are very much appreciated. Numerous other staff of the FAO have also contributed to this work through their reviews, editorial oversight, and publication. In the last decade or so, the methodology of surface irrigation engineering has moved from the empirical to the quantitative. This has been accomplished by the concerted efforts of numerous researchers and practitioners, some of whom are acknowledged in the REFERENCES. However, many others have made substantial contributions. Of these, perhaps the graduate students at the universities where surface irrigation technology has been extended have been the most unheralded. To those who have worked with the author, special thanks.

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1. The practice of irrigation

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1. The practice of irrigation 1.1 The perspective and objectives of irrigation 1.2 Irrigation methods and their selection 1.3 Advantages and disadvantages of surface irrigation

1.1 The perspective and objectives of irrigation A reliable and suitable irrigation water supply can result in vast improvements in agricultural production and assure the economic vitality of the region. Many civilizations have been dependent on irrigated agriculture to provide the basis of their society and enhance the security of their people. Some have estimated that as little as 15-20 percent of the worldwide total cultivated area is irrigated. Judging from irrigated and non-irrigated yields in some areas, this relatively small fraction of agriculture may be contributing as much as 3040 percent of gross agricultural output. Effective agronomic practices are essential components of irrigated systems. Management of the soil fertility, cropping selection and rotation, and pest control may make as much incremental difference in yield as the irrigation water itself. Irrigation implies drainage, soil reclamation, and erosion control. When any of these factors are ignored through either a lack of understanding or planning, agricultural productivity will decline. History is absolutely certain on this point. Irrigated agriculture faces a number of difficult problems in the future. One of the major concerns is the generally poor efficiency with which water resources have been used for irrigation. A relatively safe estimate is that 40 percent or more of the water diverted for irrigation is wasted at the farm level through either deep percolation or surface runoff. These losses may not be lost when one views water use in the regional context, since return flows become part of the usable resource elsewhere. However, these losses often represent foregone opportunities for water because they delay the arrival of water at downstream diversions and because they almost universally produce poorer quality water. One of the more evident problems in the future is the growth of alternative demands for water such as urban and industrial needs. These uses place a higher value on water resources and therefore tend to focus attention on wasteful practices. Irrigation science in the future will undoubtedly face the problem of maximizing efficiency. Irrigation in arid areas of the world provides two essential agricultural requirements: (1) a moisture supply for plant growth which also transports essential nutrients; and (2) a flow of water to leach or dilute salts in the soil. Irrigation also benefits croplands through cooling the soil and the atmosphere to create a more favourable environment for plant growth.

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The method, frequency and duration of irrigations have significant effects on crop yield and farm productivity. For example, annual crops may not germinate when the surface is inundated causing a crust to form over the seed bed. After emergence, inadequate soil moisture can often reduce yields, particularly if the stress occurs during critical periods. Even though the most important objective of irrigation is to maintain the soil moisture reservoir, how this is accomplished is an important consideration. The technology of irrigation is more complex than many appreciate. It is important that the scope of irrigation science not be limited to diversion and conveyance systems, nor solely to the irrigated field, nor only to the drainage pathways. Irrigation is a system extending across many technical and non-technical disciplines. It only works efficiently and continually when all the components are integrated smoothly.

1.2 Irrigation methods and their selection 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6 1.2.7 1.2.8 1.2.9

Compatibility Economics Topographical characteristics Soils Water supply Crops Social influences External influences Summary

There are three broad classes of irrigation systems: (1) pressurized distribution; (2) gravity flow distribution; and (3) drainage flow distribution. The pressurized systems include sprinkler, trickle, and the array of similar systems in which water is conveyed to and distributed over the farmland through pressurized pipe networks. There are many individual system configurations identified by unique features (centre-pivot sprinkler systems). Gravity flow systems convey and distribute water at the field level by a free surface, overland flow regime. These surface irrigation methods are also subdivided according to configuration and operational characteristics. Irrigation by control of the drainage system, subirrigation, is not common but is interesting conceptually. Relatively large volumes of applied irrigation water percolate through the root zone and become a drainage or groundwater flow. By controlling the flow at critical points, it is possible to raise the level of the groundwater to within reach of the crop roots. These individual irrigation systems have a variety of advantages and particular applications which are beyond the scope of this paper. Suffice it to say that one should be familiar with each in order to satisfy best the needs of irrigation projects likely to be of interest during their formulation. Irrigation systems are often designed to maximize efficiencies and minimize labour and capital requirements. The most effective management practices are dependent on the type of irrigation system and its design. For example, management can be influenced by the use of automation, the control of or the capture and reuse of runoff, field soil and topographical variations and the existence and location of flow measurement and water control structures. Questions that are common to all irrigation systems are when to irrigate, how much to apply, and can the efficiency be improved. A large number of considerations must be taken into account in the selection of an irrigation system. These will vary from location to location, crop to crop, year to year, and farmer to farmer. In general these considerations will include the compatibility of the system with other farm operations, economic feasibility, topographic and soil properties, crop characteristics, and social constraints (Walker and Skogerboe, 1987).

1.2.1 Compatibility

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1. The practice of irrigation

The irrigation system for a field or a farm must function alongside other farm operations such as land preparation, cultivation, and harvesting. The use of the large mechanized equipment requires longer and wider fields. The irrigation systems must not interfere with these operations and may need to be portable or function primarily outside the crop boundaries (i.e. surface irrigation systems). Smaller equipment or animal-powered cultivating equipment is more suitable for small fields and more permanent irrigation facilities.

1.2.2 Economics The type of irrigation system selected is an important economic decision. Some types of pressurized systems have high capital and operating costs but may utilize minimal labour and conserve water. Their use tends toward high value cropping patterns. Other systems are relatively less expensive to construct and operate but have high labour requirements. Some systems are limited by the type of soil or the topography found on a field. The costs of maintenance and expected life of the rehabilitation along with an array of annual costs like energy, water, depreciation, land preparation, maintenance, labour and taxes should be included in the selection of an irrigation system.

1.2.3 Topographical characteristics Topography is a major factor affecting irrigation, particularly surface irrigation. Of general concern are the location and elevation of the water supply relative to the field boundaries, the area and configuration of the fields, and access by roads, utility lines (gas, electricity, water, etc.), and migrating herds whether wild or domestic. Field slope and its uniformity are two of the most important topographical factors. Surface systems, for instance, require uniform grades in the 0-5 percent range.

1.2.4 Soils The soil's moisture-holding capacity, intake rate and depth are the principal criteria affecting the type of system selected. Sandy soils typically have high intake rates and low soil moisture storage capacities and may require an entirely different irrigation strategy than the deep clay soil with low infiltration rates but high moisture-storage capacities. Sandy soil requires more frequent, smaller applications of water whereas clay soils can be irrigated less frequently and to a larger depth. Other important soil properties influence the type of irrigation system to use. The physical, biological and chemical interactions of soil and water influence the hydraulic characteristics and filth. The mix of silt in a soil influences crusting and erodibility and should be considered in each design. The soil influences crusting and erodibility and should be considered in each design. The distribution of soils may vary widely over a field and may be an important limitation on some methods of applying irrigation water.

1.2.5 Water supply The quality and quantity of the source of water can have a significant impact on the irrigation practices. Crop water demands are continuous during the growing season. The soil moisture reservoir transforms this continuous demand into a periodic one which the irrigation system can service. A water supply with a relatively small discharge is best utilized in an irrigation system which incorporates frequent applications. The depths applied per irrigation would tend to be smaller under these systems than under systems having a large discharge which is available less frequently. The quality of water affects decisions similarly. Salinity is generally the most significant problem but other elements like boron or selenium can be important. A poor quality water supply must be utilized more frequently and in larger amounts than one of good quality.

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1. The practice of irrigation

1.2.6 Crops The yields of many crops may be as much affected by how water is applied as the quantity delivered. Irrigation systems create different environmental conditions such as humidity, temperature, and soil aeration. They affect the plant differently by wetting different parts of the plant thereby introducing various undesirable consequences like leaf burn, fruit spotting and deformation, crown rot, etc. Rice, on the other hand, thrives under ponded conditions. Some crops have high economic value and allow the application of more capital-intensive practices. Deep-rooted crops are more amenable to low-frequency, high-application rate systems than shallow-rooted crops.

1.2.7 Social influences Beyond the confines of the individual field, irrigation is a community enterprise. Individuals, groups of individuals, and often the state must join together to construct, operate and maintain the irrigation system as a whole. Within a typical irrigation system there are three levels of community organization. There is the individual or small informal group of individuals participating in the system at the field and tertiary level of conveyance and distribution. There are the farmer collectives which form in structures as simple as informal organizations or as complex as irrigation districts. These assume, in addition to operation and maintenance, responsibility for allocation and conflict resolution. And then there is the state organization responsible for the water distribution and use at the project level. Irrigation system designers should be aware that perhaps the most important goal of the irrigation community at all levels is the assurance of equity among its members. Thus the operation, if not always the structure, of the irrigation system will tend to mirror the community view of sharing and allocation. Irrigation often means a technological intervention in the agricultural system even if irrigation has been practiced locally for generations. New technologies mean new operation and maintenance practices. If the community is not sufficiently adaptable to change, some irrigation systems will not succeed.

1.2.8 External influences Conditions outside the sphere of agriculture affect and even dictate the type of system selected. For example, national policies regarding foreign exchange, strengthening specific sectors of the local economy, or sufficiency in particular industries may lead to specific irrigation systems being utilized. Key components in the manufacture or importation of system elements may not be available or cannot be efficiently serviced. Since many irrigation projects are financed by outside donors and lenders, specific system configurations may be precluded because of international policies and attitudes.

1.2.9 Summary The preceding discussion of factors affecting the choice of irrigation systems at the farm level is not meant to be exhaustive. The designer, evaluator, or manager of irrigation systems should be aware of the broader setting in which irrigated agriculture functions. Ignorance has led to many more failures or inadequacies than has poor judgement or poor training. As the remainder of this guide deals with specific surface irrigation issues, one needs to be reminded that much of the engineering practice is art rather than science. Experience is often a more valuable resource than computational skill, but both are needed. It is a poor engineering practice that leaves perfectly feasible alternatives just beyond one's perspective.

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1.3 Advantages and disadvantages of surface irrigation 1.3.1 Advantages 1.3.2 Disadvantages The term 'surface irrigation' refers to a broad class of irrigation methods in which water is distributed over the field by overland flow. A flow is introduced at one edge of the field and covers the field gradually. The rate of coverage (advance) is dependent almost entirely on the differences between the discharge onto the field and the accumulating infiltration into the soil. Secondary factors include field slope, surface roughness, and the geometry or shape of the flow cross-section. The practice of surface irrigation is thousands of years old. It collectively represents perhaps as much as 95 percent of common irrigation activity today. The first water supplies were developed from stream or river flows onto the adjacent flood plain through simple check-dams and a canal to distribute water to various locations where farmers could then allocate a portion of the flow to their fields. The low-lying soils served by these diversions were typically high in clay and silt content and tended to be most fertile. The land slope was normally small because of the structure of the flood plain itself. With the advent of modern equipment for moving earth and pumping water, surface irrigation systems were extended to upland areas and lands quite separate from the flood plain of local rivers and streams. These lands tend to have more variable soils and topographies, are usually better drained, and may be naturally less fertile. Thus, these lands usually require greater attention to design and operation.

1.3.1 Advantages Surface irrigation offers a number of important advantages at both the farm and project level. Because it is so widely utilized, local irrigators generally have at least minimal understanding of how to operate and maintain the system. In addition, surface systems are often more acceptable to agriculturalists who appreciate the effects of water shortage on crop yields since it appears easier to apply the depths required to refill the root zone. The second advantage of surface irrigation is that these systems can be developed at the farm level with minimal capital investment. The control and regulation structures are simple, durable and easily constructed with inexpensive and readily-available materials like wood, concrete, brick and mortar, etc. Further, the essential structural elements are located at the edges of the fields which facilitates operation and maintenance activities. The major capital expense of the surface system is generally associated with land grading, but if the topography is not too undulating, these costs are not great. Recent developments in surface irrigation technology have largely overcome the irrigation efficiency advantage of sprinkler and trickle systems. An array of automating devices roughly equates labour requirements. The major trade-off between surface and pressurized methods lies in the relative costs of land levelling for effective gravity distribution and energy for pressurization. Energy requirements for surface irrigation systems come from gravity. This is a significant advantage in today's economy. Another advantage of surface systems is that they are less affected by climatic and water quality characteristics. Even moderate winds can seriously reduce the effectiveness of sprinkler systems. Sediments and other debris reduce the effectiveness of trickle systems but may actually aid the performance of the surface systems. Salinity is less of a problem under surface irrigation than either of these pressurized systems.

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There are other advantages specific to individual regions that might be mentioned. Surface systems are better able to utilize water supplies that are available less frequently, more uncertain, and more variable in rate and duration. The gravity flow system is a highly flexible, relatively easily-managed method of irrigation.

1.3.2 Disadvantages There is one disadvantage of surface irrigation that confronts every designer and irrigator. The soil which must be used to convey the water over the field has properties that are highly varied both spatially and temporally. They become almost undefinable except immediately preceding the watering or during it. This creates an engineering problem in which at least two of the primary design variables, discharge and time of application, must be estimated not only at the field layout stage but also judged by the irrigator prior to the initiation of every surface irrigation event. Thus while it is possible for the new generation of surface irrigation methods to be attractive alternatives to sprinkler and trickle systems, their associated design and management practices are much more difficult to define and implement. Although they need not be, surface irrigation systems are typically less efficient in applying water than either sprinkler or trickle systems. Many are situated on lower lands with heavier soils and, therefore, tend to be more affected by waterlogging and soil salinity if adequate drainage is not provided. The need to use the field surface as a conveyance and distribution facility requires that fields be well graded if possible. Land levelling costs can be high so the surface irrigation practice tends to be limited to land already having small, even slopes. Surface systems tend to be labour-intensive. This labour need not be overly skilled. But to achieve high efficiencies the irrigation practices imposed by the irrigator must be carefully implemented. The progress of the water over the field must be monitored in larger fields and good judgement is required to terminate the inflow at the appropriate time. A consequence of poor judgement or design is poor efficiency. One sometimes important disadvantage of surface irrigation methods is the difficulty in applying light, frequent irrigations early and late in the growing season of several crops. For example, in heavy calcareous soils where crust formation after the first irrigation and prior to the germination of crops, a light irrigation to soften the crust would improve yields substantially. Under surface irrigation systems this may be unfeasible or impractical as either the supply to the field is not readily available or the minimum depths applied would be too great.

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2. Surface irrigation systems

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2. Surface irrigation systems 2.1 Introduction to surface irrigation 2.2 Surface irrigation methods 2.3 Requirements for optimal performance 2.4 Surface irrigation structures

2.1 Introduction to surface irrigation 2.1.1 Definition Surface irrigation has evolved into an extensive array of configurations which can be broadly classified as: (1) basin irrigation; (2) border irrigation; (3) furrow irrigation; and (4) uncontrolled flooding. As noted previously, there are two features that distinguish a surface irrigation system: (a) the flow has a free surface responding to the gravitational gradient; and (b) the onfield means of conveyance and distribution is the field surface itself. A surface irrigation event is composed of four phases as illustrated graphically in Figure 1. When water is applied to the field, it 'advances' across the surface until the water extends over the entire area. It may or may not directly wet the entire surface, but all of the flow paths have been completed. Then the irrigation water either runs off the field or begins to pond on its surface. The interval between the end of the advance and when the inflow is cut off is called the wetting or ponding phase. The volume of water on the surface begins to decline after the water is no longer being applied. It either drains from the surface (runoff) or infiltrates into the soil. For the purposes of describing the hydraulics of the surface flows, the drainage period is segregated into the depletion phase (vertical recession) and the recession phase (horizontal recession). Depletion is the interval between cut off and the appearance of the first bare soil under the water. Recession begins at that point and continues until the surface is drained. Figure 1. Time-space trajectory of water during a surface irrigation showing its advance, wetting, depletion and recession phases.

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2. Surface irrigation systems

The time and space references shown in Figure 1 are relatively standard. Time is cumulative since the beginning of the irrigation, distance is referenced to the point water enters the field. The advance and recession curves are therefore trajectories of the leading and receding edges of the surface flows and the period defined between the two curves at any distance is the time water is on the surface and therefore also the time water is infiltrating into the soil. It is useful to note here that in observing surface irrigation one may not always observe a ponding, depletion or recession phase. In basins, for example, the post-cut off period may only involve a depletion phase as the water infiltrates vertically over the entire field. Likewise, in the irrigation of paddy rice, an irrigation very often adds to the ponded water in the basin so there is neither advance nor recession - only wetting or ponding phase and part of the depletion phase. In furrow systems, the volume of water in the furrow is very often a small part of the total supply for the field and it drains rapidly. For practical purposes, there may not be a depletion phase and recession can be ignored. Thus, surface irrigation may appear in several configurations and operate under several regimes.

2.1.2 Scope of the guide The surface irrigation system is one component of a much larger network of facilities diverting and delivering water to farmlands. Figure 2 illustrates the 'irrigation system' and some of its features. It may be divided into the following four component systems: (1) water supply; (2) water conveyance or delivery; (3) water use; and (4) drainage. For the complete system to work well, each must work conjunctively toward the common goal of promoting maximum onfarm production. Historically, the elements of an irrigation system have not functioned well as a system and the result has too often been very low project irrigation efficiencies. The focus of surface irrigation engineering is at the water use level, the individual irrigated field. For design and evaluation purposes, these guidelines will note elements of the conveyance and distribution system, especially those near the field such as flow measurement and control, but will leave detailed treatment to other technical sources.

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2. Surface irrigation systems

Figure 2. Typical irrigation system components (redrafted from USDA-SCS, 1967)

2.1.3 Evolution of the practice Although surface irrigation is thousands of years old, the most significant advances have been made within the last decade. In the developed and industrialized countries, land holdings have become as much as 10-20 times as large, and the number of farm families has dropped sharply. Very large mechanized farming equipment has replaced animal-powered planting, cultivating and harvesting operations. The precision of preparing the field for planting has improved by an order of magnitude with the advent of the laser-controlled land grading equipment. Similarly, the irrigation works themselves are better constructed because of the application of high technology equipment. The changes in the lesser-developed and developing countries are less dramatic. In the lesser-developed countries, trends toward land consolidation, mechanization, and more elaborate system design and operation are much less apparent. Most of these farmers own and operate farms of 1-10 hectares, irrigate with 20-40 litres per second and rely on either small mechanized equipment or animal-powered farming implements. Probably the most interesting evolution in surface irrigation so far as this guide is concerned is the development and application of microcomputers and programmable calculators to the design and operation of surface irrigation systems. In the late 1970s, a high-speed microcomputer technology began to emerge that could solve the basic equations describing the overland flow of water quickly and inexpensively. At about the same time, researchers like Strelkoff and Katapodes (1977) made major contributions with efficient and accurate numerical solutions to these equations. Today in the graduate and undergraduate study of surface irrigation engineering, microcomputer and programmable calculator utilization is, or should be, common practice. Microcomputers and programmable calculators provide several features for today's irrigation engineers and technicians. They allow a much more comprehensive treatment of the vital hydraulic processes occurring both on the surface and beneath it. One can find optimal designs and management practices for a multitude of conditions because designs historically requiring days of effort are now made in seconds. The effectiveness of existing practices or proposed ones can be predicted, even to the extent that control systems operating, sensing and adjusting on a real-time basis are possible.

2.2 Surface irrigation methods 2.2.1 2.2.2 2.2.3 2.2.4

Basin irrigation Border irrigation Furrow irrigation Uncontrolled flooding

The classification of surface methods is perhaps somewhat arbitrary in technical literature. This has been compounded by the fact that a single method is often referred to with different names. In this guide, surface methods are classified by the slope, the size and shape of the field, the end conditions, and how water flows into and over the field. Each surface system has unique advantages and disadvantages depending on such factors as were listed earlier like: (1) initial cost; (2) size and shape of fields; (3) soil characteristics;

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2. Surface irrigation systems

(4) nature and availability of the water supply; (5) climate; (6) cropping patterns; (7) social preferences and structures; (8) historical experiences; and (9) influences external to the surface irrigation system.

2.2.1 Basin irrigation Basin irrigation is the most common form of surface irrigation, particularly in regions with layouts of small fields. If a field is level in all directions, is encompassed by a dyke to prevent runoff, and provides an undirected flow of water onto the field, it is herein called a basin. A basin is typically square in shape but exists in all sorts of irregular and rectangular configurations. It may be furrowed or corrugated, have raised beds for the benefit of certain crops, but as long as the inflow is undirected and uncontrolled into these field modifications, it remains a basin. Two typical examples are shown in Figure 3, which illustrate the most common basin irrigation concept: water is added to the basin through a gap in the perimeter dyke or adjacent ditch. Figure 3. Typical irrigated basins (from Walker and Skogerboe, 1987) a. large basin in the USA b. paddy basin in Asia There are few crops and soils not amenable to basin irrigation, but it is generally favoured by moderate to slow intake soils, deep-rooted and closely spaced crops. Crops which are sensitive to flooding and soils which form a hard crust following an irrigation can be basin irrigated by adding furrowing or using raised bed planting. Reclamation of salt-affected soils is easily accomplished with basin irrigation and provision for drainage of surface runoff is unnecessary. Of course it is always possible to encounter a heavy rainfall or mistake the cutoff time thereby having too much water in the basin. Consequently, some means of emergency surface drainage is good design practice. Basins can be served with less command area and field watercourses than can border and furrow systems because their level nature allows water applications from anywhere along the basin perimeter. Automation is easily applied. Basin irrigation has a number of limitations, two of which, already mentioned, are associated with soil crusting and crops that cannot accommodate inundation. Precision land levelling is very important to achieving high uniformities and efficiencies. Many basins are so small that precision equipment cannot work effectively. The perimeter dykes need to be well maintained to eliminate breaching and waste, and must be higher for basins than other surface irrigation methods. To reach maximum levels of efficiency, the flow per unit width must be as high as possible without causing erosion of the soil. When an irrigation project has been designed for either small basins or furrows and borders, the capacity of control and outlet structures may not be large enough to improve basins.

2.2.2 Border irrigation Border irrigation can be viewed as an extension of basin irrigation to sloping, long rectangular or contoured field shapes, with free draining conditions at the lower end. Figure 4 illustrates a typical border configuration in which a field is divided into sloping borders. Water is applied to individual borders from small hand-dug checks from the field head ditch. When the water is shut off, it recedes from the upper end to the lower end. Sloping borders are suitable for nearly any crop except those that require prolonged ponding. Soils can be efficiently irrigated which have moderately low to moderately high intake rates but, as with basins, should not form dense crusts unless provisions are made to furrow or construct raised borders for the crops. The stream size per unit width must be large, particularly following a major tillage operation, http://www.fao.org/docrep/t0231e/t0231e04.htm#TopOfPage[6/18/2013 7:17:58 PM]

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although not so large for basins owing to the effects of slope. The precision of the field topography is also critical, but the extended lengths permit better levelling through the use of farm machinery. Figure 4. Typical border irrigated field

2.2.3 Furrow irrigation Furrow irrigation avoids flooding the entire field surface by channelling the flow along the primary direction of the field using 'furrows,' 'creases,' or 'corrugations'. Water infiltrates through the wetted perimeter and spreads vertically and horizontally to refill the soil reservoir. Furrows are often employed in basins and borders to reduce the effects of topographical variation and crusting. The distinctive feature of furrow irrigation is that the flow into each furrow is independently set and controlled as opposed to furrowed borders and basins where the flow is set and controlled on a border by border or basin by basin basis. Furrows provide better on-farm water management flexibility under many surface irrigation conditions. The discharge per unit width of the field is substantially reduced and topographical variations can be more severe. A smaller wetted area reduces evaporation losses. Furrows provide the irrigator more opportunity to manage irrigations toward higher efficiencies as field conditions change for each irrigation throughout a season. This is not to say, however, that furrow irrigation enjoys higher application efficiencies than borders and basins. There are several disadvantages with furrow irrigation. These may include: (1) an accumulation of salinity between furrows; (2) an increased level of tailwater losses; (3) the difficulty of moving farm equipment across the furrows; (4) the added expense and time to make extra tillage practice (furrow construction); (5) an increase in the erosive potential of the flow; (6) a higher commitment of labour to operate efficiently; and (7) generally furrow systems are more difficult to automate, particularly with regard to regulating an equal discharge in each furrow. Figure 5 shows two typical furrow irrigated conditions. Figure 5. Furrow irrigation configurations (after USDA-SCS, 1967) (a) graded furrow irrigation system

(b) contour furrows

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2.2.4 Uncontrolled flooding There are many cases where croplands are irrigated without regard to efficiency or uniformity. These are generally situations where the value of the crop is very small or the field is used for grazing or recreation purposes. Small land holdings are generally not subject to the array of surface irrigation practices of the large commercial farming systems. Also in this category are the surface irrigation systems like check-basins which irrigate individual trees in an orchard, for example. While these systems represent significant percentages in some areas, they will not be discussed in detail in this paper. The evaluation methods can be applied if desired, but the design techniques are not generally applicable nor need they be since the irrigation practices tend to be minimally managed.

2.3 Requirements for optimal performance 2.3.1 Inlet discharge control 2.3.2 Wastewater recovery and reuse There is substantial field evidence that surface irrigation systems can apply water to croplands uniformly and efficiently, but it is the general observation that most such systems operate well below their potential. A very large number of causes of poor surface irrigation performance have been outlined in the technical literature. They range from inadequate design and management at the farm level to inadequate operation of the upstream water supply facilities. However, in looking for a root cause, one most often retreats to the fact that infiltration changes a great deal from irrigation to irrigation, from soil to soil, and is neither predictable nor effectively manageable. The infiltration rates are an unknown variable in irrigation practice. In those cases where high levels of uniformity and efficiency are being achieved, irrigators utilize one or more of the following practices: (1) precise and careful field preparation; (2) irrigation scheduling; (3) regulation of inflow discharges; and (4) tailwater runoff restrictions, reduction, or reuse. Land preparation is largely a land grading problem which will be discussed in Section 5. Irrigation scheduling is a theme covered separately by several publications such as the FAO Irrigation and Drainage Paper 24 (Rev) by Doorenbos and Pruitt (FAO, 1977). The attention here then is focused on inflow regulation and tailwater control.

2.3.1 Inlet discharge control Surface irrigation systems have two principal sources of inefficiency, deep percolation and surface runoff or tailwater The remedies are competitive. To minimize deep percolation the advance phase should be completed as quickly as possible so that the intake opportunity time over the field will be uniform and then cut the inflow off when enough water has been added to refill the root zone. This can be accomplished with a high, but non-erosive, discharge onto the field. However, this practice increases the tailwater problem because the flow at the http://www.fao.org/docrep/t0231e/t0231e04.htm#TopOfPage[6/18/2013 7:17:58 PM]

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downstream end must be maintained until a sufficient depth has infiltrated. The higher inflow reaches the end of the field sooner but it increases both the duration and the magnitude of the runoff. There are three options available to solve this problem, at least partially: (1) dyke the downstream end to prevent runoff as in basin irrigation; (2) reduce the inflow discharge to a rate more closely approximating the cumulative infiltration along the field following the advance phase, a practice termed 'cutback'; or (3) select a discharge which minimizes the sum of deep percolation and tailwater losses, i.e., optimize the field inflow regime. Examples of these alternative practices are discussed and illustrated in Section 5. In this configuration, the head ditch is divided into a series of level bays which are differentiated by a small change in elevation. Water levels are regulated in two bays simultaneously so that the lower bay has sufficient head to produce an advance phase flow in the furrows while in the upper bay the head is only sufficient to produce the cutback flow. Thus, the system operates by moving the check-dam from bay to bay along the upper end of the field. Two very recent additions to the efforts to control surface irrigation systems more effectively are the 'Surge Flow' system (Figure 6) developed at Utah State University, USA and the 'Cablegation' system developed at the US Department of Agriculture's Snake River Water Conservation Research Center in Kimberly, Idaho, USA. These systems will be dealt with in more detail in a later section. Figure 6. One of the innovations in surface irrigation, the Surge Flow system

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2.3.2 Wastewater recovery and reuse The tailwater deep percolation trade-off can also be solved by collecting and recycling the

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runoff to improve surface irrigation performance. Reuse systems have not been widely employed historically because water and energy have been inexpensive. Even today it is often more economical to regulate the inflow rather than to collect and pump the runoff back to the head of the field or to another field, tailwater reuse systems are more cost-effective when the water can be added to the flow serving lower fields and thereby saving the cost of pumping.

2.4 Surface irrigation structures 2.4.1 Diversion structures 2.4.2 Conveyance, distribution and management structures 2.4.3 Field distribution systems Surface irrigation systems are supported by a number of on- and off-farm structures which control and manage the flow and its energy. In order to facilitate efficient surface irrigation, these structures should be easily and cheaply constructed as well as easy to manage and maintain. Each should be standardized for mass production and fabrication in the field by farmers and technicians. It is not the intent of this guide to be comprehensive with regard to the selection and design of these structures since other sources are available, but it is worthwhile to note some of these structures by way of presenting a larger view of surface irrigation. The structural elements of a surface system perform several important functions which include: (1) turning the flow to a field on and off; (2) conveying and distributing the flow among fields; (3) water measurement, sediment and debris removal, water level stabilization; and (4) distribution of water onto the field.

