Effective Rainfall, 1974 Dastane FAO 25
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FAO paper no 25 for effective rainfall in irrigated agriculture by Dastane....
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Effective rainfall in irrigated agriculture agriculture
Table of Contents
by N.G. Dastane FAO Consultant
project coordinator Indian agricultural research institute New Delhi FAO IRRIGATION AND DRAINAGE PAPER
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 an y country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. M-56 ISBN 92-5-100272-X The copyright in this book is vested in the Food and Agriculture Organization of the United Nations. The book may not be reproduced, in whole or in part, by any method or process, without written permission from the copyright holder. Applications Applications for such permission, with a statement of the purpose and extent of the reproduction desired, 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 1978
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
Foreword Chapter I. Introduction
1. Effective rainfall and its significance 2. The pathway of rain water 3. Concepts of effective rainfall 4. Definition of effective rainfall 5. Effectiveness of rainfall 6. Factors influencing effective rainfall Chapter II. Measurement of effective rainfall
1. Components of effective rainfall and their meas urement 1.1 Rainfall and Irrigation 1.2 Surface Run-off 1.3 Rooting Depth 1.4 Deep Percolation Losses 1.5 Evapotranspiration 2. Empirical methods of determining effective rainfall 2.1 Soil Moisture Changes 2.2 Daily Soil Moisture Balance Method 2.3 Integrating Gauge 2.4 The Ramdas Method 2.5 Lysimeters 2.6 Drum Technique for Rice 3. Determining effective rainfall from formulae 3.1 Renfro Equation 3.2 U.S. Bureau of Reclamation Method 3.3 Potential Evapotranspiration/Precipitation Ratio Method (India) 3.4 USDA, SCS Method 3.5 Empirical Relationships 3.5.1 Crops other than rice 3.5.2 Rice Measurement in rice 4. Evaluation of methods Chapter III. Application of effective rainfall data is i s irrigation and drainage projects
1. Irrigation project design 2. Irrigation project operation 3. Drainage projects 3.1 Drainage of Excess Water 3.2 Drainage for Leaching of Salts
4. Rice cultivation 5. The effect of groundwater 6. Effective rainfall in unirrigated and low rainfall, rainfall , areas Chapter IV. Increasing the proportion of effective rainfall its i ts effectiveness effectiveness and further lines of work
1. Increasing effective rainfall 1.1 Reducing Surface Run-off 1.2 Increasing Infiltration (except in rice culture) 1.3 Building Water Storage Structures 1.4 Minimizing Peep Percolation Losses 2. Increasing the effectiveness of rainfall 3. Further lines of work needed in the field of effective rainfall in agriculture 3.1 Collecting Data by Setting Up Crop Lysimeters 3.2 Verification of Empirical Methods 3.3 Development of Empirical Methods in Different Areas 3.4 Utilizing Information of Effective Rainfall in Agricultural Practic es 3.5 Increasing Effective Rainfall Under Field Conditions 3.6 Increasing the Effectiveness of Rainfall Bibliography
Foreword In view of the vital importance of water in man's life, precise knowledge of not only total rainfall but also of the utilisable or effective part of rainfall is essential for the economic and efficient planning of water resources. There is abundant literature on the subject of total rainfall. The topic of effective rainfall has remained somewhat neglected, because of the several disciplines involved. Literature on effective rainfall in general and on its role in agriculture in particular is limited. This paper on concepts, assessment and the application of effective rainfall in irrigated agriculture has been prepared at the recommendation of the Ad Hoc Consultation Meetings on Crop Water Requirements held under the auspices of the Land and Water Development Division, FAO, Rome. Dr. N.G. Dastane, Project Co-ordinator, Indian Agricultural Research Instit ute, New Delhi, India, very willingly accepted the task of preparing this paper as FAO Consultant. Helpful suggestions Mere received from Panel Members: Messrs. Mes srs. A. Aboukhaled, Lebanon; G. van den Berg and P.E. Ritjema of the Netherlands; J. Damagnez, Prance; O.N. Ashford, WMO, Geneva; W.O. Pruitt, U.S.A.; and C.E. Houston and J. Doorenbos, FAO, Rome. This publication presents a number of concepts of effective rainfall currently in use. A distinction is made between effective rainfall and the effectiveness of rainfall being two different entities. The definition of effective rainfall has been examined and a . modified concept of effective rainfall is proposed for use in irrigated agriculture. A brief history as well as the criteria for the assessment and factors influencing effective rainfall are presented. In addition, methods of measuring effective rainfall and its different components, as well as merits and limitations of each of these methods are discussed. Also the application of derived data on effective rainfall, together with several calculations and reference are given for guidance in developing and using the information in planning as wall as in practice of water management. Finally, a number of of practices to increase effective rainfall are listed. I am pleased to have this opportunity of expressing to Dr. Dastane and the members of the FAO Consultative Group on Crop Water Requirements my sincere appreciation for the time and effort they have devoted to the preparation of this valuable publication. Edouard Saouma Director Land and Water Development Division
1. Effective rainfall and its significance The primary source of water for agricultural production for most of the world is rainfall. Three main characteristics of rainfall are its amount, frequency and intensity, the values of which var y from place to place, day to day, month to month month and also year to year. Precise knowledge of these three main characteristics is essential for planning its full utilization. Information of the amount, intensity and distribution of monthly or annual rainfall for the most important places in the world is generally available. Long-term records of daily rainfall have been compiled for years; norms and standard deviations have been worked out; floods and droughts have been defined and climatic zones of potential evapotranspiration less precipitation have been mapped from rainfall patterns and crop studies. Investigations using electronic computers are continuously in progress and efforts are being made to predict future trends in order to refine planning. planning. In spite of voluminous data on weather, all is not yet known that should be known about rainfall. Certain simple entities have baffled planners pla nners right up to the present. One of these is "effective rainfall". In its simplest sense, effective rainfall means useful or utilizable rainfall. Rainfall is not necessarily useful or desirable at the time, rate or amount in which it is received. Some of it may be unavoidably wasted while some may even be destructive. Just as total rainfall varies, so does the amount of effective rainfall. The importance of precise knowledge on the subject of effective rainfall needs little elaboration. The useful portion of rainfall is stored and supplied to the user; the unwanted part needs to be conveyed or removed speedily. Most rain water is used in i n agriculture for crop production. Therefore, the first point which arises is whether the available rainfall is adequate and well distributed for crop-raining. If it is inadequate, can it be supplemented with timely irrigation? If not, what type of agriculture should be practised? If irrigation can be supplied, bow should it be designed, operated and maintained? What are the water requirements of crops during the growing season as well as during different periods of growth and development and how far are these needs met by rainfall? How can excess rain water be disposed of and how can waste be reduced by changing management practices? Finally, what would all this cost? Without the necessary information on these basic points, no irrigation project can be planned and productively and economically executed. The greater greater the precision of long-term data on rainfall patterns and the greater the care taken in interpreting them, the higher will be the efficiency of water management projects. Pharande and Dastane (1964) listed salient points in t he practical application in the t he field of agriculture of data on effective rainfall as follows: - designing irrigation projects on a sound economic basis; - fixing cropping patterns and working out the irrigation requirements of crops; - operating irrigation projects efficiently from year to year; - preparing schedules of other farm operations in irrigated agriculture; - planning cropping patterns in unirrigated or rainfed areas; - designing drainage and land reclamation projects; - planning soil and water conservation programmes; - interpreting field experiments accurately; - classifying regions climatologically for agriculture. Meteorologists can neither solve nor evaluate the problem of effective rainfall merely from tables of frequency, amount and intensity of rainfall or from physical phenomena in the atmosphere. It is a task in which several disciplines and sub-disciplines sub-disci plines overlap. For example, in the field of agriculture,
soil types, cropping patterns and social, economic and management factors all have a direct impact on the extent of effective and ineffective rainfall. Because of such complexities, there is confusion in concepts, definitions and measurements and their interpretation. The nomenclature and methods of measurement need to be standardised as well as interpreted for a better understanding of effective rainfall and to convert total rainfall into effective rainfall to the maximum possible extent. In this paper on effective rainfall, present concepts are reviewed and defined, present methods of its assessment are examined, estimating esti mating procedures for evaluation in applied fields, especiall y that of irrigated agriculture, are suggested and future li nes of work on this subject are pinpointed.