2.4.1 Diversion structures Most surface irrigation systems derive their water supplies from canal systems operated by public or semi-public irrigation departments, districts, or companies. Some irrigation water is supplied in piped delivery systems and some directly pumped from groundwater. Diversion structures perform several tasks including (1) on-off water control which allows the supply agency to allocate its supply and protects the fields below the diversion from untimely flooding; (2) regulation and stabilization of the discharge to the requirements of field channels and watercourse distribution systems; (3) measurement of flow at the turnout in order to establish and protect water entitlements; and (4) protection of downstream structures by controlling sediments and debris as well as dissipating excess kinetic energy in the flow. A typical turnout structure is shown in Figure 7. Figure 7. Typical turnout from a canal or lateral (from walker end Skogerboe, 1987)

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2.4.2 Conveyance, distribution and management structures Conveying water to the field requires similar structures to those found in major canal networks. The conveyance itself can be an earthen ditch or lateral, a buried pipe, or a lined ditch. Lined sections can be elevated as shown in Figure 8, or constructed at surface level. Pipe materials are usually plastic, steel, concrete, clay, or asbestos cement, or they may be as simple as a wooden or bamboo construction. Lining materials include slip-form cast-in-place, or prefabricated concrete (Figure 9), shotcrete or gunite, asphalt, surface and buried plastic or rubber membranes, and compacted earth. Figure 8. Elevated concrete channel in Iran Figure 9. Slip-form concrete lining in the USA The management of water in the field channels involves flow measurement, sediment and debris removal, divisions, checks, drop-energy dissipators, and water level regulators. Some of the more common flow control structures for open channels are shown in Figure 10. Associated with these are various flow measuring devices like weirs, flumes, and orifices. The designs of these structures have been standardized since they are small in size and capacity. Designs for flow measurement and drop-energy dissipator structures need more attention and construction must be more precise since their hydraulic responses are quite sensitive to their dimensions. Figure 10. On-farm water management structures (from Skogerboe et al., 1971) a. a simple drop structure

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b. a typical check-divider

2.4.3 Field distribution systems After the water reaches the field ready to be irrigated, it is distributed onto the field by a variety of means, both simple and elaborately constructed. Most fields have a head ditch or pipeline

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running along the upper side of the field from which the flow is distributed onto the field. In a field irrigated from a head ditch, the spreading of water over the field depends somewhat on the method of surface irrigation. For borders and basins, open or piped cutlets as illustrated in Figure 11 are generally used. Furrow systems use outlets which can be directed to each furrow. Figure 11. Head ditch outlets for borders and basins (after Kraatz and Mahajan, FAO, 1975)

Figure 12 shows a system in which siphon tubes are used as a means of serving each furrow. Field distribution and spreading can also be through portable pipelines running along the surfaces or permanent pipelines running underground. Basins and borders usually receive water through buried pipes serving one or more gated risers within each basin or border. A typical riser outlet, known as an alfalfa valve, is shown in Figure 13. The most common piped method of furrow irrigation uses plastic or aluminium gated pipe like that shown in Figure 14. The gated pipe may be connected to the main water supply via a piped distribution network with a riser assembly like the one shown in Figure 13, directly to a canal turnout, or through an open channel to a piped transition. Figure 12. Siphons for furrow irrigation Figure 13. An alfalfa valve riser

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Figure 14. Gated pipe for furrows

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3. Field measurements

Produced by: Natural Resources Management and Environment Department

Title: Guidelines for designing and evaluatin surface irrigation systems...

More details

3. Field measurements 3.1 Field topography and configuration 3.2 Determining water requirements 3.3 Infiltration 3.4 Flow measurement 3.5 Field evaluation The evaluation of surface irrigation at the field level is an important aspect of both management and design. Field measurements are necessary to characterize the irrigation system in terms of its most important parameters, to identify problems in its function, and to develop alternative means for improving the system. System characterization necessitates a series of basic field measurements before, during, and after the irrigation. The objectives of the evaluation will dictate whether the field measurements are comprehensive or are simplified for special purposes. In some cases, there are alternative methodologies and equipment for accomplishing the same ends. The selection provided herein is based on a limited selection found to be most useful during numerous field evaluations and, in some measure, the practicality in the international sense. Five classes of field measurements are presented: (1) field topography and configuration; (2) water requirements; (3) infiltration; (4) flow measurement; and (5) irrigation phases.

3.1 Field topography and configuration All field evaluations should include a relatively simple assessment of the field topography and layout. These measurements are well enough known that only their brief mention is required. There is first of all the field's primary elevations. This information requires that a surveying instrument be used to measure elevations of the principal field boundaries (including dykes if present), the elevation of the water supply inlet (an invert and likely maximum water surface elevation), and the elevations of the surface and subsurface drainage system if possible. These measurements need not be comprehensive nor as formalized as one would expect for a land levelling project. The field topography and geometry should be measured. This requires placing a simple reference grid on the field, usually by staking, and then surveying the elevations of the field surface at the grid points to establish slope and slope variations. Usually one to three lines of stakes placed 20-30 metres apart or such that 5-10 points are measured along the expected flow line will be sufficient. For example, a border or basin would require at most three stake lines, a furrow system as little as one, depending on the uniformity of the topography. The survey should establish the distance of each grid point from the field inlet as well as the field

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dimensions (length of the field in the primary direction of water movement as well as field width). There are important items of information that should be available from the survey: (1) the field slope and its uniformity in the direction of flow and normal to it; (2) the slope and area of the field; and (3) a reference system in the field establishing distance and elevation changes. It is also worthwhile at this stage of the evaluation to record the location and extent of major soil types (this may require sampling and some laboratory analyses). The cropping pattern should be determined and, if a crop is on the field at the time of the evaluation, any obvious differences in growth and vigour should be noted. Similarly, the cultivation practices should be recorded.

3.2 Determining water requirements 3.2.1 3.2.2 3.2.3 3.2.4

Evapotranspiration and drainage requirements Soil moisture principles Soil moisture measurements An example problem on soil moisture

The irrigation system may not be designed to supply the total amount of moisture required for crop growth. In some cases, precipitation or upward flow from a water table may contribute substantially towards fulfilling crop water requirements. It is also unrealistic to expect that irrigation can be practiced without losses due to deep percolation, or tailwater runoff. The fraction of the water that is used should be maximized, but this fraction cannot be 100 percent without other serious problems developing such as a salt build-up in the crop root zone. The dependency on irrigation in an area requires some analyses of the water balance. Water balance may have three perspectives. The first is the balance of agricultural demands within a watershed as depicted in Figure 15. The outcome of such an analysis establishes the safe yield of water from various sources and thereby indicates the area of a project, the priorities among projects, and the configuration of the large systemic components of the project. An evaluation at the field level presumes that this information is available, and it should be generally understood in as much as the limits of on-farm irrigation may be dictated by the magnitude and distribution of the total water supply. Figure 15. The perspective of water balance at the river basin level (from Walker, 1978)

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The second water balance perspective, illustrated in Figure 16, is the water balance within the farm or command area. An individual field is generally irrigated in concert with others in the command or farm through sharing the water delivered through a canal turnout or a well. Fields also typically share drainage channels. Water balance at the farm or command area level is established on a field's access to water, its priority, timing and duration. Again, a field evaluation presumes that these factors have been formulated and can be determined. Figure 17 illustrates the perspective of water balance at the field level. Figure 16. A perspective of the on-farm water balance

Figure 17. The perspective of water balance at the field level

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The water balance within the confines of a field is a useful concept for characterizing, evaluating or monitoring any surface irrigation system. In using this aspect of water balance, an important consideration is the time frame in which the computations are made, i.e. whether the balance will use annual data, seasonal data, or data describing a single irrigation event. If a mean annual water balance is computed, then it becomes reasonable that the change in

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root zone soil moisture storage could be assumed as zero. In some irrigated areas, precipitation events are so light that the net rainfall can be reasonably assumed to equal the measured precipitation. Under other circumstances, various other terms can be neglected. In fact, the time base and field conditions are often selected to eliminate as many of the parameters as possible in order to study the behaviour of single parameters. One of the more important is crop evapotranspiration. The upward movement of groundwater to the root zone can usually be ignored if the water table is at least a metre below the root zone. Then if the soil moisture is measured before and after a period when there is no precipitation or irrigation, the depletion from the root zone is a viable estimate of crop water use. There are two particularly important components in the field water balance which impact design and evaluation. The first is the irrigation requirement of the crop, or its evapotranspiration and leaching needs. This is a design parameter and will be briefly described here, but a detailed treatment is left to the FAO Irrigation and Drainage Paper 24, Crop Water Requirements, by Doorenbos and Pruitt (FAO, 1977). The second important component deals with field evaluation and concerns the nature of moisture content changes in the soil profile.

3.2.1 Evapotranspiration and drainage requirements Evapotranspiration, ET, is dependent upon climatic conditions, crop variety and stage of growth, soil moisture depletion, and various physical and chemical properties of the soil. A two step procedure is generally followed in estimating ET: (1) the seasonal distribution of reference crop "potential evapotranspiration", Etp , which can be computed with standard formulae; and (2) the Etp is adjusted for crop variety and stage of growth. Other factors like moisture stress can be ignored for the purposes of design computations. There are perhaps twenty commonly used methods for calculating evapotranspiration, ranging in complexity from the Blaney-Criddle Method using primarily mean monthly temperature to more complete equations such as the Penman Method requiring radiation, temperature, wind velocity, humidity and other factors comprising the net energy balance at the crop canopy. The actual crop water demand depends on its stage of development and variety. Generally it is estimated by multiplying Etp by a crop growth stage coefficient, k CO . Values of k CO have been published by Jensen (1973), Kincaid and Heermann (1974) and Doorenbos and Pruitt (FAO, 1977) for a wide range of crops grown worldwide. Some irrigation water should be applied in excess of the storage capacity of the soil to leach salts from the rooting region, although this does not have to be achieved during each irrigation event. It can usually be applied on an annual basis. As a matter of practicality, the normally occurring deep percolation under most surface irrigation systems exceeds the leaching fraction necessary for salt balance, particularly for the first and second irrigations each season when deep percolation losses are typically greatest. In addition, precipitation helps leach salts throughout the year. Nevertheless some irrigated areas maintain a salt balance in the root zone with excess leaching during only years of plentiful water supplies, which may occur as infrequently as every three to eight years.

3.2.2 Soil moisture principles Important soil characteristics in irrigated agriculture include: (1) the water-holding or storage capacity of the soil; (2) the permeability of the soil to the flow of water and air; (3) the physical features of the soil like the organic matter content, depth, texture and structure; and (4) the

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soil's chemical properties such as the concentration of soluble salts, nutrients and trace elements. The total available water, TAW, for plant use in the root zone is commonly defined as the range of soil moisture held at a negative apparent pressure of 0.1 to 0.33 bar (a soil moisture level called 'field capacity') and 15 bars (called the 'permanent wilting point'). The TAW will vary from 25 cm/m for silty loams to as low as 6 cm/m for sandy soils. Some typical values of TAW, field capacity, permanent wetting point and miscellaneous features have been given in various texts. A typical summary is shown in Figure 18. Figure 18. Relationships between soil types and total available soil moisture holding capacity, field capacity and wilting point (from Walker and Skogerboe, 1987)

Other important soil parameters include its porosity, f , its volumetric moisture content, q ; its saturation, S; its dry weight moisture fraction, W; its bulk density, g b; and its specific weight, g s. The relationships among these parameters are as follows. The porosity, f , of the soil is the ratio of the total volume of void or pore space, Vp , to the total soil volume V: f = Vp /V (1) The volumetric water content, q , is the ratio of water volume in the soil, VW, to the total volume, V: q = Vb /V (2) The saturation, S, is the portion of the pore space filled with water: S = VW/Vp (3) These terms are further related as follows: q = S * f (4) When a sample of field soil is collected and oven-dried, the soil moisture is reported as a dry weight fraction, W: http://www.fao.org/docrep/t0231e/t0231e05.htm#TopOfPage[6/18/2013 7:18:18 PM]

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(5) To convert a dry weight soil moisture fraction into volumetric moisture content, the dry weight fraction is multiplied by the bulk density, g b; and divided by specific weight of water, g w which can be assumed to have a value of unity. Thus: q = g b W/g

w

(6)

The g b is defined as the specific weight of the soil particles, g s, multiplied by the particle volume or one-minus the porosity: g b = g b * (1 - f ) (7) The volumetric moisture contents at field capacity, q fc , and permanent wilting point, q wp , then are defined as follows: q fc = g b Wfc /g

w

q wp = g b Wwp /g

(8) w

(9)

where Wfc and Wwp are the dry weight moisture fractions at each point. The total available water, TAW is the difference between field capacity and wilting point moisture contents multiplied by the depth of the root zone, RD (refer to Table 1): TAW = (q fc - q wp ) RD (10) Table 1 AVERAGE ROOTING DEPTHS FOR COMMONLY GROWN CROPS Crop

Root Depth (metres)

Alfalfa

1.5

Almonds

1.8

Apricots

1.8

Artichokes

1.4

Asparagus

1.5

Bananas

0.9

Beans

0.9

Beets

0.8

Broccoli

0.5

Cabbage

0.5

Cantaloupes

1.5

Carrots

0.9

Cauliflower

0.6

Celery

0.4

Cherries

2.0

Citrus

1.4

Corn (maize)

1.3

Cotton

1.2

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Cucumber

1.1

Eggplant

0.9

Figs

1.5

Grains and flax

1.2

Grapes

1.5

Groundnuts.

0.7

Ladino clover

0.6

Lettuce

0.3

Melons

1.3

Milo (Sorghum)

1.2

Mustard

1.1

Olives

1.5

Onions

0.3

Palm Trees

0.9

Peaches

1.6

Pears

1.6

Peas

0.8

Peppers

0.9

Pineapple

0.5

Potatoes

0.9

Prunes

1.5

Pumpkins

1.8

Radishes

0.5

Safflower

1.5

Soybeans

1.0

Spinach

0.6

Squash (summer)

0.9

Strawberries

0.5

Sudan grass

1.8

Tomatoes

1.5

Turnips

0.9

Walnuts

2.0

Watermelon

1.2

Summarized from Marr (1967) and Doorenbos and Pruitt (FAO, 1977) The Soil Moisture Deficit, SMD, is a measure of soil moisture between field capacity and existing moisture content, q i, multiplied by the root depth: SMD = (q fc - q i) * RD (11) A similar term expressing the moisture that is allotted for depletion between irrigations is the 'Management Allowed Deficit', MAD. This is the value of SMD where irrigation should be scheduled and represents the depth of water the irrigation system should apply. Later this will be referred to as Zreq indicating the 'required depth' of infiltration.

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3.2.3 Soil moisture measurements The soil moisture status requires periodic measurements in the field, from which one can project when the next irrigation should occur and what depth of water should be applied. Conversely, such data can indicate how much has been applied and its uniformity over the field. As noted in the previous subsections, bulk density, field capacity and the permanent wilting point are also needed. There are numerous techniques for evaluating soil moisture. Perhaps the most useful are gravimetric sampling, the neutron probe and the touch-and-feel method. i. Gravimetric sampling Gravimetric sampling involves collecting a soil sample from each 15-30 cm of the soil profile to a depth at least that of the root penetration. Typical samplers are shown in Figure 19. The soil sample of approximately 100-200 grammes is placed in an air tight container of known weight (tare) and then weighed. The sample is then placed in an oven heated to 105° C for 24 hours with the container cover removed. After drying, the soil and container are again weighed and the weight of water determined as the before and after readings. The dry weight fraction of each sample can be calculated using Eq. 5. Knowing the bulk density, one can determine moisture contents from Eq. 6 and the soil moisture depletion from Eq. 11. Figure 19. Small equipment used for collecting soil samples from the field a. sampling auger b. sampling tube ii. The neutron Probe The neutron probe and scaler for making soil moisture measurements are illustrated in Figure 20. The neutron probe is inserted at various depths into an access tube and the count rate is read from the scaler. The manufacturers of neutron probe equipment furnish a calibration relating the count rate to volumetric soil moisture content. Field experience suggests that these calibrations are not always accurate under a broad range of conditions so it is advisable for the investigator to develop an individual calibration for each field or soil type. Most calibration curves are linear, best fit lines of gravimetric data and scaler readings but may in some cases be slightly curvilinear (van Baval et al., 1963). Figure 20. A neutron probe and scaler for soil moisture measurements (after Walker and Skogerboe, 1987) The volume of soil actually monitored in readings by the neutron probe depends on the moisture content of the soil, increasing as the soil moisture decreases. The accuracy of soil moisture determinations near the ground surface is affected by a loss of neutrons into the atmosphere thereby influencing measurements prior to an irrigation more than afterwards. As a consequence, soil moisture measurements with a neutron probe are usually unreliable within 10-30 cm of the ground surface. iii. Touch-and-feel As a means of developing a rough estimate of soil moisture, the Touch-and-feel method can be used. A handful of soil is squeezed into a ball. Then the appearance of the squeezed soil can be compared subjectively to the descriptions listed in Table 2 to arrive at the estimated depletion level. Merriam (1960) has developed a similar table which gives the moisture deficiency in depth of water per unit depth of soil. Over the years various investigators have

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compared actual gravimetric sample results to the Touch-and-Feel estimates, finding a great deal of error depending on the experience of the sampler. Table 2 GUIDELINES FOR EVALUATING SOIL MOISTURE BY FEEL Feel or Appearance of Soil Percent Fine sandy loams to silt Depletion Loamy sands to fine sandy loams loams

Silt loams to clay loam

0 (field capacity)

no free water on ball* but wet outline on hand

same

same

0-25

makes ball but breaks easily and does not feel slick

makes tight ball, ribbons easily, slightly sticky and slick

easily ribbons slick feeling

25-50

balls with pressure but easily breaks

pliable ball, not sticky or slick, ribbons and feels damp

pliable ball, ribbons easily slightly slick

50-75

will not ball, feels dry

balls under pressure but is powdery and easily breaks

slightly balls still pliable

75-100

dry, loose, flows through fingers powdery, dry, crumbles

hard, baked, cracked, crust

* A "Ball" is formed by squeezing a soil sample firmly in one's hand A "Ribbon" is formed by squeezing soil between one's thumb and forefinger. iv. Bulk density Measurements of bulk density are commonly made by carefully collecting a soil sample of known volume and then drying the sample in an oven to determine the dry weight fraction. Then the dry weight of the soil, Wb is divided by the known sample volume, V, to determine bulk density, g b: g

b

= Wb V (12)

Most methods developed for determining bulk density use a metal cylinder sampler that is driven into the soil at a desired depth in the profile. Bulk density varies considerably with depth and over an irrigated field. Thus, it is generally necessary to repeat the measurements in different places to develop reliable estimates. v. Field capacity The most common method of determining field capacity in the laboratory uses a pressure plate to apply a suction of -1/3 atmosphere to a saturated soil sample. When water is no longer leaving the soil sample, the soil moisture in the sample is determined gravimetrically and equated to field capacity. A field technique for finding field capacity involves irrigating a test plot until the soil profile is saturated to a depth of about one metre. Then the plot is covered to prevent evaporation. The soil moisture is measured each 24 hours until the changes are very small, at which point the soil moisture content is the estimate of field capacity. vi. Permanent wilting point Generally, at the permanent wilting point the soil moisture coefficient is defined as the moisture content corresponding to a pressure of -15 atmospheres from a pressure plate test. Although actual wilting points can be somewhere between -10 and -20 atm, the soil moisture content varies little in this range. Thus, the -15 atm moisture content provides a reasonable estimate

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3. Field measurements

of the wilting point.

3.2.4 An example problem on soil moisture A cylindrical soil sample 10 cm in diameter and 10 cm long has been carefully taken so that negligible compaction has occurred. It was weighed before oven drying (1284 grammes) and after (1151 g). What soil parameters can be identified? 1. Bulk Density: g b = Wb / V (12) = 1151 g / [(3.14 * (10 cm)2 /4) * 10 cm] = 1.466 g/cm 3 2. Dry Weight Moisture Fraction: (5) = (1284 g / 1151 g) / 1151 g = 0.116 3. Volumetric Moisture Content: = (1.466 g / 1..0 g/cm 3 ) * 0.116 = 0.170 (6) 4. Water Content Expressed as a Depth: Depth of Water = q * Depth of Soil = 0.17 cm of water per cm of soil. Now suppose the soil sample is carefully rewetted to the saturation point, utilizing 314 9 of water to do so. What other soil properties are identified? 5. Porosity: f = Vp / V (1) =

= 0.40

6. Initial Soil Saturation: S = q / f = 0.170 / 0.40 = 0.425 (4) 7. Specific Weight of the Soil Particles: g S = g b / (1 - f ) = 1.466 / 0.60 = 2.44 g/cm 3 (7) Finally, suppose the sample is allowed to drain under conditions where it does not dry due to evaporation until the water in the sample is under a negative pressure of -1/3 atm so that one can assume it is at field capacity. The water draining from the sample was collected and weighed 160 g. What other evaluations are now possible? 8. Field Capacity Volumetric Moisture Content:

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3. Field measurements

q

fc

= g b Wfc / g w (8)

9. Soil Moisture Depletion at the Time of Sampling: SMD = (q fc - q i)* RD = (0.196 - 0.170) RD = 0.026 RD (11) If the root depth is 100 cm, SMD = 2.6 cm

3.3 Infiltration 3.3.1 3.3.2 3.3.3 3.3.4

Infiltration functions Typical infiltration relationships Measuring infiltration An example infiltrometer test

Infiltration is the most important process in surface irrigation. It essentially controls the amount of water entering the soil reservoir, as well as the advance and recession of the overland flow. Typical curves of infiltration rate, I, and cumulative infiltration, Z, are shown in Figure 21. Irrigation of initially dry soil exhibits an infiltration rate with a high initial value which decreases with time until it becomes fairly steady, which is termed the 'basic infiltration rate'. Infiltration is a complex process that depends upon physical and hydraulic properties of the soil moisture content, previous wetting history, structural changes in the layers and air entrapment. Figure 21. Typical infiltration rate and cumulative infiltration function

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In surface irrigation, infiltration changes dramatically throughout the irrigation season. The water movements alter the surface structure and geometry which in turn affect infiltration rates. The term 'intake' is often used interchangeably with 'infiltration', particularly where the geometry of the field influences the infiltration process.

3.3.1 Infiltration functions Both the procedures for interpreting field data and those covering surface irrigation design require that infiltration be described mathematically. There are a number of mathematical equations to choose from, probably none as versatile as the two so-called Kostiakov-Lewis relationships. The simplest approximation of cumulative infiltration is written: Z = k ra (13) in which, Z = cumulative infiltration in units of volume per unit length per unit width; r = intake opportunity time; and k and a = empirical constants. Equation 13 is simple, easy to define, and widely used. Its major disadvantage is its inadequacy in describing infiltration over long time periods. The infiltration rate based on Eq. 13 is: I = a k ra-1 (14) Since a is always less than unity, I approaches zero at infinite time. This is a condition not typically encountered in the field. Some soils do, however, have extremely small infiltration rates after a period of time and Eqs. 13 and 14 can be used effectively. http://www.fao.org/docrep/t0231e/t0231e05.htm#TopOfPage[6/18/2013 7:18:18 PM]

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A more general infiltration function is the extended form of Eq. 13: Z = k ra + fo r (15) in which fo is the long term steady, or 'basic' infiltration rate in units of volume per unit length per unit time and width. Equation 15 easily reduces to Eq. 13 if the soil intake rate at long times approaches a zero value or if the irrigation event is short compared to the time required for the infiltration to reach a steady rate. Equation 15 gives a better long term approximation on many soils. One case in particular where Equation 15 has been shown superior to Eq. 13 is when estimates of field runoff are being made. The infiltration rate using Eq. 15 is: I = a k ra-1 + fo (16)

3.3.2 Typical infiltration relationships The Soil Conservation Service of the US Department of Agriculture developed a series of 'intake families' to assist field technicians and engineers. The curves were based on field measurements made over a period of years at numerous locations. They are given in the SCS National Engineering Handbook, Chapters 4 and 5 dealing with border and furrow irrigation. To be consistent with the analysis contained in this guide, a number of modifications were made to the SCS intake family concept. First, the curves were redefined in the format of Eq. 15 by defining a basic intake rate, fo , for each family, and then recomputing the values of a and k. Figure 22 shows the intake curves which result (Gharbi, 1984). Table 3 gives the k, a, and fo coefficients for each intake curve along with the typical soil type. The units employed are m 3 /m of length per 'characteristic' width of the field. For borders and basins, the 'characteristic' width is l metre. For furrows it is the wetted perimeter of the furrow crosssections. Thus, to use the functions for furrow irrigation, it is necessary to estimate the wetted perimeter for the inlet discharge, divide this value by the furrow to furrow spacing, and then adjust the k and fo values by multiplying each by the resulting perimeter to spacing ratio. The k and fo values should not be reduced below 50 percent as would be the case for widely spaced furrows. Figure 22. Kostiakov-Lewis intake relationships based on the US Dept. of Agriculture's intake series Table 3 KOSTIAKOV-LEWIS INTAKE PARAMETERS (after Gharbi, 1984) curve no. k m/min a

a

f o m/min ave. 6 hr intake rate soil type

.05

.00426

.258

.000022

2

.10

.00383

.317

.000035

4

.15

.00360

.357

.000046

5

.20

.00346

.388

.000057

6

.25

.00337

.415

.000068

7

.30

.00330

.437

.000078

8

.35

.00326

.457

.000088

9

.40

.00323

.474

.000098

10

.45

.00321

.490

.000107

12

.50

.00320

.504

.000117

13

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clay

clay loam

silty loam

3. Field measurements

.60

.00320

.529

.000136

15

.70

.00321

.550

.000155

17

.80

.00324

.568

.000174

20

.90

.00328

.584

.000193

22

1.00

.00332

.598

.000212

25

1.50

.00361

.642

.000280

35

2.00

.00393

.672

.000339

45

sandy loam sandy

The segregation of the intake families by soil type is qualitative, but it serves the field technician or irrigation engineer during preliminary design or evaluation work. The relationships given in Figure 22 and Table 3 are not intended as substitutes for field measurements when they can be made. These measurements are among the most important tasks that should be undertaken as part of surface irrigation work.

3.3.3 Measuring infiltration Infiltration is one of the most difficult parameters to define accurately. The importance of infiltration combined with the difficulties in obtaining reliable data suggests that the field technician should expect to spend considerable time evaluating infiltration. The irrigation engineer should ensure that infiltration has been adequately defined. Four commonly employed techniques for measuring infiltration are noted here. These are (1) cylinder infiltrometers; (2) ponding; (3) blocked recirculating infiltrometers; and (4) a deduction of infiltration from evaluation of the advance phase and the tailwater hydrograph. i. Cylinder infiltrometers Haise et al. (1956) presents one of the most complete instructions on the use of cylinder infiltrometers. A metal cylinder (Figure 23) with a diameter of 30 cm or more and a height of about 40 cm is driven into the soil, using a driving plate set on top of the infiltrometer and a heavy hammer. Figure 23. A schematic of a ring infiltrometer and driving plate (after Haise et al., 1956)

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The procedure for installing the cylinder infiltrometers is to begin by examining and selecting possible sites carefully for signs of unusual surface disturbance, animal burrows, stones that might damage the cylinder, etc. Areas that may have been affected by unusual animal or machinery traffic should be avoided. The individual cylinders used for a single test should be set within a 0.2 ha area so that they can conveniently be run simultaneously. Then the cylinder is set in place and pressed firmly into the soil, after which the driving plate is placed over the cylinder and tampered with the driving hammer until the cylinder is driven to a depth of about 15 cm. The cylinder should be driven so that the driving plate is maintained in a level plane which will require that it be checked frequently to keep it properly oriented. It is generally suggested that a buffer ring around the infiltrometer be installed so that water infiltrating from the infiltrometer will percolate vertically and thereby preserve the integrity of the measurement. There have been, however, a large number of comparisons between buffered and non-buffered readings and generally one concludes that the spatial variability in the field is so much larger than the effect of buffering that it is not worthwhile to add the substantial effort of installing and operating the buffer. In addition, most comparisons cannot

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detect the effect of the buffer. Given the accuracy of the method in depicting representative infiltration, any error caused by omitting the buffer is insignificant. Thus, the buffered cylinder infiltrometer testing is not recommended in this guide. After the infiltrometer has been installed, the test is conducted in the following manner. The volume of the cylinder above the soil is carefully measured (diameter, depth, etc.). A gauge of almost any type is fixed to the inner wall so that the water level changes that occur can be measured. A pre-measured volume, about 80-90 percent of the infiltrometer capacity of water similar to the irrigation water, is added quickly to the infiltrometer. When the water surface is quieted, an initial reading should be taken. The infiltration that occurs during the period between the start of the test and this first measurement is the difference between the computed initial level and the first actual reading: (17) Additional measurements should be recorded at periodic intervals, 5 to 10 minutes at the start of the tests, expanding to 30 to 60 minute intervals after 3 or 4 readings, but the observation frequencies should be adjusted to infiltration rates (see Figure 24 for a convenient recording form). Measurements should be continued until the intake rates are constant over a 1 to 2 hour period. Figure 24. Data recording form for ring infiltrometer tests (after Haise et al., 1956) When the water level has dropped about one-half of the depth of the cylinder, water should be added to return the surface to its approximate initial elevation. The depth should be maintained in the cylinder between 6 and 10 cm throughout the test. When water is added, it is necessary to record the level before and after filling. The interval between these two readings should be as short as possible to avoid errors due to infiltration during the refilling period. Where the infiltration rate indicated by a single cylinder is unusually high, there is a possibility that either the cylinder has been improperly installed or it has been installed over a crack or root tube in the soil. These possibilities should be checked at the conclusion of a test. Analysis of the data is usually made by plotting the data on logarithmic paper (cumulative depth, Z, on the vertical axis, cumulative time, t, on the horizontal axis). If the test is run long enough to establish a steady infiltration rate, as it should be approximately, this plot will not be linear. To evaluate the infiltration function, select readings near the later part of the test and take the slope as the basic intake rate, fo . Then use the slope of the first few data points on the logarithmic paper to define the slope, a. The intercept of the horizontal axis at 1.0 is the k value. This procedure assumes that at short times the contribution to cumulative infiltration from the steady state, fo term in Eq. 15 is negligible. These assumptions are better for the heavy soils than for the light soils. ii. Ponding methods Ponds can be created using bunds or dykes around an area on the ground surface and operated in the same manner and by using the same procedures discussed above for cylinders. The ponding; method can be used in small basins and other larger ground surface areas to evaluate the infiltration rates of a larger fraction of the field. The disadvantage of this technique is that edge effects can be significant.