2. The pathway of rain water The pathway of rain water is shown in Fig 1 which als o indicates the different factors which need to to be considered. Before rain strikes the earth's surface, there is some evaporation in the atmosphere (A in Fig 1). This amount is never measured, but it increases air a ir humidity, lowers the temperature and so reduces evapotranspiration from field crops. Vapour may also be blown away by winds to the surrounding area. This A fraction is thus t hus partially useful bat is not taken into consideration in a ny calculation. Rain may be intercepted by vegetation (B in Fig 1). Some of it Ba y be absorbed and retained by leaves; this is lost as evaporation (B1), Some may drip from the leaves onto the soil s urface (B2), The rate of vegetal interception is high in the beginning and declines with time. B1 and B2, which are measured in the total rainfall r ainfall received, are useful for crop cr op growth since they reduce transpiration and depletion of soil moisture. When showers are light the entir e rainfall may be intercepted by vegetation. In sway studies light showers have been treated incorrectly as ineffective rainfall. On striking the soil surface (C), some water infiltrates the soil (C2), ( C2), some may stagnate on the surface (C3), while some may flow over the surface as run-off (C1). Factors influencing infiltration and surface run-off are many and are interrelated. interr elated. Rain water lost by run-off may be pumped back and re-used at the site where it was received (C1.1), or it may be used elsewhere downstream (C1.2.1). In overall water development pla nning this latter amount of surface run-off still remains part of effective rainfall. Some rain water stagnates on the soil surface (C3). This amount is lost by b y evaporation in due course or by infiltration. It can be useful for meeting meeti ng the water needs of crops but may be harmful and create drainage problems. Of the water which infiltrates into the soil (C2) some may be retained around the soil particles as a thin film and is thus stored in the root zone z one (C2.1) while the rest may move beyond the root zone (C2.2). Of the stored soil water (C2.1), part may be ineffective when received during the nongrowing season. Also during the growing season rainfall can cause harm such as f lower and fruit drop, delay in harvesting and lowering of the quality of the produce. This useless or harmful water (C2.1.1) is ineffective rainfall. A part of the water stored in the root zone may be actuall y utilized in raising a crop. It may be useful for evapotranspira tion or in any other form (C2.1.2). A part of the stored water may remain unused due to the harvest of t he crop (C2.1.3) as moisture balance for the next season. Water may be lost beyond the root zone by deep percolation to groundwater storage or streams (C2.2). A certain fraction of loss by deep percolation is useful, even essential, in arid and semi-arid
regions for washing down the salts (C2.2.1). This portion is therefor also useful. If there is no salinity problem, all water lost by deep percolation beyond the the root zone may be useful only through through recharge of the underground aquifer. Fig 1: PATHWAY OF RAIN WATER
3. Concepts of effective rainfall The term effective rainfall has been interpreted differently not only by specialists in different fields but also by different workers in the same field. The city civil engineer is interested intereste d in providing a drinking water supply from his storage tank or lake. According to him, that amount of the total rainfall which enters his reservoir is the effective part (C1.2.2 in Fig 1). To an irrigation engineer the rain which reaches his storage reservoir directly and by surface run-off from the surrounding area indirectly is the effective portion (also C1.2.2 in Fig 1); but even though the concept is the same in both this and the former case, the values of effective rainfall are different for the same total rainfall. According to a hydro-electrical engineer, the rainfall which is useful for running the turbines that generate electricity is the effective portion of the total received. This comes under C1.2.1 in Fig. 1. Geohydrologists would define as effective that portion of rainfall which contributes to groundwater storage. The extent of the rise in the water table or well levels would be the effective rainfall. This quantity comes under C1.2.2 and C2.2 in Fig 1. To drainage engineers rainfall lost either by surface run-off or by deep percolation is of great significance. According to some planners, the surface run-off from a field nay be used at a higher or lower level (the former by pumping) before the water leaves the catchment area. Such water may be useful for agriculture or industry, navigation or recreation. All of it is effective in an integrated plan for utilization of water resources. It is equivalent to B, C1.1, G1.2.1, C2.1.2, G2.2.1, C3. In forests, dry leaves and twigs often form f orm a thick layer on the soil surface, called the Ao horizon. This litter intercepts a considerable amount of rain and enhances the water storage c apacity of the soil. It is a contributing factor in evapotranspiration and is therefore effective rainfall from the forestry point of view. In the field of irrigated agriculture, interests differ at the irrigation project level, at the farm level and at the field level. Some canal operating operati ng engineers feel that rain which saves an irrigation application is effective, although the rain may ma y not necessarily be effective: light showers of, s ay, less than 10 mm often make no difference in irrigation schedules and hence are treated as ineffective by canal engineers. Agriculturists may consider as effective that portion of the total rainfall which directly satisfies crop water needs and also the surface run-off which can be used for crop production on their farms by being pumped from ponds ponds or wells. This fraction is equivalent to B, C1.1, C2.1.2 C2.1.2 and C3. In the field of dry-land agriculture, when the land is left fallow, effective rainfall r ainfall is that which can be conserved for the following crop.
An individual farmer considers that effective rainfall r ainfall is that t hat quantity which is useful in raising crops planted on his soil, under his management. Water which moves out of the field by run-off or by deep deep percolation beyond the root zone of his crop is ineffective. But on the other hand, if he receives runoff from outside high level surroundings then it may add to the moisture stock and it nay be useful for crop production. Consequently, farmers with different crops will arrive at different values in assessing effective rainfall. rainfa ll. This is a dynamic rather than a static notion. B, C2.1.2 and C3 come into this category. Since there are such varied interpretations of what may be regarded as effective rainfall, it is difficult to create a definition to suit all the interested disciplines. Rainfall which is ineffective according to one discipline is effective according ac cording to another. In his calculations a planner interprets the term broadly as that which is utilizable by any sector while the individual user user interprets it narrowly to mean only that fraction which satisfies his particular need. Even in the single field of the water requirements of crops different workers may mean different entities. Hayes and Buell (1955) stated that effective precipitation is that quantity which is available for pl ant growth and amounts to total precipitation minus run-off and evaporation. This definition is not completely satisfactory since aspects prior to sowing are neglected and the term 'evaporation' is ambiguous and confusing here. Ogrosky and Mockus (1964) defined effective rainfall as the total rainfall during the growing season minus that occurring after soil saturation or irrigation, when the additional water is lost by deep percolation or by run-off. Water is needed even before sowing sowing and for land preparation; this definition overlooks this aspect. Also, rainfall prior to harvesting can be ineffective even if the soil is unsaturated. The definition cannot be applied for rice either where rainfall, after the soil is saturated, can also be effective. Hershfield (1964) defined effective rainfall as that part of the total rainfall during the growing season which is available to meet the consumptive water requirements of a crop. This is a narrow definition in that water is required to meet not only consumptive but also non-consumptive needs such as land preparation, puddling and leaching of salts. According to the Soil Conservation Service of U.S.D.A. (1967), effective rainfall is that which is received during the growing period of a crop and is available to meet c onsumptive water requirements. It does not include surface run-off or deep percolation losses. This definition is similar sim ilar to that of Hershfield above and suffers from f rom the same limitations. Miller and Thompson (1970) defined effective precipitation as the ratio of precipitation to evaporation at a given place. This definition is confusing as actually it refers to t o effectiveness of precipitation and not to effective precipitation. The two terms 'effective rainfall' and 'effectiveness of rainfall' are not synonyms s ynonyms but signify two distinct things. The term effectiveness denotes degree of utility or efficiency of the rainfall with respect to the aridity of the place. Effective rainfall refers to the useful fraction of the total rainfall received. Thornthwaite (193l) has discussed the concept of precipitation effectiveness at length, giving formulae for classifying world climates. A given amount of rainfall will have varying degrees of effectiveness under different conditions of aridity or even when it is received at different stages of crop growth. While working out agronomic practices for seasonal field crops, De and Ray (1973) used a moisture index based on two factors, namely, annual rainfall and potential evapotranspiration. This is an erroneous approach since instead of annual rainfall, effective seasonal or effective growing season rainfall should have been considered.
It is evident that the above concepts are not identical in interpretation and have serious limi tations from the point of view of crop production. The points in concepts of effective rainfall needing critical attention can be listed as follows: what period should be taken into consideration? Should it be restricted to the growing season or include the pre-sowing period, when tillage operations or moisture conservation practices commence? which needs should be met by effective rainfall; c onsumptive needs only or also those of leaching, land preparation and puddling? assuming the soil is unsaturated and the rains which fall cause flower shedding and damage to a cr op, should they be considered as effective? should the amount be usable in general by any type of vegetation or by a specific crop species? when several crops of different growth habits are being cultivated simultaneously, how should rainfall received under rainfed and irrigated conditions be accounted for? can the concept be applied to rice which requires ample water supply? suppl y? should values be the same for different purposes such as project design and project operation? how should annual variation in the amount of rainfall be accounted for?
4. Definition of effective rainfall It would be useful to clarify the term water requirements of a crop before effective rainfall is fully defined. By the tern water requirement of a crop is meant the amount of water needed to raise it and this includes water to meet both consumptive and special needs, such as land preparation, land submergence, leaching and so on. In view of this current concept, it follows that from the production point of view, the annual or seasonal effective rainfall as far as the water requirement of crops is concerned, should be interpreted as that portion of total annual or seas onal rainfall which is useful directly and/or indirectly for crop production at the site where it falls but without pumping. It therefore includes water intercepted by b y living or dry vegetation (B), that lost by evaporation from the soil surface (C3), the precipitation lost l ost by evapotranspiration during growth (C2.1.2), that fraction which contributes to leaching, percolation (C2.2.1) or facilitates other cultural operations either before or after sowing without any harm to yield yield and quality of the principal crops. Effective Rainfall, (ER) = B + C2.1.2 + C2.2.1 + C3. Consequently ineffective rainfall is that portion which is lost by surface run-off (C1), unnecessar y deep percolation losses (C2.2.2), the moisture remaining in the soil after the harvest of the crop (C2.1.3) and which is not useful for next season's crop. The effect of rainfall in lowering temperature and increasing humidity and its effect b y advection on dry areas is not included. The period here is not just the growing season but the period from the st art of the first tillage til lage operation until the harvest. It avoids the term soil saturation and therefore may also be applied to rice cultivation. As long as the rain water is useful in some aspect of crop production, it is a part of the effective rainfall. It is not possible to consider each individual crop. Those crops which occupy the la rgest proportion of land during the season may be taken as referenc e to express the effective rainfall. The smaller the unit of land area, the greater the accuracy in the value of effective rainfall. The destructive aspect of rainfall has also been accounted for in the definition by the cla use 'without any harmful effect on yield and quality'. If rain c auses lodging or any other type of damage, it must
not be regarded as effective even if the soil is dry. The rainfall has to be 'useful' in some s ome way or another and should not cause the slightest damage to crop production. This concept of effective rainfall is suggested for use in planning and operation of irrigation projects. It can also be used by economists in estimating agricultural production from meteorological data. The irrigation water supply in a given year should be planned to complement rainfall. Since annual rainfall varies from year to year, an irrigation project cannot be planned on one year's data; r ecords are needed over a long period to calculate effective rainfall on the basis of probability of occurrence. It is emphasized that the values of effective rainfall will vary for different purposes, such as project planning and project operations, dry fanning, drainage designs, and for special conditions such as a shallow water-table and salinity. salinit y. The concept that effective rainfall is that t hat which is useful or usable in any phase of crop production should be borne clearly in mind.
5. Effectiveness of rainfall The effectiveness of precipitation or rainfall efficiency has been defined as the extent of its utility in a given region. How good is a given amount of rainfall in terms of production? A rainfall of 100 mm in a temperate zone is much m uch more useful and productive for agriculture than in a tropical a rid region. The effectiveness of rainfall can be evaluated in three ways: in relation to other meteorological variables; as an input, in terms of meeting crop water needs as an output in terms of production per unit of water and/or financial returns. In assessing the effectiveness of rainfall, relationships between the climate and vegetation have been worked out in terms of moisture indices. Transeau (1905) used a quotient of annual rainfall divided by computed free water evaporation to give the moisture index. Lang (1920) introduced a factor in which rainfall was divided by temperature. De Martonne (1926) introduced the term aridity index (A) which was equal to precipitation (P in cm) divided by temperature (t) in degrees centigrade, or A - P/10 (t + 10), Meyer (1926) put forth that the effective soil moisture was proportional to the precipitation (mm) divided by saturation deficit (mm of mercury) of the air. Thornthwaite (1931) introduced the concept of the precipitation effectiveness index (PE) which is computed from the monthly values of precipitation and evaporation. The evaporation is expressed in terms of temperature.
where, P = monthly precipitation in inches; T = temperature in °F; and n = months = 12 On these values climatic regions were classified as follows:
PE Index
Climate
More than 128
Wet
64 - 127
Humid
32 - 63
Sub-humid
16 - 31
Semi-arid
Less than 16
Arid
In 1948, Thornthwaite modified the concept by changing the evaporation parameter to potential evapotranspiration which was derived from temperature. The potential evapotranspiration (PET) was defined as the amount of water lost by b y a field, completely covered with green vegetation in an a ctive stage of growth under non-limiting non-limiti ng moisture supply. PET -= 1.6 (10 t/I) a PET = monthly potential evapotranspiration in cm; t = mean monthly temperature in C; I = annual heat index, which is i = monthly heat index, which is (t/5) 1.514 a = co-efficient which varies with the heat index and is given by a = 0,000000675 I³ - 0.0000771 I² + 0.01792 I + 0.49239 The values are then corrected for day length and number of days in a month. The use of tables and charts simplifies complicated calculations. This concept was used in a water budget method for working out daily soil moisture balance for planning irrigation practices. It can be seen that the approach does not meet the requirements from an agricultural production point of view. Rainfall should satisfy not only the consumptive needs of crops but also leaching, land preparation needs, percolation needs (as in rice) etc. It is therefore more appropriate and precise to use the total water needs of the crop in computing effectiveness of rainfall, instead of evaporation or potential evapotranspiration values. In simple words, effectiveness of precipitation (needs satisfied expressed in %) = [(available rain water) / (needed water for potential production)] X 100 For example, if the values of monthly rainfall, stored rainfall, PET and total water needs are 100, 60, 120 and 180 mm respectively, the effectiveness of precipitati on will be (60/180 x 100) = 33% and not (60/120 x 100) = 50% Different amounts of effective rainfall influence their effectiveness when expressed from meteorological and agronomic points of view as is shown below. Parameters
Cases
Parameters
1 2 3
4
5
6
Monthly rainfall (A)
0 10 10 10
10
10
rainfall (B)
0 1 3
6
8
10
Potential ET (C)
6 6 6
6
6
6
Water need (D)
8 8 8
8
8
8
Stored/effective
Effective rainfall % (B/A) 0 10 30 60
80
100
Effectiveness of rainfall (B/C) meteorologically % 0 16 50 100 Excess Excess Excess (B/D) agronomically %
0 12 37 75
100 Excess
Effectiveness of rainfall can also be assessed in terns of monetary returns. This is a rather complicated analysis. The returns depend upon the type of production, use of other inputs such as fertilizers, plant protection measures, soil fertility, weather parameters other than rainfall, infrastructure developed in the area, farm management efficiency and market prices. Ml these factors are again variable with time and technology applied and therefore assessing effectiveness of rainfall in terms of financial returns in a given situation requires careful analysis of all factors involved. Values of effectiveness of rainfall vary from plus to minus according to its timing with respect to crop growth stage. Untimely rains can directly damage field crops in several ways: by delaying sowing, interculturing, threshing or drying operations in the field; by causing flower drop and fruit drop drop in certain cultivated species; by causing lodging of of plants; by lowering the quality of the produce. produce. An example is given in Table 1. Table 1: INFLUENCE OF RAINFALL ON WHEAT IN INDIA AT DIFFERENT STAGES OF GROWTH Time of shower from sowing
Crop stage
Yield (Quintals/ha) Absolute Relative
No rain
-
8
100
3 weeks
Crown root formation
14
175
6 weeks
Tillering
12
150
11 weeks
Flowering
10
125
15 weeks
Grain formation
9
112
The effectiveness of rainfall varies with different species, and therefore for comparison among different crops, it may be convenient here to express expres s production in terms of financial returns per unit depth of rainfall. Crop species is thus a factor of paramount importance in assessing the effectiveness of rainfall. This evaluation needs to be done cautiously. Since the cropping patterns and varieties vary with time, the effectiveness of rainfall will also vary from time to time.
6. Factors influencing effective rainfall Several factors influence the proportion of effective effectiv e rainfall in the total received rece ived and these may act singly or collectively and interact with each other. Any factor which affects infiltration, run-off run -off or evapotranspiration affects the value of effective rainfall (Table 2). Rainfall Characteristics
A soil has a definite and limited water intake rate and moisture holding capacity. Hence greater quantities as well as Intensities of rainfall normally reduce the effective fraction, increasing run-off and lessening infiltration. Similarly, uneven distribution decreases the extent of effective rainfall while an even spread enhances it. A well distributed dist ributed rainfall in frequent light showers is more conducive to crop growth than heavy downpours. For example, annual rainfall is lower than 100 mm in the Middle Eastern desert countries, so it i t may all become effective. In countries like India and Pakistan, intensity, frequency and amount are high daring July and August and hence the effecti ve fraction is very low. From November to April, however, most of the rainfall is effective in these countries due to its low intensity, frequency and amount. Table 2: FACTORS INFLUENCING EFFECTIVE RAINFALL THROUGH INFILTRATION, INFILTRATION, SURFACE RUN-OFF AND EVAPOTRANSPIRATION EVAPOTRANSPIRATION Factor
Relevant characteristics characteristics
Rainfall
Amount, intensity, frequency, distribution over area as well as time;
Other meteorological parameters
Temperature, radiation, relative humidity, wind velocity;
Land
Topography, slope, type of use;
Soil
Depth, texture, structure, bulk density, salt and organic matter content;
Soil water
Head, suspended matter, turbidity due to clay or colloids, viscosity, temperature, nature of dissolved salts (Na+, N03-);
Groundwater
Depth from surface, quality;
Management
Type of tillage, degree of levelling, type of layout (bunding, terracing, ridging), use of soil conditioners;
Channel
Size, slope, shape, roughness and back water effect;
Crops
Nature of crops, depth of root system, degree of ground cover, stage of growth, crop rotations,
Other Meteorological Parameters Potential evapotranspiration is primarily governed by evaporative demand under conditions of abundant water supply. An approximation of evaporative demand can be obtained front the integrated effect of four parameters: temperature, radiation, wind velocity and humidity. Increase in the first three and decrease decreas e in the fourth parameter enhance evaporation. Such conditions encourage greater deficits of moisture in the soil and therefore the proportion of effective rainfall in the total increases. The mean monthly values of temperature, radiation, wind velocit y and humidity fluctuate less from year to year than tha n total rainfall. Today in some countries, weekly or fortnightl y maps of potential evapotranspiration are available which can be used in assessing effective rainfall. Land Characteristics The time interval between receipt of rain water and its recessation recessat ion by soakage is known as 'opportunity time'. Water stays longer on flat and levelled land and thus has a longer opportunity time than on sloping land where there is a rapid run-off. Sloping, rolling and undulating lands thus influence opportunity time of rain water for uniform infiltration and consequently the eff ective rainfall fraction. The use to which land is put in the surrounding area - agriculture, road and building construction, play-grounds - also affects the amount of effective rainfall. Soil Characteristics
Soil is an important medium between water and plants, acting as a reservoir for the moisture supply s upply to crops. Hence its properties of absorption, retention, release and movement of water influence the degree of effective rainfall. Intake and water movement in soil are expressed in terms of infiltration rate and permeability. For maximum absorption of rain and reduction of surface run-off, the values of these properties should be as high as possible. Permeability depends upon the texture, structure and and compactness or bulk density of the soil. The higher the bulk density, the lower the permeability. The fraction of effective rainfall increases with increased water holding capacity in a soil. The amount of water held and retained by a soil depends upon its depth, texture, structure and organic matter content; the finer the texture, te xture, the greater the storage capacit y. The amount of water available to plants varies considerably in different soils. It may be about 10 mm per metre depth in sandy soils to about 100 mm in clayey soils. The greater the soil depth, the higher the proportion of effective rainfall in the total. Initial moisture status in a soil governs the extent of effective rainfall considerably. When a shower falls just after irrigation, ir rigation, it becomes surplus water and is lost through deep percolation or run-off, but if the soil is dry it is recharged with moisture, resulting in a saving in irrigation water. The proportion of effective rainfall is lower in irrigated than in unirrigated areas where there is often a greater deficit of moisture in the soil. Soil Water Characteristics On striking the soil surface, the rain water wate r often becomes run-off water and changes its it s physical and chemical properties during its flow. Hater characteristics influencing effective rainfall are the head or depth of water received directly or indirectly, turbidity, viscosity, temperature, and nature of the salts, such as sodium, nitrates, etcetera, dissolved in it. These properties influence infiltration and through this the effective rainfall quantum. Groundwater Characteristics The amount of effective rainfall is greater when the water table is deep than when it is shallow. Water moves upwards in the soil by capillarity, thus reducing the deficit of moisture and hence the amount of effective rainfall. The levels of water tables normally fluctuate. Before the onset of rain, the water table may be quite deep; during the rainy season, it may rise to the surface. There are horizontal flows in the sub-soil to or from adjoining regions. Because of these variati ons, the contribution of groundwater to the needs of the crop is variable and the proportion of effective rainfall varies inversely with this contribution. If the groundwater is saline, it can be harmful to crop plants especiall y when it is near the soil surface. The proportion of effective rainfall may then increase since salts are diluted. Management Practices Any management practice which influences run-off, infiltration, permeability or evapotranspiration also influences the degree of effective effecti ve rainfall. Bunding, terracing, ploughing, ridging and mulching reduce run-off and increase effective rainfall; so do well-planned irrigation schedules, while arbitrary or random practices may reduce it. Drainage Channel Characteristics
Size, shape, slope and roughness of a channel influence the speed of surface run-off to streams and consequently the time allowed for infiltration and also for direct evaporation at the site of rainfall. Hence these factors are also important in influencing effective rainfall. Crop Characteristics Crops with high water consumption create greater deficits of moisture in the soil; therefore effective rainfall is directly proportional to the rate of water uptake by the plant. Crop characteristics influencing the rate of water uptake are the degree of ground cover, rooting depth and stage of growth. Evapotranspiration is generally high during vegetative growth and the flowering period and then may decline toward maturity. Soil moisture stored in deeper layers can be tapped only when roots penetrate to these depths. Deep-rooted crops therefore increase the proportion of effective rainfall in a given area; hence the nature of the crop is an important factor in determining its extent. Rainfall just before harvesting is for most crops a waste or a nuisance and may need to be considered as ineffective. Rainfall which reduces the yield (such as downpours which often cause lodging in cereals when the latter are at the grain formation stage) must be regarded as ineffective, and similarly, rains which result in deterioration det erioration or actual destruction of a crop. The crop is an im portant factor in interpreting the basic data. Hence the seasonal needs of major crops in a given area should be taken into account when the extent of effective rainfall is assessed.