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This problem can be overcome by giving special care to the sealing of the pond perimeter with compacted clay or installing a plastic barrier. The operational and data gathering procedures, including the forms for recording data, are the same as for cylinder infiltrometers. The application of the ponding; technique to furrows requires a slightly different infiltrometer configuration. The total infiltration in a furrow consists of water moving laterally through the furrow sides as well as vertically downward. Bondurant (1957) developed a 'blocked' furrow infiltrometer which recognizes this special feature of furrows. Two sharp edged plates are driven into both ends of a furrow section to isolate a short length. The furrow geometry is then determined (as will be discussed later in this section) so the depth of water can be determined at time zero. Again, a known volume of water is added to the test section and readings begin. Since the furrow cross-sectional area declines with depth, it is best to maintain a fixed water level and record the water necessary to do so as shown in Figure 25. For the data analyses, the reservoir readings need to be adjusted for the difference in surface area between the furrow section and the reservoir, i.e. the reservoir readings of cumulative infiltration need to be multiplied by the ratio of the furrow surface area to the reservoir cross-sectional area. The remainder of the data analysis is the same. Figure 25. Blocked furrow infiltrometer (after Walker and Skogerboe, 1987) The issue of using buffer furrows must be considered. Where buffering is not considered important in cylinder or pond infiltration measurements, it can be important in furrow cases. Judgement must be exercised on this point. For silt and clay soils, the basic intake rate will generally be reached before the wetting fronts of adjacent furrows meet in the soil between furrows and the buffering is probably not necessary. In sandy soils, this may not be the case so the basic intake rate may be influenced by the soil moisture distribution after the wetting fronts meet. Thus, the buffering would be necessary to determine accurate readings. iii. Recycling infiltrometers Another innovation for evaluating infiltration, primarily in furrows is the recycling infiltrometer. The advantage of this technique is that both the geometric and hydraulic conditions encountered during irrigation are simulated during the test. This provides a better approximation of actual field situations than the static methods described above. The dynamic changes in soil-water interface must be realistically simulated. The movement of suspended particles develops a different surface condition than under the static water surface. The static case tends to form a more impermeable soil layer than would occur under usual conditions of overland flow. The recycling infiltrometer for furrows is shown in Figure 26. A sump is installed at each end of a furrow section 5 to 6 metres in length. The sumps should be carefully buried in the ground so that the sump inverts correspond with the furrow bed elevation. Water is pumped from the water supply reservoir via a hose into the furrow inflow sump. It then advances across the furrow test section and is collected in the tailwater sump. Another pump then moves the water back into the water supply reservoir. The discharge in the system can be regulated by various valves to maintain a constant water level in the tailwater sump. Figure 26. Recycling furrow infiltrometer (after Malano, 1982) a. recycling furrow test section b. supply reservoir c. tailwater sump and pumpback system

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d. recording pressure sensor on supply reservoir As with the blocked furrow technique, the furrow cross-section must be measured so that the relationship of the surface area to that of the reservoir is known. Thus, the cumulative infiltration function is developed in the same way it is for cylinder and pond measurements, i.e. the cumulative infiltration is the reservoir readings corrected by the ratio of surface areas in the furrow and the reservoir. The time required to complete the advance phase can be minimized by increasing the furrow inflow discharge rate for a few minutes. The decline in the reservoir volume is a direct reading of the cumulative infiltration into the furrow.

3.3.4 An example infiltrometer test Table 4 gives one set of cylinder infiltrometer data taken from a field study. A plot of cumulative depth versus cumulative time is given in Figure 27. Figure 27. Plot of cumulative time and infiltration for the example problem

From the last four readings, a linear slope of the plot is calculated as follows: time

Z

DZ

t

fo

fo

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(min) (mm) 960 1140 1320 480

mm/min m/min

50 6

180

0.033

0.000033

5

180

0.028

0.000028

4

160

0.025

0.000025

56 61 65

Table 4 EXAMPLE CYLINDER INFILTROMETER DATA Time Readings

Gauge Readings

Clock Cumulative Gauge Cumulative hrs

min

mm

mm

0800

0

187

0

0801

1

183

4

0802

2

182

5

0804

4

181

6

0806

6

180

7

0810

10

179

8

0820

20

177

10

0830

30

176

11

0900

60

173

14

1000

120

169

18

1100

180

166

21

1200

240

163

24

1400

360

158

29

1600

480

153

34

1800

600

149

38

2400

960

137

50

0300

1140

131

56

0600

1320

126

61

0840

1480

122

65

One can see that the linear slope is changing with time even after more than 24 hours and thus the contribution to Z from the nonlinear portion is still evident. Nevertheless, these are the data available and fo can be selected as 0.000025 m/min or .000025 m 3 /min per unit width per unit length. This value can be expected to be slightly higher than in reality due to the problem noted above, but the error is not large. Now the non-linear term in Eq. 15 can be determined by examining the first points of the data. Since fo is now estimated, an adjustment can be made as follows: time

Z

Z-f o r

min mm mm 0

0

0

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1

4

3.975

2

5

4.95

4

6

5.90

6

7

6.85

10

8

7.75

20

10

9.50

A regression can be run through the Z-fo r versus t data to arrive at a and k values. k is read directly from the table as 3.975 mm/mm a or 0.003975 m 3 /min a /unit width/unit length. The value of a is found by fitting the end points: log(9.50) = a log (20) log(3.975) = a log (1) and by simultaneous solution:

Thus, Z = 0.003975 r .291 + 0.000025 r

3.4 Flow measurement 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5

Cutthroat flumes Example of cutthroat flume calibration Rectangular thin-plate weirs Example of rectangular sharp crested weir analysis V-notch weirs

There are many useful flow measuring devices available for measuring water as part of surface irrigation evaluation and continued monitoring during the operation phases of the system. For on-farm monitoring and evaluation flumes and weirs are usually the most helpful. Flumes include the Parshall flume, the H-flume, the cutthroat flume, the V-notch flume and the trapezoidal flume. Weirs might include rectangular, triangular and Cipolletti sharp-crested weirs and various broad-crested weirs. It would be beyond the space available in this guide to describe each of these in sufficient detail to be useful to those working in the field. Most have ratings for a specific size and geometry which are supplied by manufacturers. The reader is referred to several references in the bibliography at the end of this guide for such information. However, three of the devices noted above are very easily fabricated in the field, have general ratings and are highly portable. These are: (1) the cutthroat flume; (2) the rectangular sharpcrested weir; and (3) the triangular, or V-notch sharp-crested weir. A fourth device is a recently developed broad-crested weir which enjoys simplicity of construction and portability, but which requires a computer generated rating unless a standard size is selected. The reader is recommended to Bos et al. (1985) for a full explanation of this flow measuring structure.

3.4.1 Cutthroat flumes The cutthroat flume shown in Figure 28 was developed by Skogerboe et al. (1967), with subsequent extensions in the ratings by Bennett (1972) and others. Because the cutthroat

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flume has the same geometric shape for all sizes, the ratings of all flumes can be interpolated within a degree of accuracy suitable for field use. Figure 28. The cutthroat flume The cutthroat flume with its level floor and simple inlet and exit is easy to construct and install in almost any field situation (see Figure 29 for the the flume's dimensions). Fabrication errors are not serious as the ratings are easily adjusted. Figure 29. Layout and geometry of the cutthroat flume The basic discharge equation for cutthroat flumes is: (18) where, Q = the discharge in cubic feet per second (1 cfs =.0283 cubic metres/sec); h u = the upstream gauge reading in feet (1 foot = 0.3048 metres); Cf = the 'free flow' coefficient; and b f = the 'free flow' exponent. The value of b f can be read directly from Figure 30. The value of the free flow coefficient Cf, is a function of the flume's length and throat width: Cf = K fW1.025 (19) where, W = the throat width in feet; and K f = the flume 'length' coefficient (Figure 30). Figure 30. The cutthroat flume rating curves (after Walker and Skogerboe, 1987) For accurate discharge measurements, the recommended ratio of flow depth to flume length (h u /L) should be less than or equal to 0.33. As the h u /L ratio increases inaccuracies increase because of higher approach velocities and more turbulent water surface profiles at the flume gauge. The cutthroat flume was designed to be a critical depth flume in which the flow is sufficiently restricted to cause a supercritical flow velocity to occur near the throat section. As with most flow measuring devices occasions arise when the downstream depth 'submerges' the throat so that the velocity remains in the subcritical regime. The point where this occurs is the 'transition' submergence, St, or the ratio of upstream to downstream depths (h u /h d ). The St values for the cutthroat flume are also plotted in Figure 30. Under submerged conditions, Eq. 18 is modified to: (20)

where, S = the 'submergence', (h u /h d );

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Cs = the 'submerged flow' coefficient; and n s = the 'submerged flow' exponent. These parameters are also dependent on the flume length and/or width and, therefore, can be obtained for any flume dimension by reading the value from Figure 30. The submerged flow coefficient, K s , is also plotted in Figure 30. This allows computation of the submerged Cs = K s W1.025 (21) If the cutthroat flume is found to be operating in the submerged regime by computing a submergence greater than St , it should be reinstalled in the channel. This involves raising the flume until a free flow regime exists. It may be necessary to reduce the throat width in this process. However, if it must operate in the submerged flow regime, Eq. 20 yields accurate readings. When the submergence approaches 90 percent, it is difficult to make sufficiently accurate gauge readings in the field.

3.4.2 Example of cutthroat flume calibration A basin evaluation is to be conducted with an estimated flow of .25 m3 /sec. A cutthroat flume is needed in the watercourse upstream of the basin to record the inflow. With this flow in the watercourse, there is only 13.5 cm of channel freeboard available with which the flume can operate. At this flow, the downstream depth below the flume would remain at 31.5 cm. The watercourse's constructed depth is 45 cm. How would the flume be selected and installed? The first step in the solution of this problem is to observe the potential submergence condition. If a flume were installed, the maximum upstream depth at the gauge would be limited to the constructed depth of the channel (45 cm). In other words, the flume could only back water up in front of it to a depth of 13.5 cm in order to generate the head necessary to pass the flow. This would assume the flume floor was installed at the same level as the channel bottom. The submergence in the flume would then be 70 percent, found by dividing the downstream depth given as 31.5 cm by the maximum upstream depth 45 cm. A quick look at Figure 30 shows that only cutthroat flumes with lengths greater than about 4.4 feet (1.34 metres) would work, but these would be heavy and cumbersome for this field study so an examination of the alternatives is necessary. Note, any smaller flume will have to be raised above the channel bottom. Suppose for convenience, a 1-metre long flume is desired. From Figure 30, the following parameters would be selected: St = 66.2 %, n f = 1.785 and K f = 4.27. Assume the floor will be elevated above the channel by d cm such that:

or d =5.29 cm, say 6 cm In other words, the eventual submergence in the flume will be 66 % and it will operate in the free flow range. From Eq. 18, Cf can be found as:

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Then from Eq. 19, W = (Cf/K f)1.0/1.025 = (5.689/4.27)0.9756 = 1.323 feet = 40.33 cm, say 41 cm Now running through the calculations again shows that using a 1-metre long flume with a throat of 41 cm and elevated 6 cm up off the bottom will pass 0.25 m 3 /sec at a total upstream depth of 44.6 cm and with a submergence of 66 percent or just in the free flow range.

3.4.3 Rectangular thin-plate weirs Weir structures are commonly used in irrigation systems near turnouts from the water delivery network to measure flows. A disadvantage of using weirs for flow measurements is that to ensure a free flow operation, the water upstream must be 'backed up' or its level increased substantially. This so-called 'head' is greater for weirs than for flumes. In addition, sediment and debris are often trapped by the weir. However, weir structures tend to provide more accurate discharge ratings than most other devices. There are some excellent references on weirs, such as USBR (1967), Bos (1976) and Ackers et al. (1978). Today's free flow ratings for rectangular sharp crested weirs are based on extensive laboratory studies by Kindsvater and Carter (1957). A definition sketch of a standard rectangular weir is shown in Figure 31. The free flow discharge equation is: Q = 2.9524 Ce Be h e 1.5 (22) where, Q = the flow in m 3 /sec; Ce = an 'effective free flow' discharge coefficient; b e = the 'effective width' of the constriction in m; and h e = the 'effective head' in m. Figure 31. Definition sketch of the rectangular sharp crested weir (after Bos, 1976)

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The coefficient, Ce , is plotted in Figure 32 as a function of b/B and h u /p, which are shown schematically in Figure 31. The units are again in metres. Figure 32. Definition of the Ce coefficient for rectangular thin-plate weirs (Kindsvater and Carter, 1957)

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The effective constriction width, be, is: b e = b + K b (23) where K b is plotted against the constriction ration b/B, in Figure 33. The effective head, he, is given simply: h e = h u + 0.0001 (24) Figure 33. Constriction width correction, K b , for rectangular thin-plate weirs (Kindsvater and Carter, 1957)

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3.4.4 Example of rectangular sharp crested weir analysis In the example of Subsection 3.4.3, would it be feasible to use a rectangular weir, and if so what would its dimensions be? Assume the watercourse is 1.5 m wide. According to Figure 31, the weir crest must be at least 5 cm above the downstream water level. This means the head upstream, h u , can be as much as 45 - (31.5) = 8.5 cm. The value of b/B could be as great as 1.0 in which case Figure 33 shows K b to be -0.0009, or from Eq. 23, b e = 1.4991 m. If h u was 8.5 cm, he from Eq. 24 would equal .0851 m, h u /p would be .085/(31.5 + 5.0) = .0023, and from Figure 32, Ce would be 0.62. From Eq. 22: Q = 2.9524(.62)(1.499)(.0851)1.5 = 0.068 m 3 /sec Thus, the weir cannot pass the necessary flow at the allowable depth. In fact, one can turn the procedure around and discover that the weir must have an hu of more than 20 cm to pass the necessary flow.

3.4.5 V-notch weirs Perhaps with other things being equal, the most accurate measuring device is the V-notch weir (Figure 34). Shen (1960) presented the generalized discharge equation for V-notch weirs as (the units are the same as for Eq. 22) Q = 2.3691 Ce tan(q /2) h u 2.5 (25) where,

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q = the central angle of the notch in degrees. The parameter K h can be obtained from Figure 35 and Ce from Figure 36. Both K h and Ce depend on the notch angle so long as h u /p < .4 and p/B < .2. Figure 34. The V-notch weir (from Bos, 1976)

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Figure 35. Head correction factor, K h , for V-notch weirs (Shen, 1960)

Figure 36. Discharge correction factor, Ce for V-notch weirs (Shen, 1960)

3.5 Field evaluation 3.5.1 3.5.2 3.5.3 3.5.4

Advance phase Ponding phase or wetting Depletion phase Recession phase

The phases of a surface irrigation event were listed previously: (1) advance; (2) wetting or ponding; (3) depletion; and (4) recession. The field measurements needed to evaluate each of

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these phases are as follows.

3.5.1 Advance phase Data from an evaluation of the advance phase are generally the most important in terms of the information they develop. In preparation for these tests, points in the field should be located with a grid of markers or stakes as outlined in Subsection 3.1. In fact, the grid put in place to establish this field topography can be used for the advance and recession phase analyses. It is often helpful to add a couple of stakes near the field inlet of borders and basins which have a marking that will allow observation of the water depths. Measurement of the cross-sectional geometry of furrows and corrugations is important in furrow evaluations. For each furrow, the cross-sectional geometry should be measured at two to three stations before and after the irrigation. A profilometer for determining the crosssections of furrows is shown in Figure 37. Individual scales on the rods of the profilometer provide data to plot furrow depth as a function of the lateral distance. These data can then be numerically integrated to develop geometric relationships such as area verses depth, wetted perimeter versus depth and top-width verses depth. Figure 37. Furrow profilometer for evaluating cross-sectional shapes During the evaluations the depth and extent of the water surface should be measured periodically at selected points. These data can be combined with flow areas to compute the surface storage. There are two important measurements necessary during the advance phase: (1) the discharge hydrograph onto the field or into the test furrows; and (2) the elapse time from introduction of the water until the advancing front reaches each of the stations along the direction of flow. It is important to maintain the inflows during tests at constant levels. Variations significantly affect the movement of the advance front. Forms for recording the readings are given in Figure 38 for furrows and Figure 39 for borders and basins. For furrow irrigation, the tests are conducted on individual furrows and the advance readings are simply distance from the furrow inlet as a function of time. For borders and basins, the technique for measuring the rate of advance is to plot contours of the advancing front at periodic intervals. If the field is bare or the crop height is small, the advancing front can be photographed to assist with contouring the advance. The field stakes should be marked in such a manner that allows easy identification for purposes of sketching and identifying locations. A typical example of advancing contours in a square-shaped basin will be shown in the next section.

3.5.2 Ponding phase or wetting The 'wetting phase' is usually more applicable for furrow and border irrigation where tailwater runoff can occur between the end of the advance phase and the cutoff. Ponding is the more common term for basin irrigation. The measurements needed during this phase are either the runoff hydrograph, if one occurs, or the ponding; if the end is dyked. The flow rate from borders is substantially higher than from a single furrow and, thus, the hydrograph should be measured with a flume or weir. During an evaluation, the water should run off the field for a sufficient length of time to develop a relatively steady runoff hydrograph, even if this means over-irrigating. The runoff hydrograph is needed to determine the basic intake rate of the soil.

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3. Field measurements

water surface elevation declines and ends when any portion of the ground surface is bare of water. The only measurement that needs to be recorded is the time the first areas are drained.

3.5.4 Recession phase For surface irrigated fields the recession phase ends when the surface water disappears at each measuring station and is recorded on forms such as shown in Figures 38 or 39. The time difference at each measuring station between the clock time or cumulative time for advance and recession is the intake opportunity time. Figure 38. Data recording form for furrow irrigation evaluations (from Ley, 1980) Figure 39. Format for recording basin irrigation evaluation data (from Walker and Skogerboe, 1987) Recession is difficult to observe because there is not a discernable receding edge. Further, the flow may recede from both ends of the field simultaneously. Nevertheless, as the water drains from the field, the time of recession at the field stations is helpful information. For the purposes of furrow irrigation, two very simple procedures are proposed. First, recession in furrows with a slope of 0.25 percent or more is relatively linear. To describe recession, it is suggested that the flow be observed mid-way along the furrow and at the end. When the flows have reduced to a nearly drained state, approximately 90-95 percent, record the time as the recession time at the two points. Then approximate the recession trajectory by plotting a curve through the following three points: (1) time of cutoff at the field inlet thus neglecting depletion; (2) time of recession at the mid-way point; and (3) time of recession at the lower end of the furrow. Secondly, if the furrow slope is less than .25 percent, assume the recession is negligible and simply record depletion time at the end of the furrow. Recession in borders is generally a much longer period of time than in furrows, but it may be quite well approximated by the three point method noted above for furrows, the only difference being to use the depletion time as the upstream point instead of the cutoff time. In basins, the post-cutoff period is usually limited to a depletion phase which will occur at the same time over the basin depending on the variations, and undulation in the surface topography.

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4. Evaluation of field data

Produced by: Natural Resources Management and Environment Department

Title: Guidelines for designing and evaluatin surface irrigation systems...

More details

4. Evaluation of field data 4.1 Objectives of evaluation 4.2 Performance measures 4.3 Intermediate analysis of field data 4.4 System evaluation 4.5 General alternatives for improvement 4.6 An example furrow irrigation evaluation

4.1 Objectives of evaluation The principal objective of evaluating surface irrigation systems is to identify management practices and system configurations that can be feasibly and effectively implemented to improve the irrigation efficiency. An evaluation may show that higher efficiencies are possible by reducing the duration of the inflow to an interval required to apply the depth that would refill the root zone soil moisture deficit. The evaluation may also show opportunities for improving performance through changes in the field size and topography. Evaluations are useful in a number of analyses and operations, particularly those that are essential to improve management and control. Evaluation data can be collected periodically from the system to refine management practices and identify the changes in the field that occur over the irrigation season or from year to year. The surface irrigation system is a complex and dynamic hydrologic system and, thus, the evaluation processes are important to optimize the use of water resources in this system. A summary of the data arising from a field evaluation is enumerated below. There are several publications describing the equipment and procedures for evaluating surface irrigation systems, but not all give a very correct methodology for interpreting the data once collected. The data analysis depends somewhat on the data collected and the information to be derived. This section will deal with two aspects of an evaluation. The first is the definition of the typical field infiltration relationship using the evaluation data describing the surface flow. The mathematical basis of the infiltration analysis will be the extended form of the Kostiakov-Lewis formula (Eq. 15). The second is the evaluation of the efficiency of the irrigation event studied. Although many performance measures have been suggested, only four will be noted herein: (1) application efficiency; (2) storage efficiency; (3) deep percolation ratio; and (4) runoff ratio. These will be defined here before detailing the analyses of infiltration and performance.

4.1.1 Field data The field measurements outlined in the previous section provide the following elements in a field evaluation: http://www.fao.org/docrep/t0231e/t0231e06.htm#TopOfPage[6/18/2013 7:18:40 PM]

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i. the inflow hydrograph (per furrow or per border or basin); ii. the advance and recession of the water over the field surface; iii. the runoff hydrograph (if the field is not dyked); iv. the soil moisture depletion prior to the irrigation; v. the volume of water on the soil surface at various times; vi. infiltration and water holding capacities of the soil; and vii. the geometry of the cross-sectional flow area. Not all these data will be needed as part of the field evaluation. In some cases, such as infiltration, one set of data can be derived from others to reduce the time and expense of the field measurements.

4.2 Performance measures 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6

Application uniformity Application efficiency Water requirement efficiency Deep percolation ratio Tailwater ratio Integration measures of performance

Among the factors used to judge the performance of an irrigation system or its management, the most common are efficiency and uniformity. These parameters have been subdivided and defined in a multitude of ways as well as named in various manners. There is not a single parameter which is sufficient for defining irrigation performance. Conceptually, the adequacy of an irrigation depends on how much water is stored within the crop root zone, losses percolating below the root zone, losses occurring as surface runoff or tailwater the uniformity of the applied water, and the remaining deficit or under-irrigation within the soil profile following an irrigation. Ultimately, the measure of performance is whether or not the system promoted production and profitability on the farm. In order to index these factors in the surface irrigated environment the following assumptions can be made, the consequences of which are that performance is based on how the surface flow will be managed: i. the crop root system extracts moisture from the soil uniformly with respect to depth and location; ii. the infiltration function for the soil is a unique relationship between infiltrated depth and the time water is in contact with the soil (intake opportunity time); and iii. the objective of irrigating is to refill all of the root zone.

4.2.1 Application uniformity When a field with a uniform slope, soil and crop density receives steady flow at its upper end, a water front will advance at a monotonically decreasing rate until it reaches the end of the field. If it is not dyked, runoff will occur for a time before recession starts following shutoff of inflow. Figure 40 shows the distribution of applied water along the field length stemming from the assumptions listed above. The differences in intake opportunity time produce applied depths that are non-uniformly distributed with a characteristic shape skewed toward the inlet end of the field. Application uniformity concerns the distribution of water over the actual field. A number of technical sources suggest the Christiansen coefficient as a measure of uniformity. Others http://www.fao.org/docrep/t0231e/t0231e06.htm#TopOfPage[6/18/2013 7:18:40 PM]

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argue in favour of an index more in line with the skewed distribution shown below. For example, Merriam and Keller (1978) propose that distribution uniformity be defined as the average infiltrated depth in the low quarter of the field, divided by the average infiltrated depth over the whole field. This term can be represented by the symbol, DU. The same authors also suggest an 'absolute distribution uniformity', DU a which is the minimum depth divided by the average depth. Thus, the evaluator can choose one that fits his or her perceptions but it should be clear as to which one is being used. Figure 40. Distribution of applied water along a surface irrigated field showing also the depth required to refill the root zone (after Walker and Skogerboe, 1987)

4.2.2 Application efficiency The definition of application efficiency, Ea , has been fairly well standardized as: (26) Losses from the field occur as deep percolation (depths greater than Zreq) and as field tailwater or runoff. To compute Ea it is necessary to identify at least one of these losses as well as the amount of water stored in the root zone. This implies that the difference between the total amount of root zone storage capacity available at the time of irrigation and the actual water stored due to irrigation be separated, i.e. the amount of under-irrigation in the soil profile must be determined as well as the losses.

4.2.3 Water requirement efficiency The water requirement efficiency, Er , which is also commonly referred to as the storage efficiency is defined as:

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(27) The requirement efficiency is an indicator of how well the irrigation meets its objective of refilling the root zone. The value of Er is important when either the irrigations tend to leave major portions of the field under-irrigated or where under-irrigation is purposely practiced to use precipitation as it occurs. This parameter is the most directly related to the crop yield since it will reflect the degree of soil moisture stress. Usually, under-irrigation in high probability rainfall areas is a good practice to conserve water but the degree of under-irrigation is a difficult question to answer at the farm level.

4.2.4 Deep percolation ratio The loss of water through drainage beyond the root zone is reflected in the deep percolation ratio, DPR, defined as: (28) High deep percolation losses aggravate waterlogging and salinity problems, and leach valuable crop nutrients from the root zone. Depending on the chemical nature of the groundwater basin, deep percolation can cause a major water quality problem of a regional nature. These losses can return to receiving streams heavily laden with salts and other toxic elements and thereby degrade the quality of water to be used by others.

4.2.5 Tailwater ratio Losses from the irrigation system via runoff from the end of the field are indicated in the tailwater ratio, TWR: (29) Runoff losses pose additional threats to irrigation systems and regional water resources. Erosion of the top soil on a field is generally the major problem associated with runoff. The sediments can then obstruct conveyance and control structures downstream, including dams and regulation structures.

4.2.6 Integration measures of performance With the five measures of performance defined above, a broad range of assessments is possible and specific remedies identified. Application efficiency is the most important in terms of design and management since it reflects the overall beneficial use of irrigation water. In later sections, a design and management strategy will be proposed in which the value of application efficiency is maximized subject to the value of requirement efficiency being maintained at 95-100 percent. This approach thereby eliminates Er from an active role in surface irrigation design or management and simultaneously maximizes application uniformity. If the analysis tends to maximize Ea , distribution uniformity is not qualitatively important and may be used primarily for illustrative purposes. Of course, some may prefer performance discussed in terms of uniformity or be primarily involved in systems where underirrigation is an objective or a problem. For these cases, uniformity is still available. The assumption of maximization of application efficiency in effect states that losses due to deep percolation or runoff are equally weighted.

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4.3 Intermediate analysis of field data 4.3.1 4.3.2 4.3.3 4.3.4

Inflow-outflow Advance and recession Flow geometry Field infiltration

Individual measurement of these seven processes listed above (inflow, advance, recession, outflow, soil moisture deficit, surface volume, and infiltration) is time consuming and therefore expensive. A number of procedures have been developed for estimating one or more of the seven from an analysis of the others. These are called the intermediate evaluations. Of the seven parameters listed above, only the inflow hydrograph and soil moisture deficit must be known in all cases. The evaluation of the remaining data can be divided into the following intermediate evaluations.

4.3.1 Inflow-outflow The flow through the field inlet onto the surface of the field can be measured to yield a hydrograph which, when integrated, determines the total volume of applied water. The inflows should be maintained at a steady rate. Tailwater runoff where not restricted with a dyke (outflow hydrograph) can be obtained in a similar manner. An example inflow-outflow hydrograph for a single furrow in northeastern Colorado, USA, is shown in Figure 41. Figure 41. Inflow-outflow hydrographs from a furrow irrigation evaluation in Colorado, USA (from Salazar, 1977)

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There are two useful parameters obtained from a comparison of inflow and outflow hydrographs. First, the integrated differences between the two hydrographs are an accurate measurement of the total volume of water infiltrating into the soil: Vz = Vin - Vtw in which Vz, Vin, and Vtw are the total volume of infiltration, inflow and runoff, respectively. The second parameter defined by the inflow-outflow hydrograph is the steady state or 'basic' infiltration rate. In Figure 41 the difference between the two hydrographs near the end of the irrigation is approximately .046 m 3 /min. If the basic intake rate, fo , in Eq. 15 can be defined at that point, it would be determined as follows: fo = (Q in - Q out )/L (31) = 0.046 m 3 /min/625 m = 0.000074 m 3 /min/m furrow where Q in and Q out are the flow rates in m 3 /min onto and off the field near the steady state condition and L is the length in m. It should be noted that to assess fo accurately, the outflow hydrograph should be steady. In Figure 41, the outflow hydrograph shows a continual rise which indicates that the first term in the right hand side of Eq. 15 is still a significant part of the total infiltration, and fo will be over-estimated. In these cases, it is essential to extend the irrigation test until inflow-outflow data near the time of cutoff are approximately constant.