1. Components of effective rainfall and their measurement The evaluation of effective rainfall involves measuring rainfall and/or irrigation, losses toy surface run-off, percolation losses beyond the root zone and the soil moisture uptake by the crop for evapotranspiration. Information is needed on rooting depth of crop plants. Components are measured directly or indirectly and either individually or in an integrated wa y. 1.1 Rainfall and Irrigation
Total rainfall can be measured directly with rain gauges. Several types of recording and nonrecording gauges are available. The instruments and their use have been described by WMO (1970). Irrigation applications can be measured with various types of notches, weirs, Parshall flumes, outthroat flumes and with direct recording water meters. The methods have been compiled and described by Dastane (1972), 1.2 Surface Run-off
With efficient farm water management, surface run-off should be kept to a minimum. Only excess water should be removed by deep percolation and sub-surface drainage or by surfac e drainage. For a given field, run-off can be assessed with measuring meas uring flumes and water stage recorders or can be computed with formulae established for different conditions, as given in the Handbook on Hydrology by Chow (1964), (1964), and USDA Field Manual for Research in Agricultural Hydrology (1968). 1.3 Rooting Depth
For determining the water losses due to deep percolat ion and water uptake by a crop, the depth of rooting must first be known. The depth of root system varies from crop to cr op and also from time to time during growth. The root system is influenced by several factors; too much wetness results in development of a shallow root system while drier soil water regimes encourage a deep one; soil texture and structure influence the depth of the root system to a great extent. Studies on rooting depths should, therefore, be made under existing and recommended cultural and irrigation practices on representative soils. The two most common methods of measuring the rooting depth are by excavation method and by studying soil moisture extraction patterns. With the excavation method the roots are carefully carefull y dug from the soil, then cleaned, dried and weighed. The length and distribution in different layers is noted. The process is simple but laborious and careful handling is needed. But what is the effective root depth; is it the longest root or the mean root length? As a first approximation, the soil depth in which 90 percent (by weight) of the roots lie, can be taken as the effective root zone for irrigation purposes. However, several investigators have pointed out that the mass of roots and their activity do not have a simple linear relationship. There are generally more roots in the surface layer. Also, sometimes water extraction can be greater from the sub-soil than the surface soil. Water is extracted from a layer where it is readily available, provided there is a certain minimum root permeation in that layer. Soil moisture depletion depletion can be determined by periodical soil sampling or by means of devices such as gypsum blocks or neutron probes. For irrigation purposes, the soil depth over over which 80 percent of the total water intake takes place can toe regarded as the effective root zone of a crop. Since the root system is dynamic and continues to extend up to the flowering time, such studies are necessary at monthly or fortnightly intervals.
1.4 Deep Percolation Losses
Deep percolation losses can toe determined directl y by using lysimeters or indirectly by computation of soil characteristics including soil water content, soil moisture tension and soil permeability. With the direct method, a representative and undisturbed soil column is enclosed in a large container or a tank fitted with an outlet at the bottom from which excess water can be drained into a measuring cylinder; this is the so-called so-ca lled drainage lysimeter. The lysimeter needs to be surrounded by a large cropped area. The method has been widely used since all the components of water gains and water losses can be measured from which a water balance can be obtained. For details, reference is made to comprehensive studies by Anonymous (1967), Harrold (1968), (1968), Hillel et al (1969), Rose et al (1966), and McGuinness and Bordne (1972). For the indirect method, the moisture holding capacity of the soil is calculated layer by layer from the field capacity, wilting point, bulk density, soil depth and rooting depth. Any excess over the water holding capacity of the soil will normally be a deep percolation loss. The values of the first four parameters can be determined using established methodologies (Dastane, 1972). With a dry subsoil deep percolation losses take place under saturated soil conditions but will continue even after field capacity capacit y has been reached. In the Great Plains of the U.S.A., the soil water storage efficiency is sometimes lower than 20 percent on fallow lands left for water harvesting. Accurate measurement of the deep percolation component under both saturated and unsaturated soil conditions is rather laborious. l aborious. Rooting depth and root concentration must b e known. A method has been described by Hillel et al (1972). 1.5 Evapotranspiration
The level of evapotranspiration is controlled mainly by three factors, namely, plant characteristics, extent of ground cover and stage of growth; water availability in the soil; and meteorological parameters or the evaporative demand. Maximum or potential evapotranspiration (ETp) (ETp) occurs when the soil water is non-limiting and the crop is in an active stage of growth with full ground cover; the level of ETp for a given plant species is then mainly governed by the meteorological conditions. Actual evapotranspiration (ETa), which is also sometimes called consumptive water use, is the actual quantity of water lost during crop growth by evaporation from land surface and by transpiration by plants. The ETa may reach ETp level if conditions permit. It is more difficult to estimate ETa than ETp since several factors play interacting roles. The ETa can be determined directly by periodic soil sampling and oven-drying; changes in soil water by the growing crop are followed and la yerwise depletions are studied in the effective root zone of the crop. It is a very laborious method and is subject to sampling errors. The ETp can be computed from the meteorological parameters such as temperature, radiation, wind velocity, humidity. Several different formulae to compute ETp are available. Some formulae in computing ETp and also ETa are listed in Table 3 along with the parameter s considered. None of the formulae suits all situations perfectly. Some improved methods need elaborate sets of meteorological data which are not always available. Prohibitive costs of installation and operation of equipment, time involved in processing the data and the variety of crops grown make it impossible to use and apply these formulae in day to day agricultural operations. A simple method such as measurement of open pan evaporation has been proposed since it represents the effect of all meteorological elements in an integrated way. Since only one parameter is to be measured, it is relatively easy to use this technique in the field. The relationship between open
pan evaporation and ETp however, however, is intricate. Local coefficients relating pan evaporation to ETp should preferably be established locally. The value of potential evapotranspiration where there is a non-limiting water supply varies from 0.4 to 0.8 times the U.S, Class A Open Pan Evaporimeter under different conditions of pan specifications and its method of installation. It offers a good approximation for practical purposes. For details, refer to FAO Irrigation and Drainage paper no. 24 (1974). Table 3: FORMULAE TO PREDICT CROP EVAPOTRANSPIRATION Variables considered (+) Formu Tempe Air Dr Dryl Sunsh Radi Wi Evapor Cr Cr Soi Corre Precipi Baro Entit la by rature hum y- ight ine ation nd imeter op op l ction tation graph y idity we hou hours/ velo da fac fac factor meas t rs cloud city ta tor tor ured bu cover lb te m p 1
2
Rohwer , 1931, USA
+
+
BlaneyMorin, 1942, USA
+
+
LowryJohnso n, 1942, USA
+
Thornt hwaite, 1943, USA
+
Penma n, 1948, UK
+
BlaneyCriddle , 1950, USA
+
Halstea d, 1951, USA
+
3
4
5
6
7
8
9 10 11
+
+
+
+
+
+
+
+
+
+
+
13
14
+
+
+
+
12
ET crop ET crop
+
ET of valle y, entire grow ing seaso n
+
ET crop, adeq uate
+
Eo or ET crop
+
Cu crop
ET crop
Haude, 1952, Germa ny
+
Turo, 1954, France
+
TuroLangbe in, 1954,
+
+
+
+
+
+
+
+
ET crop
+
ET crop +
France
annu al river basin
Halkais . 1955, USA Thornt hwaiteMather, 1955
Eo or ET crop,
+
+
+
+
+
+
USA
CU crop +
ET crop and soil water balan ce
van Bavel, 1956, USA
+
+
Hargre aves, 1956, USA
+
+
Ivanov, 1957, USSR
+
+
Makkin k, 1957, Hollan d
+
Rijtom a, 1957. Hollan d
+
+
McIlro
+
+
+
+
+
+
+
ET crop
Eo or ET crop ET crop under adeq uate moist ure
+
+
ET grass
+
+
+
+
+
+
+
+
+
ET crop
+
ET
y, 1961, Austral ia
crop
Olivier, 1961. UK
+
JensenHaise, 1963, USA
+
Christia nson, 1966, USA
+
+
+
+
+
+
+
+
+
+
+
+
+
+
Dastan e, 1967, India Linacro , 1967, Austral ia
+
+
ET crop
+
Eo
+
+
+
+
+
+
+
Basic water needs for crop/ land unit
ET crop +
ET crop
ET = Evapotranspiration CU = Consumptive use of water Eo = Evaporation from U.S. Class A avaporimeter placed in a grass field.
In deriving relationship between ETa and soil water content, four main approaches can be distinguished, as shown in Fig 2: Fig 2: RELATION BETWEEN ACTUAL TO POTENTIAL EVAPOTRANSPIRATION AND SOIL MOISTURE CONTENT
1 a constant ET and sudden reduction at wilting point (WP); 2 a decreasing linear rate of ET with decrease in soil water content; 3 a linear cum exponential decreasing rate of ET with decrease in soil water content; 4 an exponential decreasing rate of ET with decrease in soil water content.