4.3.2 Advance and recession The intake opportunity time is the interval during which water will infiltrate at a specified location. It begins when the water flow first reaches the point (advance) and ends when the water eventually drains from the point (recession). Because infiltration is assumed to be uniform over the field, the variation in intake opportunity time is also an indication of application uniformity. The time required for the water to advance to the end of the field length or to cover the field completely is an important consideration in managing surface irrigation systems. As will be seen in Section 5, the advance time dictates in large measure when the inflow must be terminated and it provides the time when field tailwater begins flowing from the field or when the field begins to pond. The advance trajectory does not have a concise mathematical description, but can be reasonably well approximated with the simple power function: (32) where x is the advance distance in m from the field inlet that is achieved in tx minutes of inflow, and p and r are fitting parameters. Elliott and Walker (1982) made several comparisons of Eq. 32 with more elaborate relationships and methods of fitting and concluded that the best results are achieved by a two-point fitting of the equation. The time of advance to a point near one-half the field length, t.5L , and the advance to the end, tL , can be simultaneously solved to define the empirical parameters, p and r: (33)

(34)

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and (35) An example use of these equations is shown in Figure 42 with the axes reversed to be consistent with the normal values of a time space trajectory. Figure 42. Advance Data (Salazar, 1977) For borders and basins, the undulations in the ground surface may have a major affect upon both advance and recession. The advancing front may be very uneven so rather than attempt to plot an average length of travel of the advancing front against time, the watered and dewatered area of the basin are plotted against time. To illustrate this type of analysis of advance-recession data, the basin field study reported by Kundu and Skogerboe (1980) can be examined. A basin 36.6 m wide and 36.6 m long was constructed immediately following land levelling. The soil was a silty clay loam. The basin was staked with a 6 x 6 m grid and irrigated with an inflow of 0.83 m 3 /min. Advance and recession contours drawn at different times during the tests are shown in Figures 43 and 44. Figure 43. Advance contours for a basin evaluation (Kundu and Skogerboe, 1980)

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Figure 44. Recession contours from basin evaluation (Kundu and Skogerboe (1980)

The data from the advance contours were plotted on a logarithmic scale in Figure 45 and were described by the following function: (36) in which Ax is the area wetted (m 2 ) in tx minutes. Figure 45. Basin advance data. (Kundu and Skogerboe, 1980) The recession data could also be plotted as a function of cumulative time but the results would not be usable since the recession occurs over the field in a somewhat varied way. In order to determine the intake opportunity time, it is necessary to record the advance and recession data at each point in the grid. Table 5 summarizes the data in terms of the spatial grid. http://www.fao.org/docrep/t0231e/t0231e06.htm#TopOfPage[6/18/2013 7:18:40 PM]

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Table 5 THE DISTRIBUTION OF INTAKE OPPORTUNITY TIME IN THE KUNDUSKOGERBOE BASIN TEST, TN MINUTES Grid Row

Grid Column 1

2

3

4

5

6

1

382 408 311 304 328 397

2

386 345 237 277 236 333

3

278 292 221 245 282 302

4

300 320 335 308 335 295

5

375 350 350 360 345 360

6

405 375 405 385 355 355

4.3.3 Flow geometry It is necessary to segregate the volume of water on the soil surface from the volume which has infiltrated into the soil during the advance phase in order to evaluate the field infiltration parameters. To do this it is necessary to describe mathematically the shape of the flow crosssection and the flow area. Probably the most useful flow equation is the Manning formula: (37) where Q is the discharge in m 3 /sec, A is the cross-sectional area of the flow in m 2 , R is the hydraulic radius in m, So is the slope of the hydraulic grade lines which is assumed to equal the field slope, if one exists, and n is a resistance coefficient. The simplest case of Eq. 37 is the sloping border in which a width of one metre is taken as representative of the flow and the relation reduces to: (38) in which y is the depth of flow in m, and Q is the flow per unit width. For basins the problem becomes slightly more complex because the field slope is zero. Under these conditions, it is often assumed that the slope of the hydraulic gradeline can be approximated by the depth at the field inlet, y o , divided by the distance over which the water surface has advanced. Equation 37 with this modification becomes: (39) where x is the advance distance at time tx , in m. Thus, the area of flow in a basin is time dependent during the advance phase and is continually changing. In sloping furrows and borders it is assumed constant with time. The geometry of flow under furrow irrigation is difficult to describe. The furrow shape is continually changing because of erosion and deposition of soil as the water moves it along, but its typical shape ranges from triangular to nearly trapezoidal. In most cases, simple power functions can be used to relate the cross-sectional area and wetted perimeter with depth. Figure 46 shows a furrow cross-section developed from the profilometer described in Section 3. The simplest way to analyse these data is to first plot the cross-section as shown, then divide the depth into 10-15 equal increments and graphically or numerically integrate area and wetted perimeter. Table 6 summarizes the writer's analysis. http://www.fao.org/docrep/t0231e/t0231e06.htm#TopOfPage[6/18/2013 7:18:40 PM]

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Figure 46. Typical furrow cross-section Table 6 EXAMPLE FURROW CROSS-SECTION ANALYSIS Furrow Depth, y Area, A Perimeter, WP cm

cm2

cm

0

0

0

1

2.90

6.137

2

10.65

10.531

3

22.00

14.393

4

36.55

18.086

5

54.10

21.632

6

74.45

25.018

7

97.45

28.319

8

122.95

31.454

9

149.35

34.581

10

179.70

37.798

Assuming a power relation between depth and both area and perimeter, a twopoint fit of the data in Table 6 will determine the parameters: (40) at y = 5 cm A = 54.1 cm2 = 5.41 x 10-3 m 2 at y = 10 cm A = 179.70 cm2 = 1.797 x 10-2 m 2 therefore,

a 1 = .01797 / 101.732 = 3.331 x 10-4 (41) at y = 5 cm WP = 21.632 cm = .2163 m at y = 10 cm WP = 37.798 cm = .378 m therefore,

b 1 = .378 / 10.805 = .05922 Equations 40 and 41 can be combined for the following expression for the hydraulic section in Eq. 37: (42)

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where, p 2 = 1.667 - .667 * b 2 / a 2 = 1.3568 and, (44)

Then Eq. 37 is written: (45) The units of depth, area and perimeter can be measured in cm for Eqs. 40 and 41 and converted to metres Eq. 45. Note that in Eq. 44, p 2 reduces to 1.667 and p 1 is equal to 1.0 when applied to border flow conditions

4.3.4 Field infiltration The most crucial and often the most difficult parameter to evaluate under the surface irrigation condition is infiltration. In general, a relatively large number of field measurements of infiltration is required to represent the average field condition. Methods which use a static water condition (such as ring infiltrometers) often fail to indicate the typically dynamic field condition. As a result there is a useful approach to obtain field representative infiltration functions based on the response of the field to an actual watering. This method determines the infiltration formula directly from the inflow-outflow and advance data, along with an assumption concerning the volume of water on the surface during the advance phase. There are of course works like that of Merriam and Keller (1978) which propose other methods of estimating the parameters in an infiltration equation. These methods have the same objectives as the technique discussed below which is to define a relationship between soil-water contact time and infiltration that accurately reflects the mass balance of water in the field confines. For instance, Merriam and Keller (1978) propose an approach using inflow and outflow data to adjust a relationship derived from ring infiltrometer tests. One should understand the fundamental processes that interact during the course of an irrigation event and apply methodologies that do not violate the nature of these processes. Figure 47 shows two infiltration curves plotted on loglog paper. They are typical of functions found from field data which can be simulated by a relationship such as Eq. 15 and have historically been approximated by relations like Eq. 13. If the evaluation is based on Eq. 13, it is only possible to describe accurately the field mass balance at the end of the advance phase. Potentially large errors will occur in estimating runoff as well as the final subsurface moisture distribution, depending on the relative 'linearity' of the infiltration process indicated on log-log paper. The larger the deviations of infiltration data from a linear relation after a logarithmic transformation, the more error that is inherent in using the wrong relationship for infiltration. The procedure discussed below is not free of the error, but it is not so burdened by a problem of structure. Figure 47. Logarithmic plot of two infiltration functions differing only in the value of the exponent

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Elliott and Walker (1982) and many colleagues before and after observed that a large number of point measurements of infiltration rates using blocked furrow and cylinder infiltrometers failed to provide a satisfactory projection of actual furrow advance or an accurate prediction of tailwater volume. These investigators concluded that perhaps a better and more effective evaluation would be to measure advance rates, hydraulic cross-sections and tailwater volumes and, from these data, deduce an average infiltration relationship. The method suggested for defining infiltration from a field evaluation of the irrigation system is based on a two-point approximation to the mass balance of water on the field during the advance phase. The solution assumes the mathematical form of both the infiltration function (Eq. 15), and the advance trajectory (Eq. 32). Utilizing these two assumptions, the volume balance equation can be written for any time (46) where Ao is cross-section area of flow at the inlet, m 2 , Q o is inlet discharge in m 3 /min/furrow or unit width, t is elapsed time since the irrigation started in min. Sz is the subsurface shape factor defined as:

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(47) The inlet cross-sectional flow area, Ao , can be computed using the uniform flow equation given in Ea . 45 rearranged as follows: (48)

Values of the Manning roughness coefficient, n, range from about 0.02 for previously irrigated and smooth soil, to about 0.04 for freshly tilled soil, to about 0.15 for conditions where dense growth obstructs the water movement. The 'two-point' method of evaluating the parameters in Eq. 15 begins by defining fo from the inflow-outflow hydrograph or by other means which will be noted in a following paragraph. Then Eq. 46 is written for two advance points using advance rate measurements to define the parameters in Eqs. 43 and 44. The two common points are the mid-distance of the field and the end of the field. Thus, for the mid-distance: (49)

and for the end of the field: (50) where t.5L is advance time to one-half the field length in min, tL is advance time to the end of the field in min, and L is field length in m. The unknowns in Eq. 49 and 50 are the parameters k and a. Solving these two equations simultaneously yields: (51) where, (52) and, (53) then Sz is found directly from Eq. 47 and the parameter k is found by: (54) Several approaches can be used for determining a value for fo in the infiltration equation. One

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4. Evaluation of field data

method utilizes the data from blocked furrow or cylinder infiltrometer tests made the day before irrigation. After an infiltrometer test has been run for several hours (the time being dependent on soil type), the essentially constant rate of infiltration can be taken to be fo . If a runoff hydrograph is not measured, such as for a basin evaluation, it is suggested that Table 3 be used to define fo based on soil type.

4.4 System evaluation 4.4.1 Furrow irrigation evaluation procedure 4.4.2 Border irrigation evaluation 4.4.3 Basin irrigation evaluation Field studies are necessary to define quantitatively the irrigation system performance in relation to not only the physical features of the system but also its design and management. Field analyses of the single irrigation may not clearly establish these relationships and, therefore, should be repeated at times when the soil, crop or operational characteristics have changed sufficiently to reveal the other facets of the irrigation system. Three typical results of surface irrigation are illustrated in Figure 48. When the inflow is cut off too soon after the advance phase, the application at some point in the field may be inadequate to refill the root zone (curve a). Or the application may just satisfy the needs in the least watered areas (curve b). But most often, the applied depths exceed the target depth, Zreq at all locations (curve c). Large differences in economic, physical, social and operational conditions occur in surface irrigated systems. Consequently it is impractical to judge any of the three cases as good or bad since situations like the need for conservation or rainfall expectations make each regime one to utilize when the time calls for it. The suggested evaluation of performance is the numerical definition of the efficiency parameters described earlier tempered by a case by case professional judgement. Figure 48. Three typical irrigation application patterns under surface irrigation (after Walker and Skogerboe, 1987) The Typical Under-Irrigation Case The Typical Complete-Irrigation Case The Typical Over-Irrigation Case

4.4.1 Furrow irrigation evaluation procedure A furrow evaluation would normally consist of activities before, during and after the irrigation. The pre-irrigation work is largely reconnaissance, equipment installation and soil moisture determination. During the irrigation, measurements of inflow, advance, runoff or ponding; and recession are made. Following the irrigation, furrow cross-sections can be determined as well as follow-up soil moisture sampling if desired. There are no formal rules for the evaluation since different personnel prefer their own order and technique. Merriam and Keller (1978) list some step by step procedures and give a convenient list of equipment and supplies that are needed. Following the field evaluation, the next step is to determine the infiltration function and then, in conjunction with the recorded intake opportunity, the distribution of water applied to the root zone. The length should be subdivided into 10 or more increments and the cumulative intake computed for each increment by: http://www.fao.org/docrep/t0231e/t0231e06.htm#TopOfPage[6/18/2013 7:18:40 PM]

4. Evaluation of field data

Zi = k [tr - (tx )i] a + fo [tr - ( tx )i] (55) in which tx is the recession time in minutes if it is determined. If not, the time of cutoff, tco , is used in Eq. 55 in place of tx . The results from Eq. 55 should then be plotted as in Figure 49 along with a line representing the application needed to refill the root zone deficit, Zreq which is the soil moisture deficit measured in the field. The plot can then be integrated graphically or numerically to define the components of application efficiency, deep percolation ratio, runoff ratio and requirement efficiency (or uniformity if desired).

4.4.2 Border irrigation evaluation The analysis of border irrigation data follows the same procedures as for furrow irrigated systems. Advance, if irregular, should be contoured and analysed as previously indicated. Simplification can be made in using flow rate per unit width instead of total border flow if the advancing front is relatively uniform. The flow cross-section is rectangular. This analysis assumes a free-draining outflow condition. For ponded conditions arising from a dyked downstream boundary, the analysis follows the basin procedure. In the evaluation of furrow systems, the infiltration during recession can often be considered negligible. For border systems, the infiltration during the recession period is significant and must be considered. Once the depletion and recession times (td and tr ) have been determined, the distribution of water applied to the soil can be plotted and the performance measures evaluated as for furrow evaluation.

4.4.3 Basin irrigation evaluation The estimates of basin application efficiencies are somewhat simplified by a small field slope and the prevention of runoff. Water first entering the basin would advance to the end dyke and then pond on the surface. As the water surface rises, it will approach a horizontal orientation. Thus, it can be seen that during the depletion and recession phases the surface water has little or no movement, and the subsurface profile is determined by adding the surface depths to the profile which developed during the advance phase. The application efficiency, deep percolation ratios and requirement efficiency can be found from the equations given earlier. The tailwater ratio is zero for basin irrigation.

4.5 General alternatives for improvement The field evaluation should identify at least some modifications that will improve efficiency and uniformity. The easily identified problems such as applying too much or too little water, the poor distribution of infiltrated water over the field, excessive tailwater runoff or significant deep percolation losses should be evident. In planning to improve irrigation performance, it must be recognized that all of the parameters are interdependent. Therefore, when considering changes in inflow, time of cutoff, or field length, one must understand that the time of advance, infiltration, tailwater runoff and deep percolation will be affected simultaneously. The flow rate used on an irrigated field will significantly affect the time of advance, the volume of runoff and the erosion hazard. Utilizing high flow rates will maximize the potential for tailwater losses (except for basin irrigation) and erosion, but minimize the time of advance and thereby the variation in opportunity time along the field length. In order to reduce the tailwater runoff from border or furrow systems, a high discharge rate can be used during the advance phase and then 'cut back' (reduced) for the wetting phase. Tailwater can be collected and http://www.fao.org/docrep/t0231e/t0231e06.htm#TopOfPage[6/18/2013 7:18:40 PM]

4. Evaluation of field data

reused as well. For whatever discharge is being used, the ideal time of cutoff, tco , occurs when the infiltrated depth in the least watered portion of the field is equal to the irrigation requirement. Discharge and time of cutoff are the two operational hydraulic parameters, with tco being the easiest for the irrigator to modify. Again, the interdependence between inflow and cutoff time must be known in order to maximize the performance of a surface irrigation system. Surface irrigation is critically dependent on the field topography. Undulations interrupt the flow of water and concentrate water in depressions. The high points tend to become saline. It is not a simple matter to apply the appropriate irrigation requirement, in fact, much greater depths are generally applied. Precision land levelling is an important aspect of improving the operation of surface irrigation systems, particularly for basins. Likewise, furrow preparation needs to yield channels of uniform depth and spacing. In short, land preparation should be considered an integral part of surface irrigation and not treated as an independent operation.

4.6 An example furrow irrigation evaluation 4.6.1 Field infiltration characteristics 4.6.2 Evaluation of system performance 4.6.3 Measures to improve performance An evaluation was conducted on an existing furrow system during the first irrigation of the season. The field characteristics were found to be as follows. The soil was a sandy loam which gravimetric soil samples indicated had a soil moisture depletion averaging 9.5 cm prior to the irrigation. The field had a uniform slope of 0.0075 with 200 metre furrows spaced at 75 cm intervals across the field. The water supply to the field was a large tube-well capable of supplying water on demand. Additional field measurements made during each evaluation were: (1) the furrow inflow hydrograph; (2) runoff hydrographs; (3) furrow shape shown in Figure 46 previously, and (4) the advance and recession trajectories. The furrow inflow was a steady value of 0.12 m3 /min during both irrigations. The remaining data are tabulated on Tables 7 and 8. The inflow to the tests was stopped at 390 minutes. Table 7 MEASURED ADVANCE AND RECESSION TRAJECTORIES Advance Distance Advance Time Recession Time (m)

(min)

(min)

0

0.0

390

47

6.0

396

112

18.0

402

151

30.0

405

200

54.8

408

Table 8 RUNOFF HYDROGRAPHS FOR INDIVIDUAL FURROWS Time Since Irrigation Started

Runoff

(min)

(Litres/sec)

54

0

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4. Evaluation of field data

57

.079

63

.264

72

.390

84

.494

102

.593

132

.694

192

.804

252

.867

312

.909

372

.939

390

.949

399

.777

402

.538

408

.0581

411

0

4.6.1 Field infiltration characteristics Analyses of the infiltration function as defined by the behaviour of water movement in the furrow involves a five step procedure. For this evaluation, these steps are as follows: i. Furrow inlet flow area, Ao , is computed from the Manning relation, Eq. 48, with the following data: Q o = 0.12 m 3 /min (measured); n = 0.04 (assumed); p 1 =.444 and p 2 = 1.357 from subsection 4.3.3; and So = the slope of 0.0075 (measured). Thus,

ii. The advance trajectory can be represented by a power function (Eq. 32). Using a two-point method based on the measured advance data: x = 200 m at tx = 54.8 min x = 112 m at tx = 18.0 min r = log(200/112)/log(54.8/18) = 0.5208 Evaluation of the parameter p is not necessary. iii. The basic intake rate, fo , is defined by Eq. 31 using: Q in = m 3 /sec (measured); and Q out = the steady state runoff, .00095 m 3 /sec (estimated from Table 8) L = the field length, 200 m (measured).

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4. Evaluation of field data

Thus, fo = (.002 - .00095) * 60/200 = 0.000315 m 3 /min/m iv. The values of k and a in the infiltration function, Eq. 15 are determined as follows. First, Eqs. 52 and 53 are defined:

and then from Eq. 51:

The subsurface shape factor s z, is described by Eq. 47:

Then, from Eq. 54: k = 0.013367 / (.7622 * 54.8 .532 ) = 0.00208 m 3 /min a /m v. The final field evaluated infiltration function for the first irrigation is: Z = 0.00213 r0.532 + 0.000315 r

4.6.2 Evaluation of system performance Using the derived infiltration function and the measured opportunity time (a recession time minus advance time at each point), the water applied to the soil reservoir was calculated and plotted in Figure 49. Also plotted is the application required to replace the root zone deficit (requirement is the depth times the furrow spacing, i.e. .095 m * .75 m = 0.0713 m3 /m). It can be ascertained graphically that both irrigations applied too much water, far more than enough to refill the root zone. Obviously in both cases the water requirement efficiency is 100 percent. Figure 49. Distributions of applied wafer curing the two test irrigations

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4. Evaluation of field data

The application efficiency for the test can be computed from the relationship: (56)

= 100 * (.0713 * 200) / (.002 * 60 *390) = 30.5% The performance of the system during the evaluation was poor, about 70 percent of all water applied was wasted from the field as runoff or deep percolation. In order to identify improvements, these losses must be separated, either by integrating the applied distribution and computing the deep percolation ratio or by integrating the runoff hydrograph and computing the tailwater ratio. For this example, the latter is chosen to reflect more confidence in measured runoff than calculated infiltration. Figure 50 shows the runoff hydrograph. Using a trapezoidal integration, the runoff per furrow during the irrigation was 16.7 m3 . The tailwater ratios were therefore:

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4. Evaluation of field data

TWR = 100 * 16.7 / (.002 * 60 * 390) = 35.7% Figure 50. Runoff hydrographs during the two evaluations

To complete the performance picture, the deep percolation ratio for the first evaluation is: DPR = 100 - Ea - TWR = 33.8% (57)

4.6.3 Measures to improve performance Losses during the irrigation were almost evenly split between tailwater and deep percolation. The most obvious way to improve the performance of this system would be to cut the inflow off when the application at the lower end of the field was approaching the required depth. If the required intake opportunity time at the end of the field is calculated and added to the advance time, the cutoff time represented by their sum is approximately 180 minutes. If this would have happened, the total water applied to the field would have been reduced from 46.8 m3 /furrow to 21.6 m 3 /furrow. The soil moisture deficit would still have been completely replenished (Ea = 100 percent), but the application efficiency would have been increased to about 66 percent. The DPR and TWR values would have been reduced to 11 percent and 20 percent respectively. Further improvements could be made by utilizing a cutback flow after the advance was completed or by adjusting the inflow rate (reducing it in this case would improve performance).

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5. Surface irrigation design

Produced by: Natural Resources Management and Environment Department

Title: Guidelines for designing and evaluatin surface irrigation systems...

More details

5. Surface irrigation design 5.1 Objective and scope of design 5.2 The basic design process 5.3 Computation of advance and intake opportunity time 5.4 Furrow irrigation flow rates, cutoff times, and field layouts 5.5 Border irrigation design 5.6 Basin irrigation design 5.7 Summary

5.1 Objective and scope of design The surface irrigation system should replenish the root zone reservoir efficiently and uniformly so that crop stress is avoided, and resources like energy, water, nutrient, and labour are conserved. The irrigation system might also be used to cool the atmosphere around sensitive fruit and vegetable crops, or to heat the atmosphere to prevent their damage by frost. An irrigation system must always be capable of leaching salts accumulating in the root zone. It may also be used to soften the soil for better cultivation or even to fertilize the field and spread insecticides. The design procedures outlined in the following sections are based on a target application, Zreq , which equals the soil moisture extracted by the crop. It is in the final analysis a trial and error procedure by which a selection of lengths, slopes, field inflow rates and cutoff times can be made that will maximize application efficiency. Considerations such as erosion and water supply limitations will act as constraints on the design procedures. Many fields will require a subdivision to utilize optimally the total flow available. This remains a judgement that the designer is left to make after weighing all other factors that he feels are relevant to the successful operation of the system. Maximum application efficiencies, the implicit goal of design, will occur when the least watered areas of the field are just refilled. Deep percolation will be minimized by minimizing differences in intake opportunity time, and then terminating the inflow on time. Surface runoff is controlled or reused. The design intake opportunity time is defined in the following way: (58) where Zreq is the required infiltrated volume per unit length and per unit width (and is equal to the soil moisture deficit) and rreq is the design intake opportunity time. For most surface irrigated conditions, rreq should be as close as possible to the difference between the recession time at each point and the associated advance time.

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5. Surface irrigation design

An engineer may have an opportunity to design a surface irrigation system as part of a new irrigation project where surface methods have been selected or when the performance of an existing irrigation system requires improvement by redesign. In a new irrigation project, it is to be hoped that the surface irrigation system design is initiated after a great deal of irrigation engineering has already occurred. The selection of system configurations for the project is in fact an integral part of the project planning process. If a new or modified surface system is planned on lands already irrigated, the decision has presumably been based, at least partially, on the results of an evaluation at the existing site. In this case, the design is more easily accomplished because of the higher level of experience and data available. In either case, the data required fall into six general categories (Walker and Skogerboe, 1987): i. the nature of irrigation water supply in terms of the annual allotment, method of delivery and charge, discharge and duration, frequency of use and the quality of the water; ii. the topography of the land with particular emphasis on major slopes, undulations, locations of water delivery and surface drainage outlets; iii. the physical and chemical characteristics of the soil, especially the infiltration characteristics, moisture-holding capacities, salinity and internal drainage; iv. the cropping pattern, its water requirements, and special considerations given to assure that the irrigation system is workable within the harvesting and cultivation schedule, germination period and the critical growth periods; v. the marketing conditions in the area as well as the availability and skill of labour, maintenance and replacement services, funding for construction and operation, and energy, fertilizers, seeds, pesticides, etc.; and vi. the cultural practices employed in the farming region especially where they may prohibit a specific element of the design or operation of the system.

5.2 The basic design process 5.2.1 Preliminary design 5.2.2 Detailed design The surface irrigation design process is a procedure matching the most desirable frequency and depth of irrigation and the capacity and availability of the water supply. This process can be divided into a preliminary design stage and a detailed design stage.

5.2.1 Preliminary design The operation of the system should offer enough flexibility to supply water to the crop in variable amounts and schedules that allow the irrigator some scope to manage soil moisture for maximum yields as well as water, labour and energy conservation. Water may be supplied on a continuous or a rotational basis in which the flow rate and duration may be relatively fixed. In those cases, the flexibility in scheduling irrigation is limited to what each farmer or group of farmers can mutually agree upon within their command areas. At the preliminary design stage, the limits of the water supply in satisfying an optimal irrigation

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5. Surface irrigation design

schedule should be evaluated. The next step in the design process involves collecting and analysing local climatological, soil and cropping patterns to estimate the crop water demands. From this analysis the amount of water the system should supply through the season can be estimated. A tentative schedule can be produced by comparing the net crop demands with the capability of the water delivery system to supply water according to a variable schedule. On-demand systems should have more flexibility than continuous or rotational water schedules which are often difficult to match to the crop demand. Whichever criterion (crop demand or water availability) governs the operating policy at the farm level, the information provided at this stage will define the limitations of the timing and depth of irrigations during the growing season. The type of surface irrigation system selected for the farm should be carefully planned. Furrow systems are favoured in conditions of relatively high bi-directional slope, row crops, and small farm flows and applications. Border and basin systems are favoured in the flatter lands, large field discharges and larger depths of application during most irrigations. A great deal of management can be applied where flexibility in frequency and depth are possible.

5.2.2 Detailed design The detailed design process involves determining the slope of the field, the furrow, border or basin discharge and duration, the location and sizing of headland structures and miscellaneous facilities; and the provision of surface drainage facilities either to collect tailwater for reuse or for disposal. Land levelling can easily be the most expensive on-farm improvement made in preparation for irrigation. It is a prerequisite for the best performance of the surface system. Generally, the best land levelling strategy is to do as little as possible, i.e. to grade the field to a slope which involves minimum earth movement. Exceptions occur where other considerations dictate a change in the type of system, say, basin irrigation, and yield sufficient benefits to off-set the added cost of land levelling. If the field has a general slope in two directions, land levelling for a furrow irrigation system is usually based on a best-fit plane through the field elevations. This minimizes earth movement over the entire field and unless the slopes in the direction normal to the expected water flow are very large, terracing and benching would not be necessary. A border must have a zero slope normal to the field water flow which will require terracing in all cases of cross slope. Thus, the border slope is usually the best-fit subplane or strip. Basins, of course, are generally 'dead' level, i.e. no slope in either direction. Thus, terracing is required in both directions. To the extent the basin is rectangular, its largest dimension should run along the field's smallest natural slope in order to minimize land levelling costs. The detailed design process starts with and ends with land levelling computations. At the start, the field topography is evaluated to determine the general land slopes in the direction of expected water flow. This need not be the extensive evaluation that is needed to actually move the earth. In fact, the analysis outlined earlier under the subject of evaluation is sufficient. Using this information along with target application depths derived from an analysis of crop water requirements, the detailed design process moves to the selection of flow rates and their duration that maximize application efficiency, tempered however by a continual review of the practical matters involved in farming the field later. Field length becomes a design variable at this stage and again there is a philosophy the designer must consider. In mechanized farming and possibly in animal power as well, long rectangular fields are preferable to short square ones in most cases except paddy rice. This notion is based on the time required for implement turning and realignment. In a long field, this time can be

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5. Surface irrigation design

substantially less and therefore a more efficient use of cultivation and harvesting implements is achieved. The next step in detailed design is to reconcile the flows and times with the total flow and its duration allocated to the field from the water supply. On small fields, the total supply may provide a satisfactory coverage when used to irrigate the whole field simultaneously. However, the general situation is that fields must be broken into 'sets' and irrigated part by part, i.e. basin by basin, border by border, etc. These subdivisions or 'sets' must match the field and its water supply. Thus, with the subdivisions established, the final land levelling is undertaken. Once the field dimensions and flow parameters have been formulated, the surface irrigation system must be described structurally. To apply the water, pipes or ditches with associated control elements must be sized for the field. If tailwater is permitted, means for removing these flows must be provided. Also, the designer should give attention to the operation of the system. Automation will be a key element of some systems. The treatment of these topics is not detailed since there are other technical manuals and literature already available for this purpose. The design methodology used in the guide relies on the kinematic-wave analysis for furrow and border advance and a fully hydrodynamic model for basin advance. These are transparent to the user of the guide, however, and further explanation for those interested can be found in Walker and Skogerboe (1987). Simple algebraic equations are used for depletion and recession. This guide has reduced the role of these hydraulic techniques to the advance phase to allow the User to participate more in the design process. The interested reader can refer to several references in the bibliography for other graphical techniques which extend beyond those given here, but as one does so, it becomes more important to understand the nature of the hydraulic assumptions.

5.3 Computation of advance and intake opportunity time 5.3.1 Common design computations The difference between an evaluation and a design is that data collected during an evaluation include inflows and outflows, flow geometry, length and slope of the field, soil moisture depletion and advance and recession rates. The infiltration characteristics of the field surface can then be deduced and the application efficiency and uniformity determined. Design procedures input infiltration functions (including their changes during the season), flow geometry, field slope and length, and determine the rates of advance and recession as well as the field performance levels for various combinations of inflow and cutoff times.