Each of the four curves given in Fig 2 is possible under a given situation. Considerable literature has accumulated on the subject during the last two decades. Refe rence is made to Slatyer Slat yer and McIlroy (1961), Linacre (1963), Konstantinov (1963), Hagan et a l (1967) and Gangopadhyaya et al (1968),
2. Empirical methods of determining effective rainfall
2.1 Soil Moisture Changes 2.2 Daily Soil Moisture Balance Method 2.3 Integrating Gauge 2.4 The Ramdas Method 2.5 Lysimeters 2.6 Drum Technique for Rice
There are several methods of assessing effective rainfall. Each method has certain merits and limitations. 2.1 Soil Moisture Changes
Water in the root zone may be measured by sampling and oven-drying the soil before and after every shower of rain. The increase in soil moisture, plus evapotranspiration loss (ETa) from the time the rain starts until the soil is sampled, is the amount of effective rainfall. After heavy rainfall evapotranspiration can be assumed to be at the potential rate during the short period from cessation of rainfall until the sampling time. This can be taken as 0.4 to 0.8 times the evaporation value of the Class A Pan as is given in FAO Irri gation and Drainage paper No. 24 (1974), or
ER = M2 - M1 + kp Eo
ER = effective rainfall Eo = U.S. Class A Open Pan evaporation value M 1 and M2 = moisture status in the effective root zone before and after rain, respectively kp = pan coefficient The method takes into account the soil and the crop characteristics. The determination is simple and accurate but it may involve errors due to soil variation; the sampling errors may range from 5 to 40 percent. The method is also laborious and time consuming. consuming. The use of neutron probes reduces the drudgery of periodic soil sampling, but these are costly methods for routine purposes and also subject to sampling errors. 2.2 Daily Soil Moisture Balance Method
A daily soil water balance is rather like a bank account. Rainfall and irrigation are on the credit side, while soil moisture depletion is on the debit side. Precise data on the maximum water holding capacity (field capacity) is necessary for this method. Any amount in excess of this capacity is a surplus and will be a deep percolation loss or run-off. When the balance reaches nil, no more withdrawal is possible and hence further depletion is treated as water deficiency. Rainfall and irrigation are directly measured while the t he evapotranspiration is computed from any of several available formulae. In irrigated agriculture, the soil water content is never allowed to fall below a certain value where water becomes a limiting factor in crop c rop production. When water is depleted to the lower limit of readily available moisture, irrigation is applied. Hence, computations may be based on potential evapotranspiration (ETp). In rainfed or partially irrigated areas, ar eas, where the soil moisture is depleted below the lower limit of readily available moisture, the computations are to be based on actual evapotranspiration. ETa can be estimated by using the Thornthwaite and Mather method (l955), that of Baier and Robertson (1966), or the relationships between ETa and ETp under decreasing soil water content as given by Tanner (1967). A sample calculation is given below. The water storage capacity of the soil has been assumed to be 100 mm; irrigation is applied at 50 percent of total depletion. All values are in mm. Of total rainfall of 625 mm, only 279 mm is effective in this case, which amounts to about 45 percent. Date Rainfall ETp Storage change in soil
Storage balance in Irrigation soil
Water surplus (Drainage)
1
100
6
94
94
-
0
2
25
8
17
100
-
11
3
0
9
-9
91
-
0
4
0
9
-9
82
-
0
5
0
8
-8
74
-
0
6
0
9
-9
65
-
0
7
100
5
95
100
8
100
4
96
100
60 -
96
9
50
8
42
100
-
42
10
0
10
-10
90
-
0
11
0
11
-11
79
-
0
12
0
11
-11
68
13
0
12
-12
56
-
0
14
0
12
-12
44
50
0
15
0
11
-11
63
16
0
11
-11
72
17
0
10
-10
62
18
0
11
-11
51
-
0
19
0
10
-10
41
50
0
20
0
10
-10
81
-
0
0
0 -
0 0
... Sample calculation continued Date Rainfall ETp Storage change in soil
Storage balance in Irrigation soil
Water surplus (Drainage)
21
0
11
-11
70
-
0
22
0
12
-12
58
-
0
23
0
12
-12
46
-
0
24
50
8
42
-
88
0
25
100
6
94
100
-
82
26
100
5
59
100
27
0
10
-10
90
-
0
28
0
10
-10
80
-
0
29
0
10
-10
70
-
0
30
0
10
-10
60
-
0
Total
625
279
188
386
95
2.3 Integrating Gauge
The method is shown in Fig 3. It consists of a rainfall receiver (R) which is connected to a water reservoir (WR), and which in turn is connected to an evaporating surface (E) representing a crop. The reservoir is provided with an overflow outlet at the top of the side wall (D). The capacit y of the reservoir is adjusted to the maximum water holding capacity of the soil in question. The rain water above this maximum capacity flows out and is measured as ineffective rainfall. The evaporating surface loses moisture continuously, creating a fall in the water level in the reservoir, reser voir, which is graduated so that the moisture balance can be read directly at any time. The device is simple, practical and useful and can easily be set up in the field. The evaporating surface represents the crop and therefore its size and the porosity of the material are important. The method is described by Stanhill (1958). Fig 3: INTEGRATING GAUGE FOR MEASURING EFFECTIVE RAINFALL
2.4 The Ramdas Method
Ramdas (1960) suggested a direct field method using a small portable device containing soil of the field, so eliminating the necessity of sampling. Fig 4: RAMDAS APPARATUS FOR MEASURING EFFECTIVE RAINFALL
The apparatus, as shown in Fig 4, consists of a c ylinder (CD) of about 30 cm in diameter, with a perforated base (BO) and a funnel (F) leading into a receiver bottle (H). All these parts are enclosed in an outer cylinder (MN). The cylinder (CD) is filled with a representative soil with the same density as that of the field. fiel d. The height is equal to the depth of the effective root zone of the crop. The apparatus is installed in the field fie ld crop where the effective rainfall is to be measured. The crop in the container is irrigated along with the field fiel d crop. Excess rain or irrigation water drains i n the receiver bottle H and is measured from time to time. The total rainfall minus the ineffective rainfall gives the value of effective rainfall. It is assumed that there is no surface run-off. Cylinders of different lengths are used consistent with the rooting depth of the different crops. With a suitable number of replications, the method is very useful. It is simple s imple and practical, and furnishes direct readings. 2.5 Lysimeters
Lysimetry is a method which provides complete information on all the components of water balance. Lysimeters can be used not only for measuring evapotranspiration but also for checking empirical
formulae for computing ET. The method is similar to the Ramdas method, but is more elaborate, refined and gives a higher accuracy. A lysimeter is a large lar ge container with soil in which crops are grown; water l osses and gains can be measured. The container is fitted with suitable inlets for irrigation and outlets for drainage. The lysimeters are buried in the field and are surrounded by the same crop as is grown inside. The size of lysimeter varies from small oil drums (Gilbert and van Bavel, 1954) to large size and deep lysimeters (Harrold and Dreilbelbis, 1958, l967 Pruitt and Angus, 1960, McIlroy and Angus, 1963). They can be either the non-weighing or weighing type. In non-weighing lysimeters, changes in water balance are meas ured volumetrically weekly or biweekly. No accurate daily estimates can be obtained. A simple design is shown in Pig 5. Irrigation water is applied to the lysimeter, A layer of pebbles is placed at the bottom to facilitate easy drainage. Excess water is collected from below at a suitable distance. A number of crops can be grown in a concentric pattern around a central drai nage chamber. A simple lysimeter can be built at low cost from a petrol drum. A tube with a small diameter is placed through the soil to the layer of pebbles. Excess water is removed at frequent intervals by using a thin metal tube open at the bottom which is connected to a receiver bottle in which suction can be applied using a reversed handpump. This is shown in Pig 6. Weighing lysimeters can provide precise information on soil moisture changes for daily or even hourly periods. The lysimeter is placed inside another tank which is in contact with the surrounding soil. The inside container is free for fo r weighing by scales. Also, the lysimeter tank can be floated in water; a suitable heavy liquid (ZnCl 2) is used whereby the change in liquid displacement is a measure for the water gain or loss l oss to or from the lysimeter tank. Apart from the t he high cost, the major problems with lysimeters are the restricted root growth, the disturbed soil structure in the lysimeter causing changes in water movement and possibly the tank temperature re gime, resulting in condensation of water on the walls of the container. Harrold and Dreilbelbis (1967) estimated that errors due to dew formation were in the order of 250 mm per annum. Other limitations include the 'bouquet effect' whereby the canopy of the plants grown in the lysimeter is above and extends over the surrounding crop, resulting in a higher evapotranspiration rate. In spite of these limitations, it is the best technique for precise studies on evapotranspiration. Fig 5: DRAINAGE LYSIMETER
Fig 6: SUCTION TYPE DRAINAGE LYSIMETER
2.6 Drum Technique for Rice