5.3.1 Common design computations Two of the design computations are the same for all surface irrigation systems. These are the estimate of required intake opportunity time and the time required for the water to complete the advance phase. A step-by-step procedure for these computations will be given here and simply referenced as such in later paragraphs. i. Computation of intake opportunity time The basic mathematical model of infiltration utilized in the guide is the following: Z = k r a + fo r (15)

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5. Surface irrigation design

where Z is the accumulated intake in volume per unit length, m 3 /m (per furrow or per unit width are implied), r is the intake opportunity time in min, a is the constant exponent, k is the constant coefficient m 3 /min a /m of length, and fo is the basic intake rate, m 3 /min/m of length. In order to express intake as a depth of application, Z must be divided by the unit width. For furrows, the unit width is the furrow spacing, w, while for borders and basins it is 1.0. Values of k, a, fo and w along with the volume per unit length required to refill the root zone, Zreq , are design input data. The design procedure requires that the intake opportunity time associated with Zreq be known. This time, represented by rreq , requires a nonlinear solution to Eq. 15. The simplest way to this solution is to plot Eq. 15 with the parameters being used in the design, such as the drawings in Figures 21 or 27. Another convenient method for those with programmable calculators or microcomputers is the Newton-Raphson procedure which is three simple steps as follows: 1. Make an initial estimate of rreq and label it T1 ; 2. Compute a revised estimate of rreq , T2 : (59)

3. Compare the values of the initial and revised estimates of rreq (T1 and T2 ) by taking their absolute difference. If they are equal to each other or within an acceptable tolerance of about .5 minutes, the value of rreq is determined as the result. If they are not sufficiently equal in value, replace T1 by T2 and repeat steps 2 and 3. ii. Computation of advance time The time required for water to cover the field, the advance time, necessitates evaluation or at least approximation of the advance trajectory. The first step is to describe the flow crosssectional area. For furrows and borders this is Eq. 48 in which the cross-sectional flow area, Ao in m 2 , and the inlet discharge per furrow or per unit width, Qo , in m 3 /min. The parameters p 1 and p 2 are empirical shape coefficients as noted previously. For border systems p 1 equals 1.0 and p 2 is 1.67. For most furrow irrigated conditions, p 2 will have a value ranging from 1.3 to 1.5. Fortunately, the furrow hydraulics are not too sensitive to variations in p 2 and a value of 1.35 will usually be adequate. The value of p 1 varies according to the size and shape of the furrow, usually in the range of .3 to .7. Figure 51 shows three typical furrow shapes and their corresponding p 1 and p 2 values. Figure 51. Typical furrow shapes and their hydraulic sectional parameters In a level slope condition, such as a basin, it is assumed that the friction slope is equal to the inlet depth, y o in m, divided by the distance covered by water, x in m. This leads to the following expression for Ao : (60)

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5. Surface irrigation design

Note Ao increases continually during the advance phase and must therefore be calculated at each time step of each advance distance as well as each flow and resistance. For sloping field conditions, Ao is assumed to be constant unless the flow, slope or resistance changes. The input data required for advance phase calculations are p 1 , p 2 field length (L), So , n and Q o . This information can be used to solve for the time of advance, tL , using either of two procedures: (1) the volume balance numerical approach; or (2) the graphical approach based on the advanced hydraulic models. iii. Volume balance advance For the volume balance numerical approach, Eq. 46 is used to describe the advance trajectory at two points: the end of the field and the half-way point. Equation 48 for the end of advance was written earlier as Eq. 50 and the half-way advance was written as Eq. 49. Equation 50 contains two unknowns, tL and r, which are related by Eq. 32. In order to solve them, a two-point advance trajectory is defined in the following procedure: 1. The power advance exponent r typically has a value of 0.1-0.9. The first step is to make an initial estimate of its value and label this value r1 , usually setting r1 = 0.4 to 0.6 are good initial estimates. Then, a revised estimate of r is computed and compared below. 2. Calculate the subsurface shape factor, s z, from Eq. 47. 3. Calculate the time of advance, tL , using the following Newton-Raphson procedure: a. Assume an initial estimate of tL as T1 T1 = 5 Ao L / Q o (61) b. Compute a revised estimate of tL (T2 ) as

(62)

c. Compare the initial (T1 ) and revised (T2 ) estimates of tL . If they are within about 0.5 minutes or less, the analysis proceeds to step 4. If they are not equal, let T1 = T2 and repeat steps b through c. It should be noted that if the inflow is insufficient to complete the advance phase in about 24 hours, the value of Q o is too small or the value of L is too large and the design process should be restarted with revised values. This can be used to evaluate the feasibility of a flow value and to find the inflow. 4. Compute the time of advance to the field mid-point, t.5L , using the same procedure as outlined in step 3. The half-length, .5L is substituted for L and t.5L for tL in Eq. 62. For level fields, the half-length and the flow area must be substituted. Equation 48 is used with L and .5L to find the appropriate values of

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5. Surface irrigation design

Ao . 5. Compute a revised estimate of r as follows: (63) 6. Compare the initial estimate, r1 , with the revised estimate, r2 . The differences between the two should be less than 0.0001. If they are equal, the procedure for finding tL is concluded. If not, let r1 = r2 and repeat steps 2-6. As an example of this series of calculations, suppose the advance time is wanted for a field with the following data: Infiltration parameters a = 0.568 k = 0.00324 m3 /min a /m f o = 0.000174 m3 /min/m inflow

Qo = 0.15 m3 /min

slope

S o = 0.001

length

L = 200 m

roughness

n = 0.04

hydraulic section

p1 = 0.55 p2 = 1.35

1. set r1 = 0.6 2.

3a. Note: If the field slope is zero, Eq. 60 would be used here for Ao and would use L in place of x.

3b.

=

146 - (+75.67) = 70.33 minutes 3c. Error = ABS (T2 - T1 ) = 75 - 70.33 = 4.67 minutes. Therefore, let T1 = 70.33 and repeat steps (3b and 3c). 3b. The second iteration yields T2 = 70.33 - (+4.2) = 66.13 minutes. Step 3c error is now 4.2 minutes so T1 = 66.13 and steps 3b and 3c are repeated. At the end of another iteration the error is less than one minute and the value of tL is found to

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5. Surface irrigation design

be 66.07 minutes. 4. The time of advance to the field's half-way point is found by following the same steps as outlined above by substituting 0.5 * L = 100 metres for the length and t.5L for the advance time to this distance. The result after two more iterations is 21.9 minutes. Note: If the field's slope is zero, the computation of t.5L must begin at Step 3a using L/2 for x. 5. 6. The error in the parameter r (.6 - .6285) is greater than the acceptable tolerance so Steps 2 through 6 are repeated. The final advance time is 65 minutes. As one easily finds, the numerical approach is justified only when one has at least a hand-held programmable calculator or microcomputer. vi. Graphical advance The graphical approach involving Figures 52a - 52f for furrows and borders and Figures 53a 53f for basins has been derived from computations using the kinematic-wave and hydrodynamic simulation models summarized by Walker and Skogerboe (1987). These models are available from a number of sources, some commercially, and are not included herein. Figure 52a. Dimensionless advance trajectories for borders and furrows having an infiltration exponent a = 0.2 Figure 52b. Dimensionless advance trajectories for borders and furrows having an infiltration exponent a = 0.3 Figure 52c. Dimensionless advance trajectories for borders and furrows having an infiltration exponent a = 0.4 Figure 52d. Dimensionless advance trajectories for borders and furrows having an infiltration exponent a = 0.5 Figure 52e. Dimensionless advance trajectories for borders and furrows having an infiltration exponent a = 0.6 Figure 52f. Dimensionless advance trajectories for borders and furrows having an infiltration exponent a = 0.7 Figure 53a. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.2 Figure 53b. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.3 Figure 53c. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.4 Figure 53d. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.5

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Figure 53e. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.6 Figure 53f. Dimensionless advance trajectories for basins having an infiltration exponent a = 0.7 The graphical procedure is as follows: 1. Define the infiltration parameters k, a, and fo the field length L; the field slope So ; the inlet discharge Q o ; surface roughness coefficient n; and the hydraulic section parameters p 1 and p 2 2. Compute the inlet flow area, Ao using Eq. 48 for furrows and borders and Eq. 60 for basins: 3. Compute the dimensionless parameter K*: (64)

4. Compute the dimensionless parameter L*: (65) 5. Enter the appropriate figures for values of the infiltration exponent, a, which bracket the design value, interpolate for the value of K*, and read the two values of : 6. Compute the time of advance: (66) 7. Average the two values to get tL for the value of a used in the design. As an example of using the graphical approach, suppose, as in the example of the numerical volume balance approach, the input data are as follows: 1. Infiltration parameters a = 0.568 k = 0.00324 m3 /min a /m f o = 0.000174 m3 /min/m inflow

Qo = 0.15 m3 /min

slope

S o = 0.001

length

L = 200 m

roughness

n = 0.04

hydraulic section

p1 = 0.55 p2 = 1.35

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2. Note: If the field slope is zero, Eq. 60 would be used here for Ao and would use L in place of x. 3.

4. 5. From Figure 52d, interpolating about 75 percent [log(2.3/1) / log(3/1) = .76] of the distance between curves K* = 1 and K* = 3 yields = 0.54. From Figure 52e, the same process yields a

= 0.50 for an average of 0.52. The advance

time is then estimated as:

Note the value using the volume balance numerical method yielded 65 minutes. Usually with careful interpolation the values of tL found from the two methods will vary less than 5 - 10 percent. v. Summary The calculation of advance time is possibly the most important design step. At the beginning of the design process, this procedure is used to test whether or not the maximum flow will complete the advance phase within a prescribed time. Then it is used to find the minimum inlet discharge, and in the case of cutback or reuse systems to find the desired flow for the system operation. It is suggested that after the maximum inflow is determined and the associated tL checked, the flow be incrementally decreased and additional values of tL determined so that a relationship between flow and advance time can be established. At the end of this procedure, the minimum flow will also have been identified as that which fails to complete the advance phase in a set time, 24 hours for example. Finally, the tL computation is used repeatedly in the search for the flow which maximizes the application efficiency.

5.4 Furrow irrigation flow rates, cutoff times, and field layouts 5.4.1 5.4.2 5.4.3 5.4.4

Furrow design procedure for systems without cutback or reuse Design procedure for furrow cutback systems Design of furrow systems with tailwater reuse Furrow irrigation design examples

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5. Surface irrigation design

ii. the cutback system; and iii. the tailwater recirculation system. These systems should be flexible to irrigate fields adequately in which the surface roughness and intake rates vary widely from irrigation to irrigation. The philosophy of design suggested in this guide is to evaluate flow rates and cutoff times for the first irrigation following planting or cultivation when roughness and intake are maximum and for the third or fourth irrigation when these conditions have been reduced by previous irrigations.

5.4.1 Furrow design procedure for systems without cutback or reuse i. Input Data: Description

Parameter

First irrigation infiltration

a, k, and f o

Later irrigation infiltration

as, ks and f os

Field length, width, slope, roughness L, Wf , S m and n Required application depth Z req Soil erosive velocity

w, p1 , and p2 V max

Water supply rate and duration

QT and T T

Number of furrows

Nf = Wf /W

Furrow spacing and shape

ii. The maximum flow velocity in furrows is suggested as about 8-10 m/min in erosive silt soils to about 13 - 15 m/min in the more stable clay and sandy soils. A maximum value of furrow inlet flow, Q max m 3 /min, that will fall within the maximum, Vmax , is: (67) The value of Q o should be adjusted so that the number of sets is an integer number, i.e. N fQ o should be an integer, but should not exceed Q max . iii. Compute the advance time, tL . iv. Compute the required intake opportunity time, rreq . v. Compute time of cutoff, tco , in min by neglecting depletion and recession: tco = rreq + tL (68) vi. Compute application efficiency, Ea : (69)

The application efficiency should be maximized subject to the limitation on erosive velocity, the availability and total discharge of the water supply, and other farming practices. The inflow should be reduced and the procedure repeated until a maximum Ea is determined.

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5.4.2 Design procedure for furrow cutback systems Any procedure which attempts to maximize application efficiencies will determine the minimal waste trade-off point between tailwater and deep percolation. Small values of inflow reduce tailwater losses but increase deep percolation losses. Large furrow flows advance over the field rapidly thereby providing the potential for greater application uniformity and less deep percolation, but also greater tailwater losses as the water flows from the field for a longer time. One method of minimizing tailwater is to reduce the furrow inflow when the advance phase is completed. Most cutback systems are designed to operate in two concurrent sets, one advance phase set and one wetting or ponding; set. The advance phase and the wetting phase are both equal in duration to the required intake opportunity time. One of the most common cutback systems is that proposed by Garton (1966) and is illustrated in Figure 54. The head ditch is divided into a series of level bays with spires or other means of diverting water into the furrows. As is shown, the differences in bay elevations correspond to the head on the outlets needed to provide the desired advance phase flow and the wetting flow simultaneously. The design procedure for the system illustrated in Figure 54 follows a sequence not entirely unlike that of the non-cutback systems but with several points of additional concern. In addition to information describing the furrow geometry, infiltration characteristics, field slope and length, and the required application, it is also necessary to know the relationship between head ditch water level and the furrow inflow: (70) where c 1 and c 2 are empirical coefficients, h is the head over the outlets, in m, and A is the outlet area in cm2 . Figure 54. Schematic drawing of the furrow cutback system proposed by Garton (1966)

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Elevation drawing showing the system of cutback furrow irrigation. In A, bay l is delivering the initial furrow flow. In B, the check dam has been removed from bay l, bay 2 is delivering the initial flow, and bay l is delivering the cutback furrow flow. In C, the check dam has been removed from bay 2, bay 3 is delivering the initial furrow flow, and bay 2 is delivering the cutback furrow flow, and bay l is shut off. The first calculation can be the required intake opportunity time using the first of the common design computations. The design should provide an advance phase flow sufficient to allow tL = rreq . Since this requirement is most likely to be a constraint under high intake conditions, the design advance flow for the first irrigation following a cultivation or planting should be the upper limit. This flow, of course, must be less than the maximum non-erosive flow. Thus, the second computation would be to compute the maximum flow from Equation 69. An intermediate design computation can be made at this point. The advance time can be calculated using the maximum furrow inflow, Q max . If tL is less than rreq , a feasible cutback design is possible and the following procedures can be implemented. If the advance associated with the maximum flow is too long, then either the required application should be increased (at the risk of crop stress) or the field length shortened. It is usually better to reduce the field length and repeat these calculations. When the design is shown to be within this constraint on flow, the next computation is to find the furrow advance discharge which just accomplishes an advance in treq minutes. If the advance time for a range of inflows has been determined as suggested earlier, identifying this flow is accomplished by interpolation within the data. If this information has not been developed, it is necessary to do so at this point. The easiest method is to change Qo iteratively until the associated advance time equals the required intake opportunity time. The cutback flow following the advance phase must be sufficient to keep the furrow stream running along the entire length. Thus, some tailwater will be inevitable but should be

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minimized. Knowing that infiltration rates will decrease during the wetting period to values approaching the basic intake rate suggests a guideline for sizing the cutback flow: Q cb = b fo tL (71) where b is a factor requiring some judgement to apply. It should probably be in the range of 1.1 to 1.5. The application efficiency of the cutback system can be thus described as: (72)

Once the advance and recession phase flows have been determined, the next step is to organize the field system into subsets. The first irrigating set must accommodate the entire field supply. The number of furrows in this set is therefore: N 1 = Q T/Q o (73) For the second set, N 2 = (Q T - N 1 Q cb )/Q o (74) and similarly, N i = (Q T - N i-1 Q cb )/Q o (75) The field must be divided into an integer number of subsets which may require some adjustment of Q T, Q o , or Q cb . And, it should be noted that irrigation of the last two sets cannot be accomplished under a cutback regime without reducing the field inflow, QT, or allowing water to spill from the head ditch during the cutback phase on the last set. To relieve the designer of a cumbersome trial and error procedure-trying to find the number of sets and the furrows per set that will work with various water supply rates, a suggested procedure is to fix the number of sets and compute the necessary field supply discharge. This is a four step procedure: i. Compute the cutback ratio for each of the field's infiltration conditions: CBR = Q cb /Q o (76) Select the largest value, and discard the other. ii. Let k be the number of sets and compute the following product stream: for k = 2 A2 = - CBR (77) for k > 2

(78)

Then the number of furrows in the first set is: N 1 = N f/(k + A) (79) iii. Calculate the number of furrows in each remaining set as: http://www.fao.org/docrep/t0231e/t0231e07.htm#TopOfPage[6/18/2013 7:19:08 PM]

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for k = 2 N 2 = N f - N 1 or, for k > 2 N 2 = (1 - CBR)N 1 (80) and, set first value of B = - CBR (81) N j = N 1 (1 + B) (82) iv. Steps ii and iii ensure that the field subdivides into an integer number of sets, but the field supply must vary according to the number of sets: Q T = N 1 Q o (83) Thus for a single specified Q o , the designer can subdivide the field into several sets and choose the configuration that best suits the farm operation as a whole. Before moving to the final design computation, the design of the head ditch, mention is made of using the cutback system under variable field conditions. Irrigations immediately after planting or cultivation will be generally higher than those encountered after the first irrigation. It will not be possible to alter the number of furrows irrigating per bay of the head ditch, so the inflow to the entire system must be adjusted. The design procedure outlined above is repeated for the appropriate value of Zreq and infiltration. Then, the system discharge is determined by Eq. 83. For the system illustrated in Figure 54, the design of the head ditch involves the calculation of the relative bay elevations. From Eq. 71, the head over the outlets during the advance phase, ha, is: (84) and during the wetting period phase, h w , is: (85) Thus, the elevational difference between bays is h a - h w . Each bay should be designed as a level channel section of length equal to the number of furrows per set times the furrow spacing. To accommodate the drop between bays, it is helpful if the field has a moderate cross-slope.

5.4.3 Design of furrow systems with tailwater reuse The application efficiency of furrow irrigation systems can be greatly improved when tailwater can be captured and reused. The design of such a system is somewhat more complex than the procedure for traditional furrow and cutback systems because of the need to utilize two sources of water simultaneously.

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The major complexity of reuse systems is the strategy for recirculating the tailwater One alternative is to pump the tailwater into the primary supply and then increase the number of operating furrows to utilize the additional flow. Or, tailwater can be used to irrigate separate sections of the field or even other fields. In any case the tailwater reservoir and pumping system need to be carefully controlled and coordinated with the primary water supply. To illustrate the design strategy for reuse systems, a design procedure for a common configuration outlined by Walker and Skogerboe (1987) is presented. The reuse system shown schematically in Figure 55 is intended to capture tailwater from one set and combine it with the supply to a second set. A similar operating scenario prevails for each subsequent pair until the last set is irrigated when some of the tailwater must be either stored until the next irrigation, dumped into a wasteway, used elsewhere or used to finish the irrigation after the primary inflows have been shut-off. Figure 55. Illustration of a typical reuse configuration

The total volume of tailwater recycled will be held to a constant volume equal to the runoff from the first set. The difference in tailwater volumes between the first and subsequent sets may be wasted. The recycled flow can thus be held constant to simplify the pump-back system and its operation.

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The reuse system design procedure is as follows: i. Input data are the same as for the cutback system. ii. Compute the required intake opportunity time, rreq , as outlined previously. iii. Compute or interpolate the inlet discharge required to complete the advance phase in approximately 30 percent of rreq , correcting if necessary for non-erosive stream velocities. See the suggestion at the end of section 5.4.1. iv. Compute the tailwater volume as follows: 1 The time of cutoff is: tco = rreq + tL (86) 2. The infiltrated depths at field inlet and outlets are: Zin = ktco a + fo tco (87) 3. A conservative estimate of the field runoff per furrow is: (88) where from Eq. 74 N f = Q T/Q o . v. Compute pump-back discharge, Q pb : Q pb = Vtw / tco (89) vi. Compute number of furrows in second or subsequent sets: (90)

vii. The field should be in evenly divided sets which may require repetition of the procedure with a modified furrow discharge.

5.4.4 Furrow irrigation design examples The Problem. Furrow irrigation designs are often needed either for new irrigation schemes or on existing projects where improvements are needed. Land consolidation has been carried out in a number of irrigation projects where implementation has included land reform policies and has resulted in field units amenable to furrow irrigation. Consider one such case where the new farm units have been organized around a 2 hectare block 200 m by 100 m. Flows of 30 litres per second are allocated to each block for 48 hours every 10 days. Initial field surveys showed that the fields needing first attention were comprised of a loam soil, sloped 0.8 percent over the 100 m direction and 0.1 percent over the 200 m direction. The furrows were placed on 0.5 m intervals across the 100 m direction (and running in the 200 m direction). The furrows were assumed to have a hydraulic section where p 1 = 0.57 and p 2 = 1.367. During the evaluations noted, the infiltration functions characteristic of the field were divided into two relationships to describe the first irrigation following cultivations and then the

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subsequent irrigations. These relationships are: Z = 0.00346 t

.388

+ 0.000057 t (first irrigations)

and Z = 0.0038 t

.327

+ 0.000037 t (later irrigations)

The evaluation used a Manning coefficient of n = 0.04 for all analyses. The crops expected were studied along with the local climate and it appeared that the best target depth of application, or Zreq , would be 8 cm. With 0.5 m furrow spacings, Zreq would be 0.04 m 3 /m/furrow. Water is in short supply so the project planners would like an estimate of the potential application efficiency with and without cutback and reuse. Initial Design Calculations. With the design algorithm in mind but considered only as a guide, let the design process begin with the limitations on the design parameters. The first of these can be the maximum allowable flow in the furrow, Q max . The soils are relatively stable so assume the maximum flow velocity could be as high as 13 m/min. Equation 67 in a previous section provides the means of evaluating the corresponding maximum flow rate: (67)

= 1.768 m 3 /min (the total field inflow could be put in each furrow in this case) The field is 100 m wide so that using a 0.5 m furrow spacing results in 100/.5= 200 furrows. The water supply of 30 l/s or 1.8 m 3 /min would service 1.8/.104 = 17.31 furrows per set or the field would be divided into 200/17.31 = 11.56 sets (obviously impractical since the sets must be comprised of an integer number of furrows and the field needs to be subdivided into an integer number of sets). A practical upper limit on the number of sets is perhaps 10 consisting of 20 furrows each and having a maximum flow of 0.09 m 3 /min. Beyond this 'upper limit' some of the following options also evenly divide the field: Number of Sets Furrows Per Set

Furrow Flow m3 /min

10

20

.09

8

25

.072

5

45

.045

4

50

.036

2

100

.018

1

200

.009

The second limitation on the design procedure is whether or not the flow will complete the advance phase in a reasonable time, say 24 hours. Particularly important in this regard is what

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minimum flow will complete the advance phase within this limit. If the maximum flow is too small to complete the advance, the furrow length must be reduced. The second common design computation described in Section 5.3.1 provides the means of determining the time of advance tL as a function of furrow inflow, Q o . The maximum inflow can be used to calculate the minimum advance time, but since the minimum flow conditions are not known, the maximum advance time must be established by examining each flow. The computation of tL for each Q o can be accomplished with either method outlined and if undertaken yields the results given in the following table which are also plotted in Figure 56. Sets

Advance Time

Furrow Discharge m3 /min

First Irrigation minutes Later Irrigations minutes

10

.09

58.2

*

8

.072

72.6

*

5

.045

130.8

101.4

4

.036

184.2

132.6

2

.018

847.8

379.2

1

.009

*

2390.4

Figure 56. Example relationships between inflow rate and advance time

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i. Design and layout for traditional furrow irrigation There are now five configurations feasible for the initial field condition and six for the later conditions. The design question at this stage is which one leads to the optimal design. The answer is determined by computing the application efficiency for each alternative. First, the required intake opportunity time for each condition is determined using the procedure outlined in Section 5.3.1. For the first field rreq = 214 minutes. Similarly for the later applications, rreq = 371 minutes. The application efficiency for each of the possible field configurations can now be computed.

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The results, shown in the table below, indicate that one good design is to divide the field into 4 individual subunits or sets of 50 furrows and utilize an inflow of 0.018 m 3 /min per furrow during the first irrigations. The resulting application efficiency would be nearly 56 percent. Figure 57 imposes this layout on the field. Then during later irrigations two sets would be irrigated simultaneously so that each furrow would receive .018 m3 /min. The application efficiency of later irrigations would be about 59 percent. E a , in Percent

Qo

Z req

10

.09

.04

32.6

**

8

.072

""

38.6

**

5

.045

""

51.5

37.7

4

.036

""

55.7

44.2

2

.018

""

41.9

59.3

1

.009

""

**

32.2

Sets

m3 /min m3 /m First Irrigations Later Irrigations

Figure 57. Final traditional furrow design layout

The frequency and duration of each irrigation needs to be checked and then the headland facilities selected and designed. During the first irrigation, the field will require just more than 35 hours to complete the irrigation (the sum of rreq + tL times the number of sets). The later watering will require 25 hours. If evapotranspiration rates were as high as .8 cm/day, the irrigation interval of 10 days waters the field well within these limits (Zreq divided by the crop use rate approximates the irrigation interval). Since the water supply is presumably controlled by an irrigation department, the design can be substantially hindered if the delivered flows are

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not as planned. It may be useful to examine briefly the performance of this design. If the actual irrigations evolve as these design computations indicate, the farmer's irrigation pattern will waste about 44 percent of his water during first irrigations and about 40 percent during later irrigations. By today's standards, these losses are large and it may be cost-effective to add cutback or reuse to the system to reduce these losses. Field operations. The question that arises at this point in the design is how to implement and operate the system on the field. How will the irrigator know what flow rates are actually running into the furrows, what the actual soil moisture depletion is, or when to terminate the flow into one set of furrows and shift the field supply to another set? There are several types of furrow irrigation systems but probably the most common are those that either use open watercourses at the head of the field and divert into furrows using spires or siphon tubes, or those that utilize aluminium or plastic gated pipe. The task of sizing these headland facilities will be noted in a later section. The problem at this point in the design is the means of accurate flow measurement and management. If the design is to be carried forward to an actual operation, the inlet must be equipped with a flow measuring device like those noted in Section 3. Then the irrigator with some simple instructions from the designer can 'share' this flow among the appropriate number of furrows and achieve a reasonably good approximation of the optimal discharge. In some cases, the outlets to each furrow can be individually calibrated and regulated. For instance, the size of the siphon tubes or spires might be selected by the designer. The irrigator can then adjust the flow by regulating the heads and/or the openings. In short, this phase of irrigation engineering is highly dependent on the experience and practicality of the engineer. There is no single 'best' way to do things. What works well in one locale, may not in another. The computational procedures and methods of field evaluation provide the best values of the parameters. The good design can only give the irrigator the opportunity to operate the system at or near optimal conditions. ii. Design of a cutback system There is another point which is hidden by the hydraulics of surface irrigation (which have been largely omitted from this guide). The movement of the water over the soil surface is very sensitive to the relative magnitude of the furrow discharge and the cumulative infiltration rates. Irrigation practices which modify the field inflow, such as cutback, may actually reduce the performance of the system. In more practical terms, if the advance rate is slowed to accommodate a cutback regime, the gains in efficiency derived from reduced tailwater may be more than offset by increases in deep percolation losses. The user of this guide might repeat the following cutback design example using data and field conditions for a lighter soil to illustrate this problem. As described earlier, the inherent limitation of the cutback design is that the advance phase and the wetting phase must have the same duration. Initial design calculations. The initial design computations for the cutback system are fundamentally the same as outlined above. The rreq for the first irrigation is 214 minutes and for the subsequent irrigations it is 371 minutes. If the two set system is envisioned (one set in the advance phase and one in the wetting), the advance time and cutoff times for the first irrigation are respectively, tL = rreq = 214 minutes and tco = tL + rreq = 428 minutes. For the subsequent irrigations, tL = 371 minutes and tco = 742 minutes. The next computation is the maximum flow, Q max . Since the field and furrow geometries have not changed, the value of Q

= 1.768 m 3 /min. Then it is necessary to compute the

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relationship between the inflow and the advance time. Rather than specifying a range of discharges and computing the associated advance times as above, the cutback design looks for a unique flow which yields the tL already determined as 214 or 371 minutes. This may appear simpler to some and more difficult to others. It is in fact the same effort with a slightly different aspect. The details of the computations are already given in the calculations of the previous example. Reading from Figure 56 for the two conditions, one finds that the necessary furrow flow, Q o , during the first irrigation would be about .0330 m 3 /min and .0184 m 3 /min for later irrigations. It is worthwhile emphasizing that the time of advance, tL , associated with a furrow inflow, Q o , must be less than the required intake opportunity time, rreq , in order for the cutback scheme to operate properly. When the maximum flow, Q max , results in an advance time greater than the value required for the system to work, the field length would have to be reduced or Zreq must be increased. Field layout. Once the advance phase inflows are established, the field design or layout commences with an estimate of the cutback flow. The one important constraint on the cutback flow is that it should not be less than the intake along the furrow and cause dewatering at the downstream end. Equation 71 was given to assist the designer in avoiding this problem, but it is only a guideline. Thus, for the first irrigation the cutback flow must be at least: Q cb = 1.1 * .000057 * 200 = 0.0125 m 3 /min In other words, the flow can only be cutback from .0125 m 3 /min to .033 m 3 /min, or to 38% of the advance phase flow. In subsequent irrigations, Q cb = 1.1 * .000037 * 200 = 0.0081 m 3 /min which is a cutback of 43 percent of the advance flow. There are several unique features of cutback systems that need to be considered at the design stage. Of particular concern is the fact that the number of furrows per set must vary over the field if the water supply rate, Q T, is to be held constant during the irrigation. The number of furrows per set can only be the same if the field supply is varied for each change in sets across the field. This is usually difficult if the water supply is being supplied by an irrigation project. However, for furrow systems to utilize cutback, the field supply must be regulated from irrigation to irrigation. To illustrate this, let us develop a field layout for the irrigations. Utilizing Eqs. 77-83, the following table can be developed for a variable field supply rate. The Q cb /Q o ratio is taken as .43 reflecting the constraint imposed by the later irrigations. This ratio must be the same for all irrigations. Number of Furrow Per Set No of Sets in Field