Dastane et al (1966) used a container or drum technique for assessing evapotranspiration, percolation, water requirements and also ineffective rainfall of a rice crop, (Fig 7). Three containers (drums) A, B, and C, of about 40 gallons capacity, 50 cm in diameter and 125 cm high) are embedded in a rice field leaving about a quarter of their height above ground level. The bottoms of containers B and C have been removed. To container C, outlet pipes are fitted at 0,5 cm intervals or a sliding strip is fitted for precise water control. Fig 7: CONTAINER TECHNIQUE FOR DETERMINATION OF EFFECTIVE RAINFALL IN RICE
The outlet pipes can he connected to a water receiver. The containers are filled fill ed with soil and rice is grown inside, along with the adjoining field crop. Water levels in the drums a re maintained at the same level as outside. The difference in the values on two successive days caused by the dail y loss of water in container A, represents evapotranspiration, while in container B, it indicates daily total needs of water. The daily difference between water levels in containers A and B is percolation loss. Container C is intended to assess ineffective rainfall. The maximum depth of submergence is governed by the height of the rice crop and height of the field bunds, whichever is less. Any rainfall
which submerges the crop beyond a certain critical height or which exceeds the height of the bunds is ineffective. As the height of the crop increases, the outlets are plugged or the sliding strip is pushed progressively upwards till the bund height becomes the limiting factor. The water level is set at a selected height in container C. This height can be adjusted with increase in growth of plants. Evapotranspiration and percolation continue and create a deficit eve ry day. When rain falls, it first makes up this deficit. When it becomes excessive, the surplus flows out through the outlet pipes. This is the ineffective rainfall. The difference between water levels in containers B and C is ineffective rainfall. If there are no rains, the water level in container C will gradually reach the soil surface and the crop will be irrigated according to routine practice. The technique is simple, inexpensive, easy and practicable. A typical example is given below using a daily balance sheet. Date
Irrigation Daily Drum Evapotranspiration ETA plus Percolation Ineffective rain A B C ETA percolation rainfall fall
1
2
3
4
6
7
8
9
10
Previous day
0
0
75 75 75
0
0
0
0
1
0
20
90 80 75
5
15
10
5
2
0
40
107 97 75
8
18
10
22
3
0
100 170 160 75
5
15
10
85
4
0
15
85 75 75
5
15
10
0
5
0
0
67 75 57
8
18
10
0
6
0
0
48 38 38
9
19
10
0
7
0
20
51 41 41
7
17
10
0
8
0
60
95 85 75
6
16
9
0
70
140 130 75
5
15
10
55
10
0
0
67 57 57
8
18
10
0
11
0
0
49 39 39
8
18
10
0
12
0
0
32 22 22
7
17
10
0
13
0
0
17
7
5
15
10
0
14
75
0
75 65 65
7
17
10
0
15
0
30
90 80 75
5
15
10
5
Total
75
355
98
248
150
182
-
5
7
-
-
10
Permissible water depth = 75 mm Column 2: from water meter reading Column 3: from rain gauge records Columns 4 to be adjusted daily to column 6 reading and 5: and to be observed next day Column 6: reading from daily observations Column 7: previous day's column 6 plus column 3 minus column 4 Column 8: previous day's column 6 plus column 3 minus column 5 Column 9: column 8 minus column 7 Column 10: column 5 minus column 6
3. Determining effective rainfall from formulae
3.1 Renfro Equation 3.2 U.S. Bureau of Reclamation Method 3.3 Potential Evapotranspiration/Precipitation Ratio Method (India) 3.4 USDA, SCS Method 3.5 Empirical Relationships
A number of empirically determined formulae can be used. They have been developed under a given set of conditions which may be very different from those under which they are to be applied. Their use elsewhere therefore remains doubtful. 3.1 Renfro Equation
Renfro, as quoted by Chow (1964), suggested the following equation for estimating effective rainfall: ER = E Rg + A ER = effective rainfall Rg = growing season rainfall A = average irrigation application E = ratio of consumptive use of water (CU) to rainfall during the growing season (Table 4)
The E value implies degree of rain likely to be utilized in meeting consumptive water needs. The greater the E value, the higher the value of effective rainfall. For example, if rainfall during the four month growing season is 400 mm, consumptive use of water is 700 mm, and average irrigation application is 100 mm, then the effective rainfall is i s equal to 0.60 x 400 + 100 - 340 mm. The method is empirical and may not suit many ma ny situations. Table 4: RATIO E FOR USE IN ESTIMATING EFFECTIVE RAINFALL IN RENFRO EQUATION (Chow, 1964) CU/R g E CU/R g E
0
0
2.4
0.72
0.2
0.10
2.6
0.75
0.4
0.19
2.8
0.77
0.6
0.27
3.0
0.80
0.8
0.35
3.5
0.84
1.0
0.41
4.0
0.88
1.2
0.47
4.5
0.91
1.4
0.52
5.0
0.93
1.6
0.57
6.0
0.96
1.8
0.61
7.0
0.98
2.0
0.65
9.0
0.99
2.2
0.69
/
I
3.2 U.S. Bureau of Reclamation Method
A method described by Stamm (1967), is recommended for arid and semi-arid regions and uses mean seasonal precipitation of the five driest consecutive years. Percentage marks are given to increments
of monthly rainfall ranging from greater than 90 percent for the first 25 mm (1 in) or fraction thereof, to 0 percent for precipitation increments above some 150 mm (6 in), as is shown in Table 5. Table 5: EFFECTIVE PRECIPITATION BASED ON INCREMENTS OF MONTHLY RAINFALL (U.S. BUREAU OF RECLAMATION METHOD) Precipitation increment range Percent Effective precipitation precipitation accumulated - range mm
in
mm
in
0.0 - 25.4
0-1
90-100
22.9 - 25.4
0.90 - 1.00
25.4 - 50.8
1-2
85 - 95
44.4 - 49.5
1.75 - 1.95
50.8 - 76.2
2-3
75 - 90
63.5 - 72.4
2.50 - 2.85
76.2 - 101.6
3-4
50-80
76.2 - 92.7
3.00 - 3.65
101.6 - 127.0
4-5
30-60
83.8 - 107.9
3.30 - 4.25
127.0 - 152.4
5-6
10 - 40
86.4 - 118.1
3.40 - 4.65
Over 152.4
Over 6
0-10
86.4 - 120.6
3.40 - 4.75
For example, if monthly rainfall during the past five years in the month of July is 100; 125; 250; 225 and 175 cm, the mean is 175 cm. From the table, the effecti ve rainfall value for the month of July will be 120.6 mm. The method does not take into account the type of soil, nature of the crop and frequency and distribution of rain. Nor does it consider degree of aridit y. The method is not considered satisfactory. 3.3 Potential Evapotranspiration/Precipitation Ratio Method (India)
This simple semi-empirical method is used in some projects in India. A ratio of potential evapotranspiration, taken as 0.8 of the U.S. Class A pan data, to the total rainfall for a certain group of days during the growing season is computed. The number of days in a group is based broadly on a soil type or soil moisture properties as well wel l as general weather conditions or evapotranspiration rates* The maximum number of days in a group is 15 during warm weather and 30 during cool weather for crops other than rice. The lower the water holding capacity of the soil and/or the higher the evapotranspiration rate, the shorter the period in the group. This is shown in Table 6. Table 6: NUMBER OF DAYS IN A GROUP FOR DIFFERENT SOIL TYPES AND CLIMATIC CONDITIONS Crop
Mean monthly ETp (mm/day)
Soil texture and water storage capacity (mm/m). Light (below Medium (40 to 40) 80)
Heavy (80 to Very heavy (over 120) 120)
Rice
3 to 12
2
3
4
7
Other
Over 6
4
7
10
15
crops
Below 6
7
10
15
30
Rainless periods are deleted from the calculations. The ratios are expressed in a percentage for each period. So the maximum value of the ratio cannot exceed 100. The monthly means are then computed and from these the grand mean ratio is obtained for the entire growing season. Precise knowledge on soil properties or aridity is not essential. There can be some under or over estimation depending upon the distribution of rainfall, but the error is small. This method is good for broad planning purposes. It is rapid and inexpensive.
A sample calculation for the case of a heavy soil in a warm season is shown below. Of the total rainfall of 670 mm during the growing season, 80 percent is effective ac cording to this method, Period
Potential evapotranspiration evapotranspiration (mm)
Rainfall (mm)
Percentage ratio
Mean ratio for the month(%)
July 1-10
60
80
75
11-20
80
0
0
21 - 31
65
60
100
Aug. 1 10
60
150
40
11-20
65
100
65
21 - 31
60
80
75
Sept. 1 10
60
60
100
11 - 20
70
40
100
21 - 30
80
20
100
Oct. 1 10
80
0
0
11 - 20
85
0
0
21 - 31
60
80
75
Total
825
670
-
322
Mean
...
...
...
80%
87
60
100
75
In the case of rice, instead of the evapotranspiration value, total water loss, which is evapotranspiration plus percolation losses, is used for computation. Taking crop characteristics into account, necessa ry correction can be further applied for undesirable or destructive kinds of rainfall such as those causing lodging, or flower or fruit drop. 3.4 USDA, SCS Method
The U.S. Department of Agriculture's Soil Conservation Service has developed a procedure for estimating effective rainfall by processing long term climatic and soil moisture data. A comprehensive analysis was made by perusing 50 years of precipitation re cords at 22 experimental stations representing different climatic and soil conditions. The soil moisture balance was worked out for each day try adding effective rainfall or irrigation to the previous day's balance and subtracti ng consumptive use. To avoid a high degree of complexity, neither the soil intake rate nor rainfall intensities are considered in this method. From total rainfall and monthly consumptive use, effective rainfall values were computed (Table 6). The values were based on a 3 in or 75 mm net irrigation application, which is equal to the available storage capacity in the root zone at the time of irrigation application. To convert this data to other net depths, factors were worked out which are shown in Table 7. For example, a crop of wheat grown on sandy loam has a net depth of irrigation application of 50 ram. With a mean consumptive use for the month of December of 100 mm and a mean rainfall of 75 mm, the effective rainfall will be 52.7 x 0.93 = 49 mm. The monthly effective rainfall cannot exceed the r ate of consumptive use. If it does, d oes, the lower value of the two is taken.
Table 7: MULTIPLICATION FACTORS TO RELATE MONTHLY EFFECTIVE RAINFALL VALUE OBTAINED FROM TABLE 8 TO NET DEPTH OF IRRIGATION APPLICATION (d), IN mm d mm factor d mm factor d mm factor
10.00 0.620 31.25 0.818 70.00 0.990 12.50 0.650 32.50 0.826 75.00 1.000 15.00 0.676 35.00 0.842 80.00 1.004 17.50 0.703 37.50 0.360 85.00 1.008 18.75 0.720 40.00 0.876 90.00 1.012 20.00 0.728 45.00 0.905 95.00 1.016 22.50 0.749 50.00 0.930 100.00 1.020 25.00 0.770 55.00 0.947 125.00 1.040 27.50 0.790 60.00 0.963 150.00 1.060 30.00 0.808 65.00 0.977 175.00 1.070 Table 8: AVERAGE MONTHLY EFFECTIVE RAINFALL AS RELATED TO MEAN MONTHLY RAINFALL AND MEAN MONTHLY CONSUMPTIVE CONSUMPTIVE USE (USDA, SCS) Monthly mean rainfall mm
Mean monthly consumptive use mm 25
50
75
100 125 150 175
200 225 250 275 300 325 350
Mean monthly effective rainfall mm
12.5
7.5
25.0
15.0 16.2 17.5 18.0 18.5 19.7 20.5 22.0 24.5 25.0 25.0 25.0 25.0 25.0
37.5
22.5 24.0 26.2 27.5 28.2 29.2 30.5 33.0 36.2 37.5 37.5 37.5 37.5 37.5
50.0
25
8.0
8.7
9.0
9.2 10.0 10.5 11.2 11.7 12.5 12.5 12.5 12.5 12.5
32.2 34.5 35.7 36.7 39.0 40.5 43.7 47.0 50.0 50.0 50.0 50.0 50.0
62.5
at 39.7 42.5 44.5 46.0 48.5 50.5 53.7 57.5 62.5 62.5 62.5 62.5 62.5 41.7
75.0
46.2 49.7 52.7 55.0 57.5 60.2 63.7 67.5 73.7 75.0 75.0 75.0 75.0
87.5
50.0 56.7 60.2 63.7 66.0 69.7 73.7 77.7 84.5 87.5 87.5 87.5 87.5
100.0
at 63.7 67.7 72.0 74.2 78.7 83.0 87.7 95.0 100 100 100 100 80.7
112.5
70.5 75.0 80.2 82.5 87.2 92.7 98.0 105 111 112 112 112
125.0
75.0 81.5 87.7 90.5 95.7 102 108 115 121 125 125 125
137.5
at 88.7 95.2 98.7 104 122
111 118 126 132 137 137 137
150.0
95.2 102 106 112
120 127 136 143 150 150 150
162.5
100 109 113 120
128 135 145 153 160 162 162
175.0
at 115 120 127 160
135 143 154 164 170 175 175
187.5
121 126 134
142 151 161 170 179 185 187
200.0
125 133 140
148 158 168 178 188 196 200
225
at 144 151 160 171 182 197
250 275
150 161 at
170 183 194
171 181 194 205
240 300
175
190 203 215
325
at 198 213 224 287
350
200 220 232
375
at 225 331
240
400
at 247 372
425
250 at 412
450
25
50
75
100 125 150 175
200 225 250
3.5 Empirical Relationships
3.5.1 Crops other than rice 3.5.2 Rice Measurement in rice
Several methods of estimating effective rainfall rainf all for irrigation schedules are in vogue in different countries. They are based on long experience and have been found to work quite satisfactorily in the specific conditions under which they were developed. 3.5.1 Crops other than rice India For a given area, effective rainfall is taken to be equal to 70 percent of the average seasonal rainfall. In another method, effective rainfall is taken as the mean value of rain, with the t he excess ever 3 in. in one day and 5 in. in 10 days omitted. Effective rainfall has also als o been taken to be equal to the lowest monsoon rainfall occurring in three out of four years. The Damodar Valley Corporation divided the 'Kharip season' (June to November) into ten-day periods. During these periods, rainfall which exceeded the water needs of the predominant predominant crop was considered as ineffective and the rest as effective. Khushlani (1956) suggested that rainfall during the life cycle of t he crop in a bad year (one with low rainfall) should be considered as the effective rainfall. In determining the water requirement of sugar cane under Bombay Deccan conditions, Rege et al (1943) considered that rainfall received only after five days from the irrigation date was effective. ef fective. The rainfall received within five day after irrigation was treated as ineffective. Sastry (1956) suggested the following equation for estimating rainfall usable by crops under Andhra Pradesh (central part of South India) conditions,
Y = usable rainfall, per days for a given period = mean dally rainfall C = a constant, co nstant, which determines the confidence limit of the minimum statistical average d = standard deviation of the dally rainfall
Estimates of usable rainfall in various periods of the monsoon season can be arrived at by selecting suitable confidence limits and statistical methods for calculating the standard deviation. Burma During the wet season, rainfall of less than 0.5 in. is considered as ineffective. Above this figure, 63 percent of the amount greater than 0,5 in. is considered considered affective rainfall. Baring the dry season, rainfall of less than 1 in. is considered as ineffective. Above this figure, 65 percent of the amount greater than 1 in. is considered as. usable by the crop (Kung, 1971). This approach would appear to be rather arbitrary. Thailand Of the November rainfall 80 percent and of the December to March rainfall 90 percent is considered an effective (Kung, 1971) 3.5.2 Rice Measurement in rice Rice thrives under conditions of abundant water supply, hence the practice of land submergence. Depth of flooding is governed by the variety grown and its height, the height of field bunds and availability of water. The water requirements of rice include evapotranspiration and percolation. Measuring effective rainfall is thus more complicated. Different empirical methods used in different countries are outlined below bel ow (Kung, 1971). India In one method, a percentage of total rainfall varying from 50 to 80 percent is assumed effective. In a second method, rainfall less than 0.25 in (6.25 mm) on any day is considered as in effective* Similarly any amount over 3 in (75 mm) per day, and rainfall in excess of 5 in (125 mm) in 10 days is treated as ineffective. Japan For submerged rice the year having the lowest rai nfall over the past 10 to 15 years is selected. se lected. Depending upon local conditions, an amount of 50 to 80 mm is considered as ineffective. The rest i s all effective. A daily record is kept of soil water in the field. For non-submerged rice a method using daily readings is also used. The daily rainfall efficiency is assumed to be 80 percent. Daily rainfall of less than 0.5 x ET is not effective. Also, if the sum of effective rainfall and the residual moisture from the previous day exceeds a predetermined value, the surplus is disregarded. A sample calculation of computation for a 12 day period is shown below for non-submerged rice. In this example ET = 3.7 mm/day, available soil Water = 30 mm. Date Rainfall Effective
Potential
Soil water Irrigation Stored soil
Remarks
(A)
rainfall (A x 0.8)
evapotranspiration
storage change (+ or -)
applied
water balance
1
0
0
3.7
-3.7
30.0
26.3
2
1.7
0
3.7
-3.7
0
22.6
3
5.4
4.3
3.7
+0.6
0
23.2
4
0
0
3.7
-3.7
0
19.5
5
1.5
0
3.7
-3.7
0
15.8
6
5.8
4.6
3.7
+0.9
0
16.7
7
0
0
3.7
-3.7
0
13.0
8
0
0
3.7
-3.7
0
9.3
9
53.2
20.7
3.7
+17.0
0
26.3
10
2.5
2.0
3.7
-1.7
0
24.6
11
20.3
5.4
3.7
+1.7
0
26.3
(30.0 - 24.6)5.4
12
35.0
3.7
3.7
0.0
0
26.3
(30.0 - 26.3)3.7
(1.7 x 0.8) less than 1.85
(1.5 x 0.8) less than 1.85
(53.2 x 0.8) + 9.3 greater than 30.0
The method must be modified in respect of rainfall ef ficiency, the moisture storage capacity capacit y of the soil, and evapotranspiration, before it can be used for other situations. Vietnam The water holding capacity of rice soils is assumed to be 50 mm. Daily rainfall below 5 mm and above 50 mm is disregarded. If daily evapotranspiration is 10 mm, a two -days' successive rainfall of up to 60 mm is taken as effective and excess over this limit is disregarded. Similarly, three days' successive rainfall of up to 70 mm is taken as effective and the excess is disregarded. The same procedure is followed for more rainy days. In different months different percentages are considered to be effective out of the total rainfall received. Period
% taken as effective
Remarks
April - September
75
Wet season
October
65
High rainfall intensity
November
80
Dry season
December - March
90
Dry and cool season
Burma Rainfall below 0.5 in is considered as ineffective. ineffe ctive. Above 0.5 in, 80 percent of the amount in excess is considered as effective.
4. Evaluation of methods
A method must account satisfactorily for surface run-off, water storage changes in the soil, evapotranspiration and crop characteristics. For field fiel d use, the method should be simple, inexpensive, rapid and accurate. It should be useful for broad regional planning or precise irrigation scheduling under a given set of conditions. For each method measurement of total r ainfall using & rain gauge or rain recorder is essential. Relative merits and limitations of the different methods discussed are shown in Table 9. Of the field methods, the first method (No.2.1) involving periodic measurement of soil water changed is accurate only when a sufficient number of replications are made; it is very laborious and cumbersome for field application. The method can be used to verify the applicability of empirical methods developed elsewhere. The soil water balance sheet method (No.2.2) involving the use of meteorological data and estimation of ETa will provide data on estimating effective rainfall if relevant input data are ar e available. The estimation of ETa, however, is more complicated and less accurate than the estimation of ETp. In the integrating gauge method (No.2.3), ceramic plates are used; the relation between evaporation from the plate and the evapotranspiration from the crop must be known but is difficult to establish. Errors are due to dust and salt accumulation on the evaporating surface, The Ramdas method (No.2.4) is simple bat may not be accurate unless a large number of replications are made. This may make the method cumbersome for field use. The use of lysimeters of large diameters (No.2.5) is the best and the most reliable method in assessment of effective rainfall. It is, i s, however, expensive for field use. It should be installed in res earch stations. The method can be used to check the accuracy of the other methods. The drum technique for rice (No.2.6) has given promising results. The method requires replications to give accurate data . Of the empirical formulae the Renfro equation method (No. 3.1) is too empirical. The U.S. Bureau of Reclamation method (NO. 3.2) neglects many essential parameters. The method based on the ratio of potential evapotranspiration to precipitation (No.3.3) will provide the best results. It can be used for broad estimation of effective rainfall in irrigation planning. The USDA, SCS SCS method (NO. 3.4) has has been based on data representing a wide range of conditions. The The method is satisfactory and can be applied for field use after verification in a given situation. The empirical methods for crops other than rice (No.3.5.1) and for rice (No.3.5.2), are based on long experience. They may be satisfactory under local conditions but need verification when applied in conditions very different from those under which they were developed. Methods described under 2.2, 3.3, 3.4, and 3.5.1 when used in planning should first be checked for accuracy using methods 2.1 or 2.5. The choice of method to be used depends upon the funds, facilities and basic data that are locally available. Table 9: RELATIVE MERITS OF DIFFERENT METHODS FOR DETERMINING EFFECTIVE RAINFALL Methods
Factors taken into account
Special Accuracy Relative equipment costs
Remarks
Run- Soil Aridity Crop off
2.1 Field studies of soil moisture
+
+
+
2.2 Daily soil water budget with Eta
-
+
+
2.3 Integrating gauge
-
+
+
2.4 Ramdas
-
+
+
+
+
Very high Medium
Good for verifying other met hods; cumbersome practicability low
+
Very high Medium
Practicability medium
+
+
Medium
Medium
Needs careful standardization
+
+
High
Medium
Practicability good
apparatus 2.3 Lysimeters
-
+
+
+
+
Very high Very high Practicability medium, good as a check on other methods
2.6 Drum technique (rice)
+
+
+
+
+
Very high Low
3.1 Renfro equation
-
B
+
-
+
Low
Negligible Too empirical
3.2 U.S. Bureau of Reclamation method
+
-
-
-
-
Low
Negligible Not suitable for wide use
3.3 Ratio of ETp to precipitation
B
B
+
-
-
Medium
Low
Satisfactory for very preliminary planning purposes
3.4 USDA, SCS method
-
B
+
B
-
Medium
Low
Good for areas with low intensity of rainfall and high soil infiltration rate
3.5.1 Empirical methods (other than rice)
B
B
B
B
-
Low to high
Negligible Practicability very high
3.5.2 Empirical methods (rice)
B
B
B
B
-
Medium
Negligible Needs verification; practicability high
+ = positive; - = negative; B = first approximation
Practicability high
Chapter III. Application Application of effective rainfall data is irrigation and drainage projects 1. Irrigation project design 2. Irrigation project operation 3. Drainage projects 4. Rice cultivation 5. The effect of groundwater 6. Effective rainfall in unirrigated and low rainfall, rainfall , areas
Knowledge of effective rainfall is vital for efficient utilization of water in irrigated agriculture. Application of information on effective rainfall is shown in the subsequent pages with sample calculations and illustrations for design of irrigation projects, operation of irrigation projects, design and operation of drainage systems, leaching of salts, rice cultivation, planning of irrigation systems using groundwater, and for rainfed agriculture.