Set Number

QT QT 1st irr Later Irr

3 3 1 2 3 4 5 6 7 8 9 m /min m /min

4

67 38 50 45

2.21

1.27

5

54 30 41 36 39

1.78

0.99

6

46 26 35 31 32 30

1.51

0.84

7

40 22 30 27 28 27 26

1.32

0.73

8

35 19 26 23 25 24 24 24

1.15

0.64

9

31 17 23 21 22 21 22 21 22

1.02

0.57

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One can see that if the water supply capacity is limited to 1.8 m 3 /min, the field must be divided into at least five sets to accommodate the first irrigation condition. The upper limit on the number of sets can be evaluated by examining the duration and frequency of the irrigations. The time of cutoff for each set during the first irrigation was determined previously as 7.1 hours (428 minutes). For the later irrigations, tco = 12.4 hours (742 minutes). For a 5 set system, the total duration of the later irrigations is, 6 * 6.2 = 37.2 hrs or 1.6 days, assuming the irrigator will operate 24 hours per day. (Note that because two sets are irrigating simultaneously under cutback with the exception of the first and last sets, the duration of the irrigation on the field is the number of sets plus 1 times the advance or required intake opportunity time.) Thus, if the 48 hour availability constraint imposed in the problem outline is maintained, a cutback system for this field is only feasible in the 5 or 6 set configuration without changing the depth of water to be added during each irrigation. For the purpose of this example, let us suppose the water supply agency will deliver water to a 5 set system needed for the cutback regime. Field implementation. For this example, the field outlets are to be spires with adjustable square slide gates having the following head-discharge characteristics: Spile Size Full-Open Area

Discharge Coefficient

(mm)

(cm2 )

19

3.61

0.00114

25

6.25

0.00136

38

14.44

0.00145

50

25.00

0.00169

Note that Q o = c A h

.5

where h is the head above the spire invert in cm, and Q o is in units

3

of m /min. The change in elevation across the 100 m headland of the field is 0.008 * 100 = 80 cm which is sufficient for the system shown in Figure 54. To make the system work, the bays need to be constructed on a level slope. The transition between bays is accomplished with a drop equal to the difference in the head between the advance phase flows and the cutback flows. They are then operated irrigation to irrigation by controlling the gate openings. For example, if the 25 mm spires are selected, the advance phase head at the full opening is: h = (.0330 / 6.25 / .00136)2 = 15.07 cm and for the cutback phase: h = (.0330 * .43 / 6.25 / .00136)2 = 2.79 cm Thus, the elevation drop between the bays should be 15.07 - 2.79 = 12.28 cm. This will necessitate elevating the head ditch approximately 30 cm above the low end of the field and providing a drop to the furrows. When irrigating the field later, the head on the gates will necessarily remain the same, but the openings must be reduced. For the advance phase, A = .0184 / 15.07 .5 / .00136 = 3.49 cm2 = 55.8% opening and similarly for the later irrigations:

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5. Surface irrigation design

A = .0184 * .43 / 2.79 .5 / .00136 = 3.48 cm2 = 55.7% opening The operation is relatively simple so long as the total field inflow rate can be regulated to compensate for the lower infiltration during later irrigations. Figure 58 illustrates the alignment of the head ditch for this cutback example design. Figure 58. Cutback example field and head ditch layout

The performance of this design is calculated as follows. For the first irrigation (Eq. 72):

and for the later irrigations:

Cutback, therefore, substantially improves the efficiency on this field over traditional methods. iii. Design of furrow reuse systems Another furrow irrigation option is to capture runoff in a small reservoir at the end of the field and either pump it back to the upper end to be used along with the primary supply or diverted to another field. The system envisioned for this reuse example will use the same head ditch configuration as the traditional or cutback system options already developed. The irrigator will introduce the canal water to the first set and collect the surface runoff from it. Then with initiation of the second set and subsequent sets, the water in the tailwater reservoir will be pumped to the head of the field and mixed with the canal supply. The field layout will be similar to the schematic system depicted in Figure 55. Initial calculations. Initial calculations begin again with the required intake opportunity. These results were determined in the previous example: rreq = 214 min during first irrigations rreq = 371 min during later irrigations

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5. Surface irrigation design

The maximum allowable furrow flows are also the same, 1.768 m 3 /min. A rule-of-thumb states that the advance time for reuse systems should be about 30 percent of the required intake opportunity time. From Figure 56, the first irrigation flow should be .082 m3 /min which will yield an advance time of .3 * .214 min = 64 min. Similarly, for subsequent irrigations, an advance time of 112 min based on a flow of 0.042 m 3 /min is selected. When the maximum non-erosive flow fails to meet the 30 percent rule, it is usually taken as the furrow flow and the rule is ignored. The application efficiency and field layout under the reuse regime are computed as before. It is first necessary to compute the deep percolation ratio and the tailwater runoff ratio for the possible range of flows. The usual procedure is to compute the deep percolation ratio and then find the tailwater ratio as 100 - Ea - DPR in percentages. As an example, the first irrigation analysis can be demonstrated. From the volume balance advance calculations or, if one prefers, the graphical approach, the time of advance to the furrow mid-point can be found as 25.9 min. From this information the values of p and r in Eq. 32 are 8.45 and .7595, respectively. Then using the power advance trajectory (Eq. 32) and the infiltration function, the distribution of applied depths can be described as in the following table. Distance From Field Inlet Computed Opportunity Time 1 Computed Application 2 (m)

(min)

(m 3 /m)

0

278.5

0.0466

20

275.4

0.0463

40

270.8

0.0458

60

265.3

0.0453

100

252.6

0.0440

120

245.6

0.0433

140

238.2

0.0425

160

230.4

0.0417

180

222.4

0.0408

200

214.0

0.0400

1

top = tco - tx , tx = (x/p) 1/r

2

application = depth * furrow spacing/m of width

Using the trapezoidal integration of the applied water, the amount infiltrated over the field length is

= 8.733 m 3 /furrow The required application is: .08 m x .50 m * 200 m = 8 m3 /furrow The total inflow to each furrow is: .082 m 3 /min * 278.5 min = 22.84 m 3 /furrow

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5. Surface irrigation design

The deep percolation and runoff ratios are thus:

TWR = 0 % (on the assumption that all is recycled) And, the application efficiency for the first set is: Ea = 100 - 3.2 - 0 = 96.8% The runoff fraction is:

The volume of tailwater per furrow is: 0.612 * 22.84 = 14 m 3 /furrow It is obvious, or should be, that recycling 61 percent of the water applied to a field is going to be relatively costly. Consequently, a wider range of furrow flows needs to be examined along with their performance characteristics. For the later irrigations of this example, the figures are as follows: DPR = 3.3 percent and Ea = 96.7 percent. The field configuration. The reuse system will collect the tailwater from the first set in the runoff reservoir and pump it back in the supply to the remaining sets. The pump-back system will operate continuously and will have some excess capacity in the reservoir even though the total runoff from subsequent sets will be greater. The field layout can be found by trial and error or calculated. If the layout is calculated, one approach is to fix a furrow flow and determine the external supply that is needed. Using the design relations in Section 5.3 one can derive the following equation for the layout. (91) in which Q T is the flow rate of the external water supply needed for the system in m3 /min, N f is the total number of furrows on the field, Q o is the design furrow inflow in m 3 /min, N s is the number of sets in the field, and TWR is the runoff ratio associated with an inflow of Q o m 3 /min. During the first irrigation, a Q o of 0.082 m 3 /min satisfied the probable requirements. Choosing six sets as the basic field subdivision, the number of furrows in the first set is: N 1 = Q T/Q o = 1.8/.082 = 22 For the first irrigation, the volume of the runoff reservoir must be: Vro = 14 m 3 /furrow * 22 furrows = 308 m 3 Recalling that for a first irrigation condition, the time of cutoff is 278.5 minutes, the capacity of the pump-back system is therefore: Q cb = 308 m 3 /278.5 min = 1.11 m 3 /min

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5. Surface irrigation design

The number of furrows per set for the subsequent sets is: (92) There are 200 furrows in the field. Five sets would contain 36 furrows; one set, the first, contains 22. This is 202 furrows so it is necessary to reduce one of the sets by two furrows. Now the system must be configured for the later irrigating conditions. If the individual furrow inflows are set at .042 m 3 /min, two sets can be irrigated simultaneously to have effectively a 3 set system, and, the number of furrows in the first is: N 1 = 1.8 / .042 = 43 The volume of the runoff reservoir needs to be 493 m 3 and the capacity of the pump-back system must be 1.02 m 3 /min. It will therefore not be necessary to regulate the pump-back system during the first irrigation to a value different than that for later irrigation. The runoff reservoir capacity, however, is governed by the later irrigation. The number of furrows in subsequent sets is 79. This layout adds up to 201 furrows so the number in the last set can be decreased to 78.

5.5 Border irrigation design 5.5.1 5.5.2 5.5.3 5.5.4

Design of open-end border systems Design of blocked-end borders An open-end border design example A blocked-end border design example

With two exceptions, the design of borders involves the same procedure as that for furrow systems. The first difference is that while the depletion and recession phases are generally neglected in furrow design, both phases must be included for borders. The second difference is that the downstream end of a border may be dyked to prevent runoff. One simplification of border analyses is that the geometry of the flow is simpler because it can be treated as wide, plane flow. The values of p 1 and p 2 are always 1.0 and 1.67, respectively.

5.5.1 Design of open-end border systems The first four design steps for open-ended borders are the same as those outlined under subsection 5.4.1 for traditional furrow systems: (1) assemble input data; (2) compute maximum flows per unit width; (3) compute advance time; and (4) compute the required intake opportunity time. Hart et al. (1980) also suggest computing a minimum flow, Qmin, based on a value that ensures adequate field spreading. This relationship is: Q min = 0.000357 L So .5 / n (93) where Q min is the minimum suggested unit discharge in m 3 /min/m and L, So , and n are variables already defined. There will be substantially more water on the surface of borders than for furrows. Consequently, it is good practice to check periodically the depth of flow at the field inlet to ensure that depths do not exceed the dyke heights. For this:

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5. Surface irrigation design

(94) where y o is the inlet flow depth in m. The border designs given here assume the advance phase is completed before the inflow is terminated. Many irrigators, in fact nearly all where the downstream end is dyked, actually cut off the inflow before the end of the advance phase. In these cases, the volume of water on the surface will continue to advance along the border until it reaches the lower end where it will run off or pond in front of the dyke. Unless the border system is extremely well designed and operated, the downstream pond often creates a substantial threat to the crop in the submerged areas and although dyked at their lower ends, most farmers provide a surface drain for excess water. Consequently, the border efficiency and uniformity are approximately the same as borders in which excess surface water simply drains off the field after the advance phase is complete. The following procedure is therefore suggested for border systems where the excess surface water is drained from the field either by a completely openended border or by a regulated outlet from a blocked-end border. After completing the first four design steps, as with furrows, open-ended border design resumes as follows: v. Compute the recession time, tr , for the condition where the downstream end of the border receives the smallest application: tr = rreq + tL (95) vi. Calculate the depletion time, td , in min, as follows: 1. Assign an initial time to the depletion time, say T1 = tr; 2. Compute the average infiltration rate along the border by averaging the rates as both ends at time T1 : (96) 3. Compute the 'relative' water surface slope: (97) 4. Compute a revised estimate of the depletion time, T2 : (98) 5. Compare T2 with T1 to determine if they are within about one minute, then the depletion time td is determined. If the analysis has not converged then let T1 = T2 and repeat steps 2 through 5. The computation of depletion time given above is based on the algebraic analysis reported by Strelkoff (1977).

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5. Surface irrigation design

vii. Compare the depletion time with the required intake opportunity time. Because recession is an important process in border irrigation, it is possible for the applied depth at the end of the field to be greater than at the inlet. If td > rreq , the irrigation at the field inlet is adequate and the application efficiency, Ea can be calculated with Eq. 69 using the following estimate of time of cutoff: tco = td - y o L / (2 Q o ) (99) If td < rreq , the irrigation is not complete and the cutoff time must be increased so the intake at the inlet is equal to the required depth. The computation proceeds as follows: tco = rreq - y o L / (2 Q o )(100) and then Ea is computed with Eq. 69. Since the application efficiency will vary with Q o several designs should be developed using different values of inflow to identify the design discharge that maximizes Ea . viii. Finally, the border width, Wo in m is computed and the number of borders, N b , is found as: Wo = Q T/Q o (101) and, N b = Wt /Wo (102) where Wt is the width of the field. Adjust Wo until N b is an even number. If this width is unsatisfactory for other reasons, modify the unit width inflow or plan to adjust the system discharge, Q T.

5.5.2 Design of blocked-end borders The computations needed to evaluate and design blocked-end borders where the flow is cut off before or shortly after the advance phase is complete are substantially more detailed than the procedures outlined above for furrow and open-end border irrigation systems. In fact, the volume balance methods given previously are relatively weak for this particular case of surface irrigation. Generally, the computations for blocked-end borders are best performed with zero-inertia or full hydrodynamic simulation models which are beyond the scope of this paper. A number of studies have been made to develop relationships among the most important variables involving border irrigation using a dimensionless approach and the higher level simulation models. The interested reader might want to refer to Strelkoff and Katapodes (1977), Strelkoff and Shatanawi (1984), Shatanawi and Strelkoff (1984), and Yitayew and Fangmeier (1984) for some of these reports. The design procedure outlined below is an extension of the approaches already given and consistent with the level of treatment given herein. The procedure given here is intended to be conservative and will yield designs capable of performing at somewhat lower application efficiencies than is perhaps possible using the more comprehensive methods.

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5. Surface irrigation design

The suggested design steps are as follows: i. Determine the input data as for furrow and border systems already discussed. ii. Compute the maximum inflows per unit width using Eq. 67 with p 1 = 1.0 and p 2 = 1.67. The minimum inflows per unit width can also be computed using Eq. 93. iii. Compute the require intake opportunity time, rreq . iv. Compute the advance time for a range of inflow rates between Qmax and Q min, develop a graph of inflow, Q o verses the advance time, tL , and extrapolate the flow that produces an advance time equal to rreq . Define the time of cut off, tco , equal to rreq . Extrapolate also the r and p values in Eq. 32 found as part of the advance calculations. v. Calculate the depletion time, td , in min, as follows: td = tco + y o L / (2 Q o ) = rreq + y o L / (2 Q o ) (103) vi. Assume that at td , the water on the surface of the field will have drained from the upper reaches of the border to a wedge-shaped pond at the downstream end of the border and in front of the dyke. vii. At the end of the drainage period, a pond should extend a distance l metre upstream of the dyked end of the border. The value of l is computed from a simple volume balance at the time of recession: (104)

where, Zo = k tda + fo td (105) and: ZL = k (td - tL )a + fo (td - tL ) (106) If the value of l is zero or negative, a downstream pond will not form since the infiltration rate is high enough to absorb what would have been the surface storage at the end of the recession phase. In this case the design can be derived from the open-ended border design procedure. If the value of l is greater than the field length, L, then the pond extends over the entire border and the design can be handled according to the basin design procedure outlined in a following section. The depth of water at the end of the border, y L , will be: y L = l So (107) viii. The application efficiency, Ea , can be computed using Eq. 56. However, the depth of infiltration at the end of the field and at the distance L-l metres from the inlet should be checked as Eq. 56 assumes that all areas of the field receive at

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5. Surface irrigation design

least Zreq . The depths of infiltrated water at the three critical points on the field, the head, the downstream end, and the location l can be determined as follows for the time when the pond is just formed at the lower end of the border: Z1 = k (td - tL-1)a + fo (td - tL-1) (108) where, tL-1 = [(L-l) / p] 1/a (109) It should be noted again by way of reminder that one of the fundamental assumptions of the design process is that the root zone requirement, Zreq , will be met over the entire length of the field. If, therefore, in computing Ea , one finds ZL1 or ZL less than Zreq , then either the time of cutoff should be extended or the value of Zreq used should be reduced. Likewise, if the depths applied at l and L significantly exceed Zreq , then the inflow should be terminated before the flow reaches the end of the border. If the inflow is cut off before the advance phase is completed, the analysis above will have to be replaced by the judgement and experience of the designer, or the more advanced models will have to be utilized.

5.5.3 An open-end border design example The problem. In subsection 5.4.4, an example of furrow design was given in which the soil was quite heavy (low infiltration rates). To generate a basis for what might be an interesting comparison of borders and furrow systems, suppose the original question for that field is extended to whether or not borders might be as good. Let us assume that the infiltration characteristics are the same except adjusted for an increased wetted perimeter. The approximate wetted perimeter for the furrows is found by returning to the flow area, perimeter, and depth relationships. At a flow of 0.09 m 3 /min, the flow area found in the furrow example was (Eq. 48):

From Eq. 40 from which the furrow shape was extracted: y = (154 cm2 / 3.331) 1/1.732 = 9.15 cm From Eq. 41: WP = 5.922 * 9.5.805 = 35.18 cm. Since the furrows were spaced at .5 m intervals, one could approximate the infiltration of a border by adjusting the k and fo values by a factor of 1.4 based on the ratio of border to furrow wetted perimeter (50/35.18). If the furrows were operated in the 100 m direction where the slope is .8 percent, the multiplication factor would be about 2.0. For this exercise, the 1.4 factor will be utilized. Thus, First Irrigation Conditions: Z = 0.00484 t

.388

+ 0.00008 t

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5. Surface irrigation design

Later Irrigation Conditions: Z = 0.0053 t

.327

+ 0.000052 t

The units of Z are again m 3 /m of length/unit width. One would not expect the border infiltration equation to more than double furrow infiltration with furrows spaced less than 1 m apart. Again Mannings n can be 0.04 for initial irrigations and .1 for later irrigations due to crop cover. Zreq is 8 cm. Basic calculations. Assuming also that the soil is relatively stable, Eq. 67 is used to calculate the maximum inflow per unit width for the first irrigation along the 200 m length where erosion is most likely:

And similarly for irrigations along the 100 m (SO = 0.008) direction:

The minimum flow suggested by Eq. 93 using later field roughness where spreading may be a problem is for the 200 m lengths: Q min = 0.000357 * 200 * .001 .5 / .10 = 0.0226 m 3 /min/m or in the 100 m direction: Q min = .000357 * 100 * .008 .5 / .10 = 0.032 m 3 /min/m The required intake opportunity times found according to the procedure suggested by Eq. 59 are: First Irrigations rreq = 388.5 min Later Irrigations rreq = 678.9 min The next basic calculation, as with furrows, must be to formulate the relationship between advance time and inflow discharge. Starting with a flow near the maximum and working downward using the processes already outlined, advance curves for both infiltration conditions and flow directions can be found. The results for this example are shown in Figure 59. Figure 59. Discharge-advance relationship for the border example problem The last of the basic calculations concerns the depletion and recession times for various values of flow. One illustration should demonstrate this procedure adequately. For an inflow of 0.06 m 3 /min/m, the advance time along the 200 m length under later conditions is about 145 min. From Eq. 48:

The time of recession at the lower end of the field, tr , is determined as: http://www.fao.org/docrep/t0231e/t0231e07.htm#TopOfPage[6/18/2013 7:19:08 PM]

5. Surface irrigation design

tr = rreq + tL = 679 + 145 = 824 min The time of depletion must be iteratively determined from Eqs. 96 - 98: a. td = tr = 824 min b.

c.

d. e. Since T1 is not close to T2 , steps b - d must be repeated with T1 set equal to 677 min: b.

c.

d. e. Again another estimate of td seems to be required by the difference found between the iterations. If steps b - d are repeated, the new value of T2 is 680 min and the procedure has converged. The time of cutoff, tco , is found from Eq. 99: tco = td - Ao L / (2 Q o ) = 680 - .0355 * 200 / .12 = 631 min. Finally the application efficiencies of the alternative flows and flow directions are found using Eq. 56. An example for the 0.072 m 3 /min/m flow along the 200 m direction during the later irrigations is:

This series of computations is repeated for the full range of discharges, field lengths and infiltration conditions. The following table gives a detailed summary of selected options for the first and subsequent irrigation conditions running in both the 200 m and 100 m directions. First Irrigations L = 200 m Sets

Border Width

Unit Flow

Advance Time

Cutoff Time

m

m3 /min

hrs

hrs

Recession Field OnTime Time hrs

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hrs

Application Efficiency Percent

5. Surface irrigation design

2

50

0.036

6.36

11.34

12.83

22.67

65.3

3

33

0.0545

3.11

8.10

9.59

24.29

60.4

4

25

0.072

2.14

7.12

8.61

28.49

52.0

5

20

0.09

1.64

6.63

8.12

33.16

44.7

Later Irrigations L = 200 m Border Width

Unit Flow

Advance Time

Cutoff Time

m

m3 /min

hrs

hrs

hrs

hrs

1

100

0.018

15.55

23.66

26.86

23.66

62.6

2

50

0.036

5.03

13.12

16.34

26.24

56.5

3

33

0.0545

3.15

11.25

14.47

33.76

43.4

Sets

Recession Field OnTime Time

Application Efficiency Percent

First Irrigations L = 100 m Border Width

Unit Flow

Advance Time

Cutoff Time

m

m3 /min

hrs

hrs

hrs

hrs

2

100

0.018

5.27

11.21

11.74

22.42

66.1

3

67

0.0269

2.35

8.30

8.83

24.89

59.8

4

50

0.036

1.44

7.39

7.92

29.55

50.1

5

40

0.045

1.03

6.98

7.51

34.91

42.4

Sets

Recession Field OnTime Time

Application Efficiency Percent

Later Irrigations L = 100 m Border Width

Unit Flow

Advance Time

Cutoff Time

m

m3 /min

hrs

hrs

hrs

hrs

1

200

0.009

12.89

23.07

24.20

23.07

64.2

2

100

0.018

3.45

13.61

14.76

27.23

54.4

Sets

Recession Field OnTime Time

Application Efficiency Percent

Field layout and configuration. The field water supply, Q T, established in the furrow example was 1.8 m 3 /min which would have a duration of 48 hours. Usually, border irrigation would require a higher discharge than furrow systems, but as a first attempt at the problem, consider the field supply fixed. The options for field layout are to align the borders in either the 200 m or the 100 m directions. The alternative configurations outlined by the data in the preceding tables indicate that there is probably not a strong advantage in irrigating in either direction and the decision can be based on other practical factors. For instance, dividing the field into two, 50 m wide borders running along the 200 m length may be preferable if farming operations are mechanized. During later irrigations, both borders would be irrigated simultaneously with the water supply. The potential application efficiency of this border design would be 63-65 percent which is better than furrow systems without cutback or reuse but not as good as the cutback or reuse options.

5.5.4 A blocked-end border design example The problem. Section 5.5.4 illustrated the open-end border design procedure. The option of dyking these borders should be considered as an option for improving application efficiency. http://www.fao.org/docrep/t0231e/t0231e07.htm#TopOfPage[6/18/2013 7:19:08 PM]

5. Surface irrigation design

From results already available, the required intake opportunity times, rreq , needed to apply a depth of 8 cm (Zreq ) were about 389 minutes and 679 minutes for initial and subsequent field conditions, respectively. Assuming the borders will run in the 200 m direction on the 0.1 percent slope as above, Figure 59 indicates the inflows that will complete the advance in the respective rreq times are 0.036 m 3 /min/m for initial irrigations and 0.0215 m 3 /min/m for later ones. The values of r and p need to be generated or extrapolated for these flow rates unless they are already generated as part of the development of Figure 59 or, in this example case, from the previous example problem. For the 0.036 m 3 /min/m inflow, the values of r and p were determined from the previous example as r =.5635 and p = 6.949. For the 0.0215 m 3 /min/m inflow, r and p were calculated using the methods outlined in section 5.3.1 rather than extrapolated with the result that r =.6032 and p = 3.916. All other inputs to this problem like infiltration coefficients and roughness are assumed to be the same as in section 5.5.3. To this point, the blocked-end border design procedure outlined in section 5.5.2 is completed through step iv. The remainder of the steps are as follows: v. Calculate the depletion time, td , in min, as follows: tco = rreq = 389 min (94) td = tco + y o L / (2 Q o ) = 389 + .0134 * 200 / (2 * .036) = 426 min (103) vi. Assume that at 426 min the water on the surface of the field has drained into the wedge-shaped pond at the downstream end of the border. vii. At 426 min, a pond should extend a distance of l metre upstream of the dyked end of the border. The value of l is: Zo = k tda + fo td = .00484 * 426 .388 + .00008 * 426 = 0.0848 m 3 /m/m (105) ZL = k (td - tL )a + fo (td - tL ) = .00484 * (426 - 389) .388 + .00008 * (426 - 389) = 0.0226 m 3 /m/m (106) (104)

Since the value of l is between zero and L a downstream pond will form and infiltrate in place to fill the root zone. The depth of water at the end of the border, y L , will be:

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5. Surface irrigation design

y L = l So = 80.8 * .001 = 0.0808 m (105) viii. The application efficiency, Ea , can be computed using Eq. 56. However before making this computation, it is instructive to compute the depths of infiltration along the border. The application at the inlet was found above to be 0.0848 m or about 8.5 cm. At the end of the border, the application is ZL from above plus y L , or .1034 m. The depth of infiltration at the distance L-1 metres from the inlet is: tL-1 = [(L - 1) / p] 1/r = (119.2 / 6.949) 1/.5635 = 155 min Z1 = k (td - tL-1)a + fo (td tL-1) (107) = .00484 * (426 - 155) .388 + .00008 * (426 - 155) = 0.064 m As one immediately determines, the middle of the field is under-irrigated. If fact, if Ea is calculated from Eq. 56, (56)

one sees that the results are distorted. The assumption that the entire field receives the required depth, Zreq , is implicit in Eq. 56. It cannot be used unless this condition is met. And since the objective of the design is to completely refill the root zone, either the time of cutoff needs to be extended or the design value of Zreq should be reduced to approximate the depth infiltrated in the least watered areas to ensure this constraint. The simplest option is to adjust Zreq to say 0.06 m and utilize the values of inflow and cutoff time developed above. If this is decided upon, the application efficiency according to Eq. 56 is 85.7% which is a substantial improvement over the open-end design. The other option is to extend the cutoff time so the ponded wedge extends further up the basin. This involves several repetitions of the design procedure given above in a trial and error search for the cutoff time that works. Given the precarious nature of the volume balance procedure for the blocked-end border case in the first place, this later option is not recommended. If a better design is sought, the more advanced simulation models will have to be used. Now other field configurations must be tested and compared. The eventual selection will be the one with the best performance over both infiltration conditions. These calculations will be left to the interested reader. One note should be made at this point however. The computer program given at the end of this paper does not include an option or blocked-end borders.

5.6 Basin irrigation design 5.6.1 An example of basin design Basin irrigation design is somewhat simpler than either furrow or border design. Tailwater is prevented from exiting the field and the slopes are usually very small or zero. Recession and depletion are accomplished at nearly the same time and nearly uniform over the entire basin. However, because slopes are small or zero, the driving force on the flow is solely the hydraulic http://www.fao.org/docrep/t0231e/t0231e07.htm#TopOfPage[6/18/2013 7:19:08 PM]

5. Surface irrigation design

slope of the water surface, and the uniformity of the field surface topography is critically important. An effort will not be made to develop a design procedure for irregularly shaped basins or where the advancing front is very irregular. Rather, the water movement over the basin is assumed to occur in a single direction like that in furrows and borders. Three further assumptions will be made specifically for basin irrigation. First, the friction slope during the advance phase of the flow can be approximated by: Sf = y o / x (110) in which y o is the depth of flow at the basin inlet in m, x is the distance from inlet to the advancing front in m, and Sf is the friction slope. Utilizing the result of Eq. 112 in the Manning equation yields:

or, (112)

The second assumption is that immediately upon cessation of inflow, the water surface assumes a horizontal orientation and infiltrates vertically. In other words, the infiltrated depth at the inlet to the basin is equal to the infiltration during advance, plus the average depth of water on the soil surface at the time the water completes the advance phase, plus the average depth added to the basin following completion of advance. At the downstream end of the basin the application is assumed to equal the average depth on the surface at the time advance is completed plus the average depth added from this time until the time of cutoff. The third assumption is that the depth to be applied at the downstream end of the basin is equal to Zreq . Under these three basic assumptions, the time of cutoff for basin irrigation systems is (assume y o is evaluated with x equal to L): (113)

The time of cutoff must be greater than or equal to the advance time. Basin design is much simpler than that for furrows or borders. Because there is no tailwater problem, the maximum unit inflow also maximizes application efficiency. Thus, the design procedure does not need to search among various flow rates for a value that meets a design criterion like finding the deep percolation-field tailwater trade-off point. Basin dimensions therefore become more a matter of practicality to the farmer than one of hydraulic necessity. As a guide to basin design, the following steps are outlined: i. Input data common to both furrows and borders must first be collected. Field slope will not be necessary because basins are usually 'dead level'. ii. The required intake opportunity time, rreq , can be found as demonstrated in the http://www.fao.org/docrep/t0231e/t0231e07.htm#TopOfPage[6/18/2013 7:19:08 PM]

5. Surface irrigation design

previous examples. iii. The maximum unit flow should be calculated along with the associated depth near the basin inlet. The maximum depth can be approximated by Eq. 112: (114)

and then perhaps increased 10-20 percent to allow some room for post-advance basin filling. If the computed value of y max is greater than the height of the basic perimeter dykes, then Q max needs to be reduced accordingly. The maximum unit flow, Q max , is difficult to assess. During the initial part of the advance phase, flow velocities will be greater than later in the advance. As a general guideline, it is suggested that Q max be based on the flow velocity in the basin when the advance phase is one-ninth completed. The basin equivalent to Eq. 67 is: (115)

Usually the design of basins will involve flows much smaller than indicated in Eq. 115. iv. Select several field layouts that would appear to yield a well organized field system and for each determine the length and width of the basins. Then compute the unit flow, Q o for each configuration as: Q o = Q T / Wb (116) where Wb is the basin width in m. As noted above, the maximum efficiency will generally occur when Q o is near Q max so the configurations selected at this phase of the design should yield inflows accordingly. v. Compute the advance times, tL , for each field layout as discussed in subsection 5.3.1, the cutoff time, tco , from Eq. 113 (if tco < tL , set tco = tL ), and the application efficiency using Eq. 56. The layout that achieves the highest efficiency while maintaining a convenient configuration for the irrigator/farmer should be selected.

5.6.1 An example of basin design The problem. A comparison of basin irrigation with the furrow and border systems in previous subsections should provide an interesting view of the three systems collectively. To remind the reader, an irrigation project is in the planning stages in which a basic field block of 2 hectares has been chosen for field design. A preliminary survey has revealed that the fields are configured in 100 m widths and 200 m lengths. The typical slopes are .8% in the 100 m dimension and .1% in the other. Soils appear to be relatively non-erosive and have been tested to yield the following infiltration functions: First Irrigations Z = 0.00484 r .388 + 0.00008 r Later Irrigations Z = 0.0053 r .327 + 0.000052 r

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5. Surface irrigation design

Z has units of m 3 /m of length/m of width, and r has units of minutes. Anticipated application depths per irrigation based on an evaluation of cropping patterns and crop water requirements are 8 cm. The water supply to the field is set by the project at 1.8 m3 /min, available for 36 hours every 10 days. Quality of water supply is good and hopefully these deliveries will be made as expected so far as rate, duration, and frequency are concerned. For the purposes of design, the Manning roughness coefficient for first irrigations will be taken as 0.04 and for the later irrigations as 0.10. This is to reflect a bare soil condition for first irrigations and a cropped surface for later irrigations. Basic calculations. The intake opportunity times for the two field conditions are the same as found earlier for borders, namely: rreq = 389 min for initial irrigations and, rreq = 679 min for later irrigations Maximum flows permissible assuming a 30 cm perimeter dyke around the basins and flows running in the 100 m direction are found from Eq. 115:

Utilizing Figures 53a-f, the advance time as a function of unit flow can be determined as indicated below. The Q o verses tL data are plotted in Figure 60. Figure 60. Discharge-advance relationships for the basin example

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5. Surface irrigation design

QO First Irrigations 0.40

0.0649

1.00

0.020

0.022

17.8

0.20

0.0471

1.22

0.040

0.050

29.4

0.10

0.0342

1.48

0.080

0.120

57.3

0.05

0.0248

1.81

0.160

0.300

93.0

0.03

0.0196

2.09

0.267

0.750

183.8

Later Irrigations 0.40

0.099

0.62

0.013

*

*

0.20

0.072

0.78

0.026

0.030

41.5

0.10

0.052

0.98

0.052

0.061

61.0

0.05

0.038

1.20

0.104

0.155

113.3

0.03

0.030

1.41

0.173

0.430

248.1

Field layout. Basins installed on sloping fields should have their longest dimension running

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5. Surface irrigation design

normal to the largest field slope in order to minimize land levelling costs. Thus, for this example where the basins have been selected with a 100 m length, they would have their direction of flow parallel to the 200 m direction. The width is a choice left to the designer. Some of the options, their dimensions and performance are summarized below. Figure 61 shows a 10 basin configuration. No. of Basins

Basin Width

Unit Flow

Advance Time

Cutoff Time

Field Irrig. Time

Application Efficiency

m

m3 /min

min

min

hrs

%

First Irrigations 4

50

0.036

140

316

21.1

70.3

6

33

0.054

90

201

20.1

73.7

8

25

0.072

68

147

19.6

75.6

10

20

0.09

55

116

19.3

76.6

12

17

0.108

45

94

18.8

78.8

20

10

0.18

31

56

18.7

79.4

Later Irrigations 4

50

0.036

175

327

21.8

68.0

6

33

0.054

105

197

19.7

75.2

8

25

0.072

80

143

19.1

77.7

10

20

0.09

68

114

19.0

78.0

12

17

0.108

60

95

19.0

78.0

20

10

0.18

43

58

19.3

76.6

Figure 61. Example basin configuration

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5. Surface irrigation design

One of the advantages of basins that immediately becomes apparent is that field division is much more flexible. Application efficiencies can be very high and nearly all options are workable in terms of the water supply.

5.7 Summary It is not possible to illustrate effectively the judgement or 'art' required to evaluate and design surface irrigation systems. The previous examples demonstrate the procedures described in this guide and, to a limited extent, alert the reader to factors he or she will need to determine on a case by case basis. There are major influences on the design process one might expect which lie far outside a mathematical treatment. For example, the size and shape of individual land holdings and their future change in response to customs for inheritance, governmental interventions such as land consolidation and resettlement, farmer preference and attitudes, harvesting and cultivating equipment limitations, etc. In short, there is not a universal algorithm for design and evaluation that eliminates the need for good judgement. On the other hand, good judgement is no substitute for the mathematical aids presented herein. One might demonstrate this by comparing the performance of a system properly designed with one where selection of inflow and cutoff time is made arbitrarily. To be skilled in design is to completely understand the relationships among the selectable and manageable variables governing surface irrigation, particularly the effects of infiltration and stream size on advance. The mathematical treatment, if followed, helps illustrate some of the more important individual processes occurring in the field. Because the irrigator has the latitude of changing flow rates and cutoff times, the field system may not respond as designed. The problem is unlike sprinkler and trickle irrigation where having selected and installed the system's piping, the hydraulics of the system's operation are defined. Consequently, surface irrigation design cannot provide a guaranteed level of performance but must rely on the farmer to operate and manage it efficiently. It is apparent therefore, that the role of extension and technical assistance to farmers is critical for surface irrigated regimes. As a final thought in this section, something should be stated regarding costs associated with surface irrigation. It would be most desirable to present a comprehensive review, but such is impractical because surface irrigation systems themselves are so widely varied. Table 9 lists a number of irrigation technologies and a figure representing the costs. The units here are $/ha but should be used only to indicate the relative magnitude of various system costs under agricultural conditions typical of the western United States. Other systems enter the picture as one moves from country to country. Table 9 TOTAL ANNUAL COSTS FOR SELECTED ON-FARM IRRIGATION SYSTEMS Description of System or Improvement Annual Costs, $/ha Concrete ditch linings

40

Gated-pipe

35

Cutback systems

100

Reuse systems with gated-pipe

150

Solid-set sprinklers

500-700 1

Hand - moved sprinklers

300-450

Wheel-line or side-roll sprinklers

200-300

Centre-pivot sprinklers

150-200

Trickle irrigation

500-1000

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The pressurized systems are often supplied by groundwater wells on-farm. The range of costs is for surface supplies (small values) and for groundwater (larger values).

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6. Land levelling

Produced by: Natural Resources Management and Environment Department

Title: Guidelines for designing and evaluatin surface irrigation systems...

More details

6. Land levelling 6.1 The importance of land preparations 6.2 Small-scale land levelling 6.3 Traditional engineering approach 6.4 Laser land levelling

6.1 The importance of land preparations Levelling, smoothing and shaping the field surface is as important to the surface system as the design of laterals, manifolds, risers and outlets is for sprinkler or trickle irrigation systems. It is a process for ensuring that the depths and discharge variations over the field are relatively uniform and, as a result, that water distributions in the root zone are also uniform. These field operations are required nearly every cropping season, particularly where substantial cultivation following harvest disrupts the field surface. The preparation of the field surface for conveyance and distribution of irrigation water is as important to efficient surface irrigation as any other single management practice the farmer employs. There are perhaps two land levelling philosophies: (1) to provide a slope which fits a water supply; and (2) to level the field to its best condition with minimal earth movement and then vary the water supply for the field condition. The second philosophy is generally the most feasible. Because land levelling is expensive and large earth movements may leave significant areas of the field without fertile topsoil, this second philosophy is also generally the most economic approach. Land levelling always improves the efficiency of water, labour and energy resources utilization. The levelling operation, however, can be the most intensively disruptive cultural practice applied to the field and several factors should be considered before implementing a land levelling project. Major topographical changes will nearly always reduce crop production in the cut areas until fertility can be replaced. Similarly, equipment traffic can so compact or pulverize the soil that water penetration is a major problem for some time. The farmer has many activities which contribute to his productivity and therefore require his skill and labour. The irrigation system should be designed with him (or her) in mind. A field levelled to high standards is generally more easily irrigated than one where undulations require special attention. New equipment is continually being introduced which provides the capability for more precise land levelling operations. One of the most significant advances has been the adaptation of laser control in land levelling equipment. The equipment has made level basin irrigation particularly attractive since the final field grade can be very precise. Comparisons with less precise techniques have clearly shown that laser-levelled fields achieve better irrigation and

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6. Land levelling

production performance. Nevertheless, for most irrigated agriculture, laser-controlled precision is unfeasible because of the high cost of such equipment unless a large number of farmers form a cooperative or a government programme is started with subsidized land levelling as one component in an effort to improve farm production.

6.2 Small-scale land levelling Most small-scale farming operations rely on animal power or small mechanized equipment which an individual can own and operate. As the irrigator waters his fields season after season he is able to observe the locations of high and low spots on the field. Then as he prepares the fields between plantings, he tries to move soil from the high spots to the low ones. Over a period of several years individual fields are smoothed enough to be watered fairly well. Figures 62 and 63 show two examples of these operations. In Figure 62, a farmer is preparing land for paddy and using the ponded water level on the field to direct him to the high and low spots. Since this is a normal land preparation practice, it does not represent an extra task for the irrigator. Figure 63 shows a similar operation using mechanized equipment for typical annual crops and again one sees that the field preparation also readies the seed bed for planting. Beyond these technologies one may observe various levels of mechanization and an array of implements. The one feature common to most small-scale land levelling is the trial and error nature of the practices and the long-term incorporation of land levelling with seed bed preparation. Another feature is that no technical or engineering inputs are needed. Figure 62. A typical land smoothing operation using animal power Figure 63. Levelling and smoothing a field as part of tractor-based farming operations

6.3 Traditional engineering approach 6.3.1 6.3.2 6.3.3 6.3.4 6.3.5

Initial considerations Engineering phase Adjusting for the cut/fill ratio Some practical problems An example problem

6.3.1 Initial considerations Initially, the field should be studied and an overall irrigation strategy identified. Once accomplished, the land levelling programme derived from traditional engineering practice can be initiated. The first step is to establish the plane of the field. This involves placing a reference grid on the field, surveying the existing topography of the field by establishing the elevations of the grid points, and calculating the new field topography by adjusting the grid elevations to correspond to the desirable plane. This is the engineering phase of the land levelling procedure. Once the surface design has been determined, a land levelling operation begins. This is typically a private contractor utilizing his equipment to move the earth into the new position on the field, and the adequacy of the land levelling is dependent on the skill of the equipment operator.

6.3.2 Engineering phase Surveying and mapping the field involves setting a uniform grid system on the field and establishing the field topography. This need not be a complicated procedure. One corner of

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6. Land levelling

the field can be chosen as a starting point and the first stake can be located one-half grid spacing from either boundary. Then a row of stakes can be measured and set using a transit or level and tape. The instrument is set up over the first stake and sighted along a line parallel to the boundary. Usually this is accomplished by going to the opposite edge and locating a stake one-half grid spacing from the edge. Then, using the instrument for alignment, the first row of stakes is measured into place. With the instrument located over the same stake and aligned along the first row, the next step is to turn the alignment 90° by either measuring a right triangle or by using the instrument angle indicators if available. The new alignment is used to locate another stake row along the other field axis. Each of the remaining stakes can be placed visually by sighting against the two stakes at the field edges. The grid spacing can be set at convenient lengths so long as it is square and consistent (this is not technically required but it simplifies the calculations). In the US, the typical grid spacing is 100 feet by 100 feet (30.5 m by 30.5 m). However this would be too large in many countries with small fields. It is suggested that the surveyor use a multiple of 10 m as a spacing and select one that divides the field into at least 5 percent subareas. The field stakes provide the basis of the field survey. The level or transit can be located in a central area and rod readings taken from each stake position. It is generally advisable to locate a benchmark near the field from which to reference the readings as elevations. In addition, readings taken from the location of water supply structures are also useful for designing the head ditches, watercourses and drainage channels. It is assumed that the basic principles of land surveying are known and practiced during this phase of the land levelling operation. An initial decision as to the method of surface irrigation will dictate field slope. Basins are designed to be level in both field directions. Borders are similar in having zero cross-slope, but may have advance slopes of up to 2 or 3 percent, depending on crop and soil conditions. Furrow irrigation systems work well with advance slopes up to 1 to 3 percent and cross-slopes of 0.5 to 1.5 percent. If the average natural slopes are greater than these ranges, terraces or benches should be planned. There are several ways to compute the new field slope including some that are inspection methods requiring some experienced judgment. A formal method, called the 'plane method,' will be used here. The plane method is a simple least squares or linear regression fit of field elevations to a twodimensional plane. Subsequent adjustments are made in the elevation of the plane centroid to compensate for variable cut/fill ratios. If the field has a basic X-Y orientation, the plane equation is written as: E(X, Y) = AX + BY + C (117) in which: E = elevation of the X, Y coordinate; A, B = regression coefficients; and C = elevation of the origin or reference point for the calculations of field topography using Eq. 117. The first step in evaluating the constants, A, B and C, is to determine the weighted average elevations of each grid point in the field. The purpose of the weighing is to adjust for any boundary stakes that represent larger or smaller areas than given by the standard grid dimension. The weighing factor is defined as the ratio of actual area represented by a grid point to the standard area. The grid point area is assumed to be the proportional area surrounding the stake or other identification of the grid point elevation. The weighing factor is:

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6. Land levelling

(118)

where: q ij = weighing factor of the grid point identified as the ith stake row and the jth stake column; Aij = area represented by the (i, j) grid point; and As = area represented by the standard grid dimension. The next step is to determine the average elevation of each row and column. For the ith row, Ei, is:

(119)

in which: N' = number of stake columns; and Eij = elevation of the (i, j) coordinate found from field measurements E(X, Y). A similar expression can be written for finding the average elevation of the jth stake column, E j:

(120)

where N" is the number of stake rows. The next step is to locate the centroid of the field with respect to the grid system. For convenience, an origin can be located one grid spacing in each direction from the first stake position, i.e. the initial stake position on the field. The distance from the origin to the centroid in the X dimension is found by:

(121)

where: X = x distance from origin to centroid; Xj = x distance from origin to the jth stake column position; and (122) Similarly,

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6. Land levelling

(123)

in which: Y = y distance from the origin to centroid; Yi = y distance from origin to the ith stake row position; and (124) The fourth step is to compute a least squares line through the average row elevations in both field directions. The slope of the best fit line through the average X-direction elevation (Ej) is A and is found by:

(125)

For the best fit slope in the Y-direction, the slope, B, is.

(126)

Finally, the average field elevation, EF , can be found by summing either Ei or Ej and dividing by the appropriate number of grid rows. This elevation corresponds to the elevation of the field centroid (X, Y). Thus, Eq. 117 can be solved for C as follows: C = EF - A X - B X (127) An adjusted elevation for each stake can be computed with Equation 110 and compared to the measured values. The differences are the necessary cuts or fills. Before these computations are undertaken, however, the slopes in both field directions must be checked to see if they are within satisfactory limits. For example, if the intended system is a border irrigation system, the cross-slope should be zero (A = 0) and the cuts and fills would need to be based on this condition. A second note concerns the fact that cuts and fills do not balance because of variations in soil density. This adjustment will be covered in a following section.

6.3.3 Adjusting for the cut/fill ratio In most cases, the best fit plane and the subsequent adjusted elevation will result in different total volumes of cuts or fills. A simple and rapid calculation of these respective volumes can be made as follows. (128) and, http://www.fao.org/docrep/t0231e/t0231e08.htm#TopOfPage[6/18/2013 7:19:29 PM]

6. Land levelling

(129) in which: Vc = volume of cuts, m 3 ; Vf = volume of fills, m 3 ; A = grid area m or n, m 3 ; Cm = depth of cut at grid point m, in metres, and F n = depth of fill at grid point n, in metres. The cut/fill ratio r is: r = Vc / Vf (130) and should be in the range of 1.1 to 1.5 depending on the soil type and its condition. The necessity of having cut/fill ratios greater than one for land levelling operations stems from the fact that in disturbing the soil, the density is changed (the fill soil is more dense because its structure has been destroyed). Selecting a cut/fill ratio remains a matter of judgement. If the value arrived at by least squares is not in the range of 1.1 to 1.5, the elevation of the field centroid, C, is raised or lowered until the value of r is appropriate. This adjustment is determined by: (131)

where r' is the cut/fill ratio required in the design. Equations 130 and 131 assume that none of the 'cut' grid points become 'fill' points or viceversa. Consequently, in some cases it will be necessary to iterate a few times to get the proper cut/fill ratio. Equation 128 is usually less formal than required for contracting purposes. Some more complete estimators include the prismoidal formula, the 'average end area method,' and the 'four corners method.' The 'four corners method' is simplest to use and is suggested by the USDA (1970). The formula for all complete grid spacings is:

(132)

in which: Ai = area of the grid square i, m 2 ; N c = number of cuts at the four corners of the grid square; and Cj and F m = cut and fill depths in m, but they are taken as absolute values so they both have the same sign, positive. At the field edges and corners, if complete grid spacings are not present, the cut volume must be computed separately. The procedure is to assume the elevations of the field boundaries are http://www.fao.org/docrep/t0231e/t0231e08.htm#TopOfPage[6/18/2013 7:19:29 PM]

6. Land levelling

the same as the nearest stake and would thereby have the same cut or fill dimensions. Equation 132 is then utilized with appropriate Ai value corresponding to the actual edge area.

6.3.4 Some practical problems The engineering design derived from the procedures above results in a field design which should provide the irrigator with a system that will satisfy his irrigation practices and yield efficient and uniform waterings if managed properly. Between the design and the operable system is the land levelling operation itself. Generally, a contractor must be retained to move the earth, after which the field topography is checked and if necessary the contractor refines his job with additional work. The skill and efficiency of the equipment operator is critical to how well the field levelling is finally accomplished. A good operator may be able to provide a field grade within plus or minus 10 cm; a poor operator perhaps double this value. The first of the practical problems is the arrangements between the irrigator and the contractor. The work should be checked and fall within the 10 cm limits before it is accepted and reimbursed. Land levelling is likely to be not only the most disruptive operation applied to the field but also the most costly. One method of reducing cut volumes, and therefore the cost, is to subdivide the field into terraces or benches. Usually, earthwork is minimized when the terrace runs parallel to the direction of highest field slope but to be sure, the cut volumes should be checked with the alternative field layouts. Operators develop field movement patterns based on their own judgement and experience. A cut-haul-fill pattern of travel that maximizes the efficiency of the land levelling operation tends to be one in which the routes are of nearly equal length. Such a strategy prevents the overuse of travel lanes and minimizes the haul and return distances. Where manually controlled equipment is used, many operators establish a bench mark grid over the field by cutting and filling strips on both sides of a stake to the desired grade. Then the median areas can be levelled to grade to better precision. Good operators make cut and fill passes which are relatively uniform and their equipment is seen to operate at fairly uniform speeds, particularly during loading passes. Earth may be used to raise the elevation of roadways, or prepare a raised pad for headland facilities. In the computation setting field cuts and fills, the volume of the earth needed for these miscellaneous requirements should be deducted in the cut/fill ratio calculation. The topography of surface irrigated fields, even after levelling, is not a static feature of the land. Year to year variations in tillage operations disturb the surface layers as well as shift their lateral position. The loose soils may settle differently depending upon equipment travel or depths of irrigation water applied. Consequently, a major land levelling operation will correct the macro-topographical problems but annual levelling or planing is needed to maintain the field surface by correcting micro-topographical variations.

6.3.5 An example problem Booher (FAO, 1974) devotes a chapter in his manual on surface irrigation to land levelling. Included is an example problem around which useful suggestions are made regarding the methods and equipment for levelling the field into a workable surface irrigated field. The problem that is developed utilizes a different approach to that suggested herein so it will be partially repeated for purposes of both illustration and comparison. The first six columns and the first eight rows of Booher's example field have been extracted and are shown in Figures 64 and 65. The locations of the field boundaries have been changed relative to the grid system to illustrate the importance of weighing grid point elevations based on the areas they represent. In the following example the standard grid spacing is 20 m by 20 http://www.fao.org/docrep/t0231e/t0231e08.htm#TopOfPage[6/18/2013 7:19:29 PM]

6. Land levelling

m and begins one-half spacing from the upper left corner of the field (represented by the grid point [i, A] in Figure 65). The standard grid area is 400 m 2, but one will note that grid points adjacent to the right field boundary represent 500 m 2 . One point, the lower right grid represents an area of 375 m 2 . Figure 64. Example problem field layout

Figure 65. Initial field elevations in metres

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6. Land levelling

The first step in the calculation of the revised field plane is to determine the grid point weighing factors using Eq. 118. Using the standard area per point as 400 m2 , the weighing coefficients, q ij , are shown in Figure 66. The row and column weights are the sum of the grid point weights and are shown to the left and at the bottom of Figure 66. Figure 66. Grid point weighing coefficients

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6. Land levelling

Using the column and row weights, Eqs. 119 and 120 are used to calculate the average elevation of the respective rows and columns. These data are included along the left and bottom of Figure 65. The field centroid is calculated with Eqs. 121 to 124 using the distances from the origin and the row and column weights. For the X coordinate of the centroid, this calculation is:

and for the Y coordinate:

Note that the origin is 10 m to the right and 10 m above the stake at grid position [i, A]. The next step is to run a linear regression through the average row and column elevations using Equations 125 and 126. These procedures are fairly standard on hand-held calculators and microcomputers so the calculations will not be shown here. The slope of the field from right to left is 0.000373 (A) and that from top to bottom is -0.002247 (B). It can also be mentioned that standard regression techniques will also yield an intercept value representing the elevation with which the best fit line through the average elevations will intercept the X and Y axis running through the origin. These intercepts can be ignored. The final calculations involving the revised field plane involve the calculation of the C value in Eq. 117 as outlined in the paragraph preceding Eq. 127. The average elevation at the centroid of the field is determined by summing the average row or column elevations. This value is also

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6. Land levelling

shown in Figure 65 as 1.557 m. From Eq. 127, then: C = 1.577 - 0.000373 * 72 - (-.002746 * 87.743) = 1.7911 m The resulting equation of the field plane defined by the procedure so far is: E(X, Y) = .000373 * X - .002746 * Y + 1.7911 If this relationship is used to recompute the elevations at each grid point, the cuts and fills are identified as the positive (fills) or negative (cuts) differences between the computed elevations and the original topography. Figure 67 shows these results as the upper number near the grid points. Figure 67. First determination of cuts and fills for the example problem

In order for the earthwork to balance in the field after levelling, the volume of cuts should exceed the fills by 10 to 30 percent. For the 6th row shown below, Eqs. 128 and 129 are evaluated as follows: http://www.fao.org/docrep/t0231e/t0231e08.htm#TopOfPage[6/18/2013 7:19:29 PM]

6. Land levelling

| +.11 +.03 -.01 -.01 -.02 0 | vi | *

*

*

*

*

* |

Volume of Cuts for Row vi = (-.01) * 400 + (-.01) * 400 + (-.02) * 400 = -16 m 3 or since the sign is irrelevant, the cut volume along row 6 is 16 m3 , and for the fills: Volume of Fills for Row vi = 400 * (.11 + .03 + .000) = 56 m3 Determining the cuts and fills of each row and then summing yields a total cut volume of 627 m 3 and a total fill volume of 1007 m 3 . Dividing the cut volume by the fill volume gives a cut/fill ratio of about 0.62, which of course is not satisfactory. Assuming the cut/fill ratio should be about 1.3, Equation 131 can be used to recompute the elevation of the field centroid, C. The change in centroid elevation is determined by summing the area of each cut station times the depth of cut. There are 17 cut points in which the grid area is 400 m 2 , 2 involving the 500 m 2 left boundary points, and 4 cuts along the 300 m grid points along the lower field boundary. Thus the area summation in the denominator of Equation 124 is 9000 m 2 . The remainder of Equation 124 is then:

This calculation assumes that none of the previous fill locations become cut locations. To test this assumption, 0.033 m is subtracted from each cut and fill depth in Figure 67 and the results are shown in Figure 68. It is noted that 2 fill locations have become cut points. Figure 68. Second determination of cuts and fills for the problem

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6. Land levelling

Recomputing the volume of cuts from Eq. 128 and the fills from Eq. 129 yields the following cut/fill ratio (Eq. 130):

This value is slightly more than the 1.3 assumed in adjusting the C value in Eq. 117 and reflects the problem of grid points changing from cuts to fills (or vice versa in other cases). If the error had been greater, another iteration would be suggested. Not in this case, however, and the final field plane is as shown in Figure 68 with the subscript cuts and fills. If the field is intended for borders and basins, the procedure is the same except that the A and/or B slopes in Eq. 117 would be zero. Similarly, if the field is to be terraced, the procedure is applied separately to the grid points in each terrace area. The last engineering step is the formal computation of the volume of cuts for contractual http://www.fao.org/docrep/t0231e/t0231e08.htm#TopOfPage[6/18/2013 7:19:29 PM]

6. Land levelling

purposes. This is illustrated for the evaluation of Eq. 132 for the area between rows v and vi. The final cut/fill depths for rows vii and viii are shown below. v |*

*

*

*

*

| +.28 +.18 +.05 +.01 0

*

|

+.05 |

|

|

|

|

vi | *

*

*

*

*

*

|

| +.08 -.01 -.04 -.04 -.05 -.04 |

It is assumed that the depth of fill at the left boundary is .28 m at row v and .08 m at row vi. Similarly, the fill and cut at the right boundary are .05 m at row v and -.04 at row vi respectively. Equation 132 is evaluated as follows: Grid Points |

*

+.28

+.28

Computations

Total

= |

*

+.08

+.08

*

*

+.28

+.18

0 m3

= .02 m3 *

*

+.08

-.01

*

*

+.018 +.05 = .89 m3 *

*

-.01

-.04

*

*

+.05

+.01 = 4.57 m3

*

*

-.04

-.04

*

*

+.01

+.0 = 8.10 m3

*

*

-.04

-.05

*

*

0

+.05 = 5.79 m3

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6. Land levelling

*

*

-.05

-.04

*

|

+.05

+.05 = 4.44 m3

*

|

-.04

-.04 Total

23.81 m3

Repeating these calculations for each grid area yields a total cut volume of 946.02 m3 which is very close to the 959 m 3 estimated with Eq. 128. It is perhaps worthwhile mentioning at this point that microcomputer programmes have been written to perform land levelling computations as illustrated above. Some of these are commercially available, some can be acquired by tracking down the programmer.

6.4 Laser land levelling The advent of the laser-controlled land levelling equipment has marked one of the most significant advances in surface irrigation technology. One such system is shown in Figure 69. It has four essential elements: (1) the laser emitter; (2) the laser sensor; (3) the electronic and hydraulic control system; and (4) the tractor and grading implement. Figure 69. Two views of land levelling equipment using laser control systems

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6. Land levelling

The laser emission device, like that pictured in Figure 70, involves a battery operated laser beam generator which rotates at relatively high speed on an axis normal to the field plane. This rotating beam thereby effectively creates a plane of laser light above the field which can be used as the levelling reference rather than the elevation survey at discrete grid points in conventional land levelling techniques. Various beam generators are equipped with selfadjustment mechanisms that allow the plane of the beam to be aligned in any longitudinal or latitudinal slope desired. This reference plane of laser light is an extremely advantageous factor in the levelling operation because it is not affected by the earth movement, does not require a field survey to establish the high and low spots, and does not require the operator to judge the magnitude of cuts and fills. The distance between the laser beam and the earth surface is defined such that deviations from this distance become the cuts and fills. With laser systems, there is little or no need for the exhaustive engineering calculations of the conventional approach. The cost of levelling is usually contracted on the basis of money per equipment hour. The laser emitter is generally located on a tripod or other tower-like structure on or near the field and at an elevation such that the laser beam rotates above any obstructions on the field as well as the levelling equipment itself. The beam is targeted and received by a light sensor mounted on a mast attached to the land grading implement. The sensor is actually a series of detectors situated vertically so that as the grading implement moves up or down, the light is detected above or below the centre detector. This information is transmitted to the control system which actuates the hydraulic system to raise or lower the implement until the light again strikes the centre detector. It is in this manner that the sensor on the mast is continually aligned with the plane on the laser beam and thereby references the moving equipment with the beam. It is important to note that the sensitivity of the laser sensor system is at least 10 to 50 times more precise than the visual judgement and manual hydraulic control of an operator on the tractor. Consequently, the land levelling operation is correspondingly more accurate. The skill of the operator is substantially less critical to the levelling which allows farmers and other personnel access to the land grading equipment.

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Figure 70. Close up view of laser beam emmitter The electronic and hydraulic control systems generally have two operating modes. In the first, or observation mode, the mast itself moves up or down according to the undulations in the field as the operator drives the equipment over the field in a grid-like fashion. The monitor in the tractor yields elevation data from which the operator can determine average field elevations and slopes. In other words, the system operates as a self-contained surveying system. In this mode, the blade of the grading implement is fixed in place and only the sensor mast moves. In the second mode, or planing mode, the mast position is fixed relative to the implement blade which is then raised or lowered in response to the land topography. The beam plane is located the appropriate distance above the field centroid and at the desired slopes. By adjusting the height of the mast sensor relative to this plane and the centroid, the cutting and filling is accomplished simply by driving the tractor over the field. However, in many cases, the depth of cuts will exceed the depth which can be cut with the power of the tractor and the operator must override the automatic controls in order to keep the equipment operating. The fourth element of the levelling system is the tractor - grading implement combination. This equipment is generally standard agricultural tractors and land graders in which the hydraulic and control systems have been modified to operate under the supervision of the electronic controller supplied with the laser emitter and sensor devices. The tractor needs to be carefully selected so that it is not under-powered and its hydraulic system is strong enough to work with the laser-imposed frequency of movements and adjustments. The grading implement can be as simple as a land plane which scrapes the earth and moves only as much as can be pushed in front of the blade or a complex piece of equipment which loads and carries earth. The former is used primarily for small levelling jobs, smoothing and repeat grading. The latter is usually better for initial levelling where cuts are larger and in the preparation of level basins where the cuts are also larger than in bordered or furrowed fields. As a final note on levelling in general and laser levelling is particular, it is probable that the importance of accurate field grading has been under estimated. The precision improves irrigation uniformity and efficiency and as a result the productivity of water and land. On large fields, the improved productivity has been shown to pay economic dividends that easily exceed the cost of the levelling. However, the equipment is expensive and quite beyond all but the largest of farmers. In the developing countries, laser-guided equipment is being demonstrated and tested. There remains the solution as to how such equipment can be made useful for the small farmer.

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7. Future developments

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Title: Guidelines for designing and evaluatin surface irrigation systems...

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7. Future developments 7.1 Background 7.2 Surge flow 7.3 Cablegation 7.4 Adaptive control systems 7.5 Water supply management

7.1 Background Surface irrigation is the historical choice of irrigators worldwide and will undoubtedly remain so. Surface practices have for the most part changed very little in centuries. Two questions arise in this connection. First, is there a need to do things differently, and second, is there a means to do so? The stimuli to discover and implement improved surface irrigation practices are numerous and varied. Perhaps the most important are population growth, urbanization and industrialization because existing water resources have largely been committed for these uses. Water shortages are often addressed as arbitrary restrictions in supply but it would be wiser to introduce practices to conserve water. Since agriculture usually imposes the largest water demand, improved irrigation efficiency should become the centrepiece of conservation strategies. Energy resources and labour availability are declining in most countries. Reduced energy supplies may tend to restrict the use of sprinkler and trickle systems. A lack of labour will prompt the adaptation and use of automation as part of the operation of irrigation systems. The immediate and long-term futures of most irrigated regions also appear to depend heavily on improved irrigation practices. The probability that major technical or operational innovations for existing surface irrigation systems will be made is low. Over the thousands of years during which surface irrigation has been practiced, the alternatives for diverting the water onto the field have been clearly identified. There are, however, four areas where significant innovations have or will be made; these are: (1) precision land levelling in order for water movements to be more uniform and manageable; (2) automation for headland facilities; (3) supervisory and adaptive or feedback control systems; and (4) water supply control and management. In precision land levelling, the most important innovation has been the laser guidance and control system applied to mechanized land levelling equipment. Precision has increased by at least a factor of 10 and results are impressive in terms of efficiency and production. This topic was discussed in Section 6 and need not be mentioned here except to conclude that the technology's expansion into the developing countries will make a significant improvement in surface irrigation.

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Automation of surface irrigation headland facilities is difficult. Each irrigation behaves differently which limits the standardization necessary for effective automation. Thus, a great deal of research and development notwithstanding, surface irrigation automation has not been widely successful. However, a series of new concepts has emerged in the last decade that offers a better opportunity. Some of the important US references on automation include Haise et al. (1980), Dedrick and Erie (1978), and Humpherys (1969, 1971, and 1983). Perhaps one of the more interesting is the 'surge flow' concept developed at Utah State University. For many years automation has attempted to manage discharge directly. These efforts have not been very successful, but the surge flow concept manages discharge indirectly by regulating on an off time and by so doing has made the management problem tractable. A similar innovation involving indirect flow regulation is 'cablegation' described by Kemper et al. (1985). Both surge flow and cablegation will be described below to illustrate the idea of new approaches to automation. They are by no means the only automating measures now available, but are illustrated here to show the reader two alternatives for improving water control by controlling time rather than discharge. Control systems and water supply management are opportunities to deal with the uncertainty associated with variable infiltration and will be considered separately below.

7.2 Surge flow 7.2.1 Effects of surging on infiltration 7.2.2 Effects of surging on surface flow hydraulics 7.2.3 Surge flow systems In 1979, Stringham and Keller (1979) reported a new approach for automating surface irrigation systems in which problems with slow advance and excessive surface runoff occur. The approach was called 'surge flow' to describe the hydraulic regime of the flow over the field. In 1986, a US patent was granted to Professors Keller and Stringham of Utah State University for the concept. A trademark registration was issued for the term 'Surge Flow' although by the time of writing this bulletin, the term has become widespread as a surface irrigation water management concept. Consequently, the use of 'surge flow' in this guide will not attempt to distinguish the proper use. Under the surge flow regime, an irrigation is accomplished through a series of individual pulses of water onto the field such that, instead of the typically found advance-wettingdepletion-recession trajectory shown in Figure 1 in normal surface irrigation conditions, it looks like that in Figure 71. Thus instead of providing a continuous flow onto the field for say six hours, a surge flow regime would apply six 1 hour 'surges'. Each surge is characterized by a cycle time and a cycle ratio. The cycle time is comprised of an on-time and an off-time related by the cycle ratio which is the ratio of on-time to the cycle time. The cycle time can range from as little as one minute to as much as several hours. Cycle ratios typically range from 0.25 to 0.75. By regulating these two parameters, a wide range of surge flow regimes can be produced which can significantly improve irrigation efficiency and uniformity. Figure 71. Typical surge flow advance-recession trajectory

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It is perhaps worth noting that surge flow, while appearing quite simple, is nevertheless an advanced irrigation technology.- The design and evaluation require a level of hydraulics beyond this guide and the equipment needed to implement surge flow fully is often feasible only in large farming operations. This is not to imply that surge flow cannot or should not be considered in developing countries, only that special adaptations will be necessary.

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Since its introduction in 1979, surge flow has been tested on nearly every type of surface irrigation system and over the full range of soil types. Results vary depending on the selection of cycle time, cycle ratio and discharge. But in nearly every case, the intermittent application significantly reduces infiltration rates and/or substantially reduces the time necessary for the infiltration rates to approach the final or 'basic' rate. To effect infiltration rates, the flow must completely drain from the field between surges. If the period between surges is too short, the individual surges overlap or coalesce and the infiltration effects are generally not created. Research shows that the surging effect on infiltration is primarily due to the consolidation of the thin layer of fine material deposited in the bottom of the furrow or on the border or basin surface by the destruction of soil aggregate and erosion caused by the water flow. As the water drains from the field between surges, the negative pressure that develops in the soil consolidates the surface layer, collapsing the larger pores, attracting small particles into the lattice between larger particles, and orienting clay and silt into a layered structure. As a result the permeability of the field surface is reduced and thereafter infiltration rates are lowered. The reductions in surface permeability seem to be more pronounced in sandy loam soils than in clay loam soils. The rate of aggregate wetting and erosion affect the thickness and extent of the surface layer. Evidence of the consolidation of the fine layer between surges can usually be observed in the field 5-15 minutes after the water has completely drained from the field. Tension cracks form between the layers of fine material and those less disturbed by the flow. When water is again introduced into the field, sediments are deposited in these cracks as they begin to swell shut, thereby further compacting the surface layer. The effect of having reduced the infiltration rates over at least a portion of the field is that advance rates are increased. Generally, less water is required to complete the advance phase by surge flow than with continuous flow. Surging is often the only way to complete the advance phase in high intake conditions like those following planting or cultivation. As a result, intake opportunity times over the field are more uniform. However, since results will vary among soils, type of surface irrigation, and the surge flow configuration, tests should be conducted in areas where experience is lacking in order to establish the feasibility and format for using surge flow.

7.2.2 Effects of surging on surface flow hydraulics The hydraulic regime of a surge flow system is composed of two parts: (1) the distinct surge phase; and (2) the coalesced surge phase. The distinct surge phase is noted above. Each pulse of water advances and recedes over a portion or all of the field as shown in Figure 71. This phase is used during the advance phase for the entire field, i.e. during the time needed to wet the entire surface of the field. Surges during the distinct phase must be of sufficient duration and discharge to fill cracks and depression storage along the pathway so that there is enough volume and energy to continue advancing at an adequate rate over the succeeding field section, but short enough to limit cumulative intake and maximize or minimize the infiltration reduction. In the coalesced phase, the individual surges run together, overlap and result in a nearly steady flow in the downstream sections of the field. In this situation, the flow rate below the point of convergence is about one-half of the instantaneous rate at the field inlet. If the cycle ratios are reduced, the flow in the continuous flow reaches will be correspondingly reduced. It is therefore possible to adjust the cycle ratios until practically no surface runoff occurs. The reader thus immediately sees the coalesced phase as being exactly equivalent to the cutback phase described in previous sections for furrow irrigation. Indeed, the original research of Stringham and Keller (1979) was directed toward the development of an alternative cutback http://www.fao.org/docrep/t0231e/t0231e09.htm#TopOfPage[6/18/2013 7:19:59 PM]

7. Future developments

method. The advantages of surge flow during the advance phase came as a welcome surprise. Thus, by combining the distinct and coalesced phases of surge flow into one system, the solution of the long-standing surface irrigation dilemma is available, a high flow for the advance phase and a low flow for the wetting phase.

7.2.3 Surge flow systems There are basically two field systems commercially available for surge flow, both limited at present to furrow irrigation. The first is shown in Figure 72 and will be described here as the 'dual line' system. Water is supplied to the field generally through a buried pipeline which connects to surface gated pipe through a riser and valve. The valve, shown schematically in Figure 73, is automated to switch the flow between two sets. Surging is accomplished by alternating the flow between the two sets. When these two are finished, the entire flow is directed to another riser and valve by the irrigator. The dual line system is in widespread use in the USA where irrigators already have a gated pipe furrow irrigation system in place. They only need to purchase the automated valve to implement fully a surge flow regime. The costs for these systems where the distribution and gated pipe already exist can be as low as US$50 per hectare. Figure 72. Schematic diagram of a dual line surge flow furrow irrigation system (redrawn from Humpherys, 1987)

Figure 73. Configuration of one automated surge flow valve for the dual line system (redrawn from Humpherys, 1987)

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7. Future developments

The second field configuration is the single line system shown in Figure 74. A single gated pipe is connected to the water supply and individual outlets along with pipe are controlled by small hydraulic, pneumatic, or electric valves which are organized in banks and sets as shown and controlled by a single controller. Figure 74. Schematic of the single line surge flow system (redrawn from Humpherys, 1987) The single line system is economic for new systems where all of the field facilities need to be provided. It also tends to be more economic where only the gated pipe is available and the decision of the irrigator is whether or not to put in a buried supply line and then use the bidirectional valve or to put automated gates on the gated pipe and use the single line concept. In many cases, the single line system will be more flexible than a dual line system in terms of irrigating an entire field. Adaptation for border and basin systems can be made by automating existing control structures and perhaps by a new control structure like that of Ismail and Westesen (1984). Single or dual line surge flow systems can also be utilized where open channel systems are present (Testezlaf et al. 1985).

7.3 Cablegation The cablegation system illustrated graphically in Figure 75 was developed by the Soil and Water Management Research Unit of the US Department of Agriculture's laboratory at Kimberly, Idaho (Kemper et al. 1985). The system involves a pipe with fixed or adjustable outlets which is placed on a precise gradient. An adjustable plug is placed inside the pipe and connected by a cable to a winch-type unit at the pipe inlet. The winch unit includes a speed control feature. Figure 75. Schematic diagram of a cablegation furrow irrigation system Hydraulically, a cablegation system operates in the free surface flow regime upstream of the travelling plug except immediately adjacent to it. In the region near the plug, the flow is slowed and expands to fill the pipe. Thus, in the uniform open channel flow region of the pipe, the water surface is below the outlets which are therefore shut off from the field. Near the plug, the water level rises above the outlets to supply the field. The unique feature of the cablegation system is the high outlet flows nearer the plug. This feature gives the advance phase discharge needed to facilitate field coverage. As the plug moves downstream, the outlet flow is cutback to allow soaking time without causing excessive surface runoff.

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Cablegation and surge flow are two examples of an alternative approach to managing surface irrigation. After years of trying to regulate discharges unsuccessfully, these two methods accomplish this end by managing time and equipment speed.

7.4 Adaptive control systems The most limiting problem associated with design and management of all types of surface irrigation systems is the fact that the infiltration characteristics are unpredictable. They change after each irrigation, from season to season, and following each cultivation. They change over a period of years as the content of organic matter changes, as salinity in both the water and soil changes, and as irrigation methods are altered. It should be clear that if infiltration rates were predictable, the time of advance and irrigation efficiency would be quite predictable and this would allow much better management and automation. The premise of the adaptive control system is that infiltration and therefore advance time, cutoff time, and application efficiency can be forecast during the early stages of the irrigation and that actions can be taken shortly thereafter if the outcome of present settings is not going to be adequate. Using a volume balance hydraulic concept similar to that discussed in Section 4.3.4, Burt et al. (1982) outlined a procedure in which infiltration coefficients could be deduced from rate of advance during the first watering set and then used to refine the set size, flow and times during subsequent sets to improve efficiency substantially. Reddell and Latimer (1986) took the next step and coupled the volume balance inference of infiltration to real time conditions with a microcomputer located near the field in which sensor readings are processed to determine when the advance phase will be completed and how the system should be set for the cutback flow. Work soon to be reported by Busman (1987) and others now working on the computer software and field verification will indicate the application of advance hydraulic models to the same problem except the infiltration will be deduced from advance sensor readings near the field inlet. This will allow settings to be changed to improve the performance of irrigation on the current set as well as those subsequent.

7.5 Water supply management It is clear from evaluating hydraulic principles that if the discharge onto a field varies from the design or values given by the manager, the performance will be significantly affected. If the discharge is reduced, it is likely that uniformity will suffer and deep percolation losses will increase. If the flow is unexpectedly increased, runoff losses will increase or ponding; on the field surface will be excessive. When the water supply is uncertain, irrigators are reluctant to invest heavily in costly agricultural inputs like high-yielding seed varieties, fertilizers and cultivation practices. When water deliveries to the farm are not timely, crop yields tend to decline due to crop stress or overwatering. The irrigator usually has very little actual control of the problems noted above unless his water supply is from a well, he is near the headworks of an irrigation project, or he is very influential in the operation and maintenance of the irrigation facilities upstream of his farm. Thus, an overriding concern in developing efficient and effective surface irrigation systems is the operation of the irrigation project itself. The management of the collection, storage and conveyance systems in a project is a critical factor in the performance and production of the surface irrigation system at the farm level. To ignore this linkage is to invite low production, waterlogging and salinity, pollution of both surface and subsurface water resources, poverty of the agricultural sector, and numerous other well-known irrigation problems. Yet, this linkage has rarely if ever been established effectively, and as one would expect, the problems are easy to identify. Irrigation project management for improved on-farm irrigation and efficiency is beyond the

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scope of this guide, but it brings into focus the future direction of water management. The technical principles of irrigation are fairly well developed, understood, and modelled. Most research and development efforts are aimed at refining and expanding engineering, soil and plant science, and economic knowledge of individual processes and interactions that are already well defined. The weakness therefore in irrigation science and application lies primarily in the management of the irrigation system as a whole and not the design and operation of the irrigation system's individual components (fields, farms, canals and watercourses, reservoirs, dams and headworks, etc.). The hydraulics of surface irrigation, for example, continue to receive research attention even though the fundamental relationships have been established long since. It is important that this research continue in-order that the application of the research be made more accurate and universal.

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References

Produced by: Natural Resources Management and Environment Department

Title: Guidelines for designing and evaluatin surface irrigation systems...

More details

References Ackers, P., W.R. White, J.A. Perkins and A.J. Harrison. 1978. Weirs and Flumes for Flow Measurement. John Wiley & Sons Ltd. Chichester, West Sussex, UK. Bennett, R.S. 1972. Cutthroat flume discharge relations. MS Thesis. Colorado State University, Fort Collins, Colorado. Unpublished document. Bondurant, J.A. 1957. Developing a furrow infiltrometer. Agric. Engineering pp. 602604. Bos, M.G., Repogle, J.A. and Clemmens, A.J. 1985. Flow Measuring Flumes for Open Channel Systems. Wiley, New York. 321p. Bos, M.G. 1976. Discharge measurement structures. Publication 20, International Institute for Land Reclamation and Improvement (ILRI), Wageningen, The Netherlands. Burt, C.M., Robb, G.A. and Hanon, A. 1982. Rapid evaluation of furrow irrigation efficiencies. Paper 82-2537 presented at the Winter Meeting of ASAE, Chicago, Illinois. Busman, J.D. 1987. Optimizing control of surface irrigation using concurrent evaluation of infiltration. PhD Dissertation, Agricultural and Irrigation Engineering, Utah State University, Logan, Utah. Unpublished document. 209p. Dedrick, A.R. and Erie, L.J. 1978. Automation of on-farm irrigation turnouts utilizing jack-gates. Trans. ASAE 21(1) 92-96. Elliott, R.L. and Walker, W.R. 1982. Field evaluation of furrow infiltration and advance functions. Trans. ASAE, 25(2):396-400. FAO. 1974. Surface irrigation, by L.J. Booher. FAO Agricultural Development Paper No. 95. Rome. 160p. FAO. 1975. Small hydraulic structures, Vol. 1 and 2, by D.B. Kraatz and V.I.K. Mahajan. Irrigation and Drainage Papers 26/1 and 26/2, Rome. 407p and 293p, respectively. FAO. 1977. Crop water requirements (Revised Edition), by J. Doorenbos and W.O. Pruitt. Irrigation and Drainage Paper 24, Rome. 144p. Garton, J.E. 1966. Designing an automatic cut-back furrow irrigation system. Oklahoma Agricultural Experiment Station, Bulletin B-651, Oklahoma State University, Stillwater, Oklahoma. Gharbi, A. 1984. Effect of flow fluctuations on free-draining and sloping furrow and border irrigation systems. MS Thesis, Agricultural and Irrigation Engineering. Utah State University, Logan, Utah. Unpublished document. 123p.

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References

Haise, H.R., Donnan, W.W., Phelan, J.T., Lawhon, L.F., and Shockley, D.G. 1956. The use of cylinder infiltrometers to determine the intake characteristics of irrigated soils. Publ. ARS 41-7, Agricultural Research Service and Soil Conservation Service, USDA, Washington DC. Haise, H.R., Kruse, E.G., Payne, M.L,. and Duke, H.R. 1980. Automation of surface irrigation: 15 years of USDA research and development at Fort Collins, Colorado. USDA Production Research Report No. 179. US Government Printing Office, Washington DC. Hart, W.E., Collins, H.J., Woodward, G., and Humpherys, A.J. 1980. Design and operation of gravity on surface systems, Chapter 13, In: Design and Operation of Farm Irrigation Systems. ASAE Monograph Number 3, St. Joseph, Michigan. 829p. Humpherys, A.S. 1969. Mechanical structures for farm irrigation. J. Irrig. and Drainage Div., ASCE, 95(IR4):463-479. Humpherys, A.S. 1971. Automatic furrow irrigation systems. Trans. ASAE 14(3):446-470. Humpherys, A.S. . Automated Air-Powered irrigation Butterfly Valves. Trans. ASAE 26(4):1135-1139. Humpherys, A.S. 1987. Surge flow surface irrigation: Section 3, Equipment. Final report of Western Regional Project W-163, Utah Agricultural Experiment Station, Utah State University, Logan, Utah. 106p. Ismail, S.M. and Westesen, G.L. 1984. Surge flow border irrigation using an automatic gate. Paper 84-2069 presented at the Winter Meeting of ASAE, Chicago, Illinois. Jensen, M.E. (ed.) 1973. Consumptive Use of Water and Irrigation Water Requirements. American Society of Civil Engineers, New York. 215p. Kemper, W.D., Kincaid, D.C., Worstell, R.V., Heinemann, W.H., and Trout, T.J. 1985. Cablegation system for irrigation: description, design, installation, and performance. USDAARS Pub. 21, US Government Printing Office, Washington DC. Kincaid, D.C. and Heermann, D.F. 1974. Scheduling irrigations using a programmable calculator. USDA, Agricultural Research Service, ARS-NC-12. US Government Printing Office, Washington DC. Kindsvater, C. E. and R.W. Carter. 1957. Discharge characteristics of rectangular thin-plate weirs. J. Hydraulics Div. ASCE, 83(HY6), Paper 1453. Kundu, S.S. and Skogerboe, G.V. 1980. Field evaluation methods for measuring basin irrigation performance. Technical Report No. 59, Water Management Research Project, Colorado State University, Fort Collins, Colorado. Ley, T.W. 1980. Sensitivity of furrow irrigation performance to field and operation variables. MS Thesis, Department of Agricultural and Chemical Engineering, Colorado State University, Fort Collins, Colorado. Unpublished document. 174p. Malano, H.M. 1982. Comparison of the infiltration processes under continuous and surge flow. MS Thesis. Utah State University, Logan, Utah. Unpublished document. Marr, J.C. 1967. Grading land for surface irrigation. Circular 408, California Agricultural Experiment Station, University of California, Davis, California. Merriam, J.L. 1960. Field method of approximating soil moisture for irrigation. Trans. ASAE 3(1):31-32.

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References

Merriam, J.L. and Keller, J. 1978. Farm irrigation system evaluation: A guide for management. Department of Agricultural and Irrigation Engineering, Utah State University, Logan, Utah. Reddell, D.L. and Latimer, E.A. 1986. Advance rate feedback irrigation system (ARFIS). Paper 86-2578 presented at the Winter Meeting of ASAE, Chicago, Illinois. Salazar, L.J. 1977. Spatial distribution of applied water in surface irrigation. MS Thesis, Department of Agricultural Engineering, Colorado State University, Fort Collins, Colorado. Unpublished document. 154p. Shatanawi, M.R. and Strelkoff, T. 1984. Management contours for border irrigation. J. Irrig. and Drainage Div., ASCE, 110(4):393-399. Shen, J. 1960. Discharge characteristics of triangular thin-plate weirs. Water Supply Paper 1617B US Department of the Interior, Geological Survey. US Government Printing Office, Washington DC. Skogerboe, G.V., Hyatt, M.L., Anderson, R.K., and Eggleston, K.O. 1967. Design and calibration of submerged open channel flow measurement structures: Part 3, Cutthroat flumes. Report WG31-4, Utah Water Research Laboratory, Utah State University, Logan, Utah. Skogerboe, G.V., Somoray, V.T., and Walker, W.R. 1971. Check-drop-energy dissipator structures in irrigation systems. Water Management Technical Report No. 9, Water Management Research Project, Colorado State University, Fort Collins, Colorado. Strelkoff, T. 1977. Algebraic computation of flow in border irrigation. J. Irrig. and Drainage Div., ASCE, IR3(103):357-377. Strelkoff, T. and Katapodes, N.D. 1977. Border irrigation hydraulics with zero-inertia. J. Irrig. and Drainage Div., ASCE, 103(IR3):325-342. Strelkoff, T. and Shatanawi, M.R. 1984. Normalized graphs of border irrigation performance. J. Irrig. and Drainage Div., ASCE, 110(4):359-374. Stringham, G.E. and Keller, J. 1979. Surge flow for automatic irrigation. Proc. ASCE Irrigation and Drainage Specialty Conference, Albuquerque, New Mexico. Testezlaf, R., Garton, J.E., Cudrak, A.J., and Elliott, R.L. 1985. An open ditch surge flow furrow irrigation system. Paper 85-2069 Presented at the Winter Meeting of ASAE, East Lansing, Michigan. US Bureau of Reclamation. 1967. Water Measurement Manual. Second Edition - Revised Reprint, US Government Printing Office, Washington DC. 327p. US Department of Agriculture, Soil Conservation Service. 1967. National Engineering Handbook, Section 15, Chapter 3, Planning farm irrigation systems. US Government Printing Office, Washington DC. US Department of Agriculture, Soil Conservation Service. 1970. National Engineering Handbook, Section 15, Chapter 12, Land leveling. US Government Printing Office, Washington DC. van Bavel, C.H.M., P.R. Nixon, and V.L. Hauser. 1963. Soil moisture measurement with the neutron method. Publ. ARS41-70. US Department of Agriculture, Agricultural Research Service, Washington DC., June. Walker, W.R. 1978. Identification and initial evaluation of irrigation return flow models. Report

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References

EPA-600/2-78-144, Robert S. Kerr Environmental Research Laboratory, US Environmental Protection Agency, Ada, Oklahoma. Walker, W.R. and Skogerboe, G.V. 1987. Surface Irrigation: Theory and Practice. PrenticeHall, Englewood Cliffs, New Jersey. 386p. Yitayew, M. and Fangmeier, D.D. 1984. Dimensionless runoff curves for irrigation borders. J. Irrig. and Drainage Div., ASCE, 110(2):179-191.

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Appendix I - Fortran 77 surface irrigation design program

Produced by: Natural Resources Management and Environment Department

Title: Guidelines for designing and evaluatin surface irrigation systems...

More details

Appendix I - Fortran 77 surface irrigation design program A diskette copy of this program source and executable codes for IBM PC and compatible microcomputers is available from the FAO. The program requires at least 256k bytes of ram memory. It can be utilized from either a fixed or floppy disk drive and is written to utilize only 80 x 25 text mode screen features. FAO does not warrant or guarantee this software for specific purpose. The user assumes full responsibility for its application. The diskettes supplied by the FAO will not be protected and may be duplicated and distributed as desired.

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FAO irrigation and drainage papers

Produced by: Natural Resources Management and Environment Department

Title: Guidelines for designing and evaluatin surface irrigation systems...

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FAO irrigation and drainage papers 1. Irrigation practice and water management, 1971 (Ar** E** F** S**) Rev. 1. Irrigation practice and water management, 1984 (E*) 2. Irrigation canal lining (New edition (1977) available in E, F and S in the FAO Land and Water Development Series) 3. Design criteria for basin irrigation systems, 1971 (E**) 4. Village irrigation programmes - a new approach in water economy, 1971 (E** F**) 5. Automated irrigation, 1971 (E** F** S**) 6. Drainage of heavy soils, 1971 (E** F* S**) 7. Salinity seminar, Baghdad, 1971 (E** F*) 8. Water and the environment, 1971 (E** F** S**) 9. Drainage materials, 1972 (E** F** S**) 10. Integrated farm water management, 1971 (E** F** S**) 11. Planning methodology seminar, Bucharest, 1972 (E** F**) 12. Farm water management seminar, Manila, 1972 (E**) 13. Water-use seminar, Damascus. 1972 (E** F**) 14. Trickle irrigation, 1973 (E** F** S**) 15. Drainage machinery, 1973 (E** F**) 16. Drainage of salty soils, 1973 (C** E** F** S**) 17. Man's influence on the hydrological cycle, 1973 (E** F** S**) 18. Groundwater seminar, Granada, 1973 (E** F* S*) 19. Mathematical models in hydrology, 1978 (E*) 20. Water laws in Moslem countries, Vol. 1, 1973 (E** F*) 20. Water laws in Moslem countries, Vol. 2,1978 (E* F*) 21. Groundwater models, 1973 (E*) 22. Water for agriculture, 1973 (E** F** S**) 23. Simulation methods in water development, 1974 (E* F** S**) 24. Crop water requirements (Revised), 1977 (C** E* F* S*) 25. Effective rainfall, 1974 (C** E** F** S**) 26. Small hydraulic structures (Vols. 1 and 2), 1975 (E* F* S*) 27. Agro-meteorological field stations, 1976 (E* F** S**) 28. Drainage testing, 1976 (E* F* S*) 29. Rev. 1. Water quality for agriculture, 198 (C** E* F* S*) 30. Self-help wells, 1977 (E*) 31. Groundwater pollution, 1979 (C** E* S*) 32. Deterministic models in hydrology, 1979 (E*) 33. Yield response to water, 1979 (C** E* F* S*) 34. Corrosion and encrustation in water wells, 1980 (E*)

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FAO irrigation and drainage papers

35. Mechanized sprinkler irrigation, 1982 (C* E* F* S*) 36. Localized irrigation, 1980 (Ar** C* E* F* S*) 37. Arid zone hydrology, 1980 (C* E*) 38. Drainage design factors, 1980 (Ar* C* E* F* S*) 39. Lysimeters, 1982 (C* E* F* S*) 40. Organization, operation and maintenance of irrigation schemes, 1982 (C*** E* F* S***) 41. Environmental management for vector control in rice fields, 1984 (E* F* S*) 42. Consultation on irrigation in Africa, 1986 (E* F*) 43. Water lifting devices, 1986 (E*) 44. Design and optimization of irrigation distribution networks, 1988 (E* F***) 45. Guidelines for designing and evaluating surface irrigation systems, 1989 (E*)

Availability: November 1989 Ar - Arabic C - Chinese E - English F - French S - Spanish * Available ** Out of print *** In preparation The FAO Technical Papers can be purchased locally through FAO sales agents or directly from Distribution and Sales Section, FAO, Via delle Terme di Caracalla, 00100 Rome, Italy.

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