1. Irrigation project design Monthly, seasonal and annual rainfall varies from year to year, so does the effective rainfall, and consequently irrigation requirements. The Water supply cannot be planned on the minimum value of effective rainfall since this would result for most years in a highly uneconomic and wasteful project. Nor can it be based on the average amount of effective rainfall since this would provide provide an adequate and assured water supply for approximately only half the time. Therefore, th e value of effective rainfall is computed on a probability basis. The percent chance of its occurrence is selected sele cted on a number of considerations, including yield predictions, cost of the system and financial returns; for instance, for a high value crop like vegetables, a water supply may be based on effective rainfall occurring nine years out of ten but for a low value cr op, five out of ten years may be adequate. The water supply to be developed is to be based on the percent chance of occurrence chosen which exceeds the gross irrigation requirement of the crop during the chosen period. If this is 10 percent, for example, the water supply for that period would be adequate in 9 years out of 10. The 90 percent chance effective rainfall will need to be determined. Data items needed are rainfall rai nfall data for several years, weekly; consumptive use data, mean monthly; available water storage capacity of soils in the scheme area, mean data for different soil types or classification units; irrigation efficiency, percentage or fraction; type of crops and acreages, their sowing and harvesting times, critical stages in crop c rop growth, and special water needs if any; an y; economic factors from which chance of occurrence of rainfall can be selected, A procedure to develop gross irrigation requirements is given below. Rainfall data can normally be obtained from available records. The rainfall values are arranged in order of magnitude and their cumulative frequency is worked out. In drawing cumulative frequency distribution of rainfall on a log-normal probability paper the following steps a re required: Step 1s obtain rainfall data for as many years as possible (columns 1 and 2);
Step 2: arrange them in order of magnitude, with the largest number firs t (columns 3 and 4); Step 3: calculate the plotting position (Fa) by using the Hazen equation (USDA, SCS, 1967): Fa = [100 (2n - 1)] / 2 y Fa = the plotting position in percent n = the rank number y = the number of years of record keeping Step 4: prepare the vertical scale of the log-normal probability paper and plot the rainfall on the vertical logarithmic scale against the Fa positions on the percent chance scale (Fig 8). Step 5: drew the best fitting line through the plotted points as shown in Fig 8. For instance, at the 80 percent chance level, the rainfall is about 350 mm. Years Annual rainfall (R a) mm n R a
Fa
(1)
(2)
(3) (4) (5)
1934
380
2 960 3.8
1935
720
3 900 6.3
1936
1000
4 880 8.8
1937
900
5 860 11.3
1938
800
6 840 13-8
1939
520
7 800. 16.3
1940
380
8 780 18.8
1941
260
9 760 21.3
1942
300
10 720 23.8
1943
620
11 700 26.3
1944
840
12 700 28.8
1945
300
13 560 31.3
1946
340
14 620 33.8
1947
660
15 600 36.3
1948
960
16 600 38.8
1949
240
17 580 41.3
1950
280
18 560 43.8
1951
400
19 540 46.2
1952
600
20 520 48.8
1953
880
21 500 51.2
1954
860
22 500 53.8
1955
780
23 480 56.2
1956
760
24 480 58.7
1957
360
25 460 61.2
1958
420
26 440 63.7
1959
540
27 420 66.2
I960
700
28 400 68.7
1961
360
29 400 71.2
196?.
400
30 380 73.7
1963
440
31 380 76,2
1964
460
32 360 78.7
1965
480
33 360 81.2
1966
480
34 340 83.7
1967
500
35 320 86.2
1968
700
36 300 88.7
1969
500
37 300 91.2
1970
560
38 280 93.7
1971
580
39 260 96.2
1972
600
40 240 98.7
Fig 8: FREQUENCY DISTRIBUTION 0F RAINFALL
A sample calculation of irrigation water requirements is given in Table 10. In Table 10, from the above data, the 50 and 75 percent (or any other) chance rainfall c an be determined, which is here 500 and 374 mm respectively. The ratio 374/500 = 0.75 is used here to determine the rainfall at 75 percent chance of occurrence for each month, assuming that the frequency distribution of yearly yearly or seasonal rainfall is the t he same as the frequency distribution of each month. From data on mean monthly water needs and 50 or 75 percent chance monthly rainfall, the 50 and 75 percent chance effective rainfall is obtained from Table 8. Gross irrigation requirements can be calculated from crop water need and carryover soil moisture data dat a and selected irrigation application efficienc y for 50 and 75 percent chance effective rainfall or, in the example in Table 10, 404 and 513 respectively. Rainfall patterns will differ from month to month. Rather than using a constant ratio derived from seasonal or yearly data, the 75 percent chance or any other percent chance monthly rainfall should preferably be determined from a rainfall frequency distribution analysis prepared for each month using the step method described above. This would also allow a selection of percent chance of rainfall occurrence for each month, with possibly a higher percentage when water is needed most, such as during the flowering stage of most crops. The calcul ations will be similar to those t hose given in Table 10, except for column 9. Table 10: SAMPLE CALCULATION OF IRRIGATION WATER REQUIREMENTS AT 50% AND 75% CHANCE OF OCCURRENCE OF RAINFALL (mm) Mo Mea nth n mon thly wate r need
Month Month ly ly rainfal effecti l at ve 50% rainfal chanc l at e of 50% occurr chanc ence e of occurr ence (Table 8)
Carr y over soil mois ture
Total avail able soil moist ure (Col umn 4 plus 5)
Net Gross Month irrigati irrigati ly on on rainfal require require l at ment ment at 75% (Colum 75% chanc n 2 irrigati e of minus on occurr 6 plus efficien ence 5 of the cy (Colu next mn 3 x month) R*) at 50% chance
Mon Carr thly y effec over tive soil rainf mois all at ture 75% chan ce (Tab le 8)
Total avail able soil moist ure (Col umn 10 plus 11)
Net Gross irrigati irrigati on on require require ment ment at (Colum 75% n 2 irrigati minus on 12 plus efficien 11 of cy the next month) at 75% chance
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Ma y
25
50
25
50
75
0
0
37
22
50
72
0
0
Jun e
75
100'
64
50
114
0
0
75
50
47
97
3
4
July 200
150
120
39
159
66
88
112
93
25
118
107
142
Aug 200 .
125
102
25
127
98
130
94
78
25
103
122
162
Sep 175 t.
75
60
25
85
115
153
56
45
25
80
129
172
Oct. 50
0
0
25
25
25
33
0
0
0
0
25
33
Tot 725 500 371 304 404 374 288 386 513 al *R = [75% chance of total rainfall (374 mm)] / [50% chance of total rainfall (500 mm)] = 0.75 An alternative method for determining first estimates on effective rainfall that will exceed a given percent chance of occurrence is suggested by USDA, SCS (1967). (1967). Data needed are average annual rainfall and average effective rainfall; factors with which the average effective rainfall needs to be multiplied to obtain its value for any given percent chance of occurrence are given in Table 11. This method should be used only when no high degree of accuracy is required. For example, if average annual rainfall is 1 250 mm, and mean effective rainfall is 1 000 mm, the 80 percent chance factor is 0.85 and effective rainfall at 80 percent chance of occurrence is 1 000 x 0.85 = 850 mm. In a given irrigation project, certain crops may ma y be provided with a water supply at 90 percent level of certainty while the others may be provided with one at 50 percent depending upon their economic value and availability of water. Normally, fruits a nd vegetables, oilseeds and industrial crops, spices and condiments require an assured supply to obtain high production levels while this may, for a given condition, be a less stringent requirement for cereals, pulses, pastures, and some millets. Irrigation projects should not be based on economic considerations alone; many other aspects, including humanitarian, are also involved in deciding on the level of water to be supplied. Table 11: FACTORS APPLICABLE TO EFFECTIVE RAINFALL Mean annual rainfall nun Percent chance of occurrence 50
60
70
50
90
75
0.80 0.68 0.56 0.45 0.33
100
.84
.72
.61
.50
.38
125
.87
.76
.65
.54
.42
150
.88
.78
.68
.57
.45
175
.89
.79
.69
.60
.48
200
.90
.81
.71
.62
.51
225
.91
.82
.73
.63
.53
250
.92
.83
.75
.65
.55
300
.93
.85
.78
.69
.58
350
.94
.86
.79
.71
.61
400
.95
.88
.81
.73
.63
450
.95
.89
.82
.74
.65
500
.96
.90
.83
.75
.67
550
.96
.90
.84
.77
.69
600
.97
.91
.84
.78
.70
650
.97
.92
.85
.79
.71
700
.97
.92
.86
.80
.72
750
.97
.93
.87
.81
.73
875
.98
.93
.88
.82
.75
1000
.98
.94
.89
.83
.77
1125
.98
.94
.90
.84
.78
1250
.98
.95
.91
.85
.79
1375
.99
.95
.91
.86
.80
1500
.99
.9 5
.91
.87
.81
1750
.99
.95
.92
.88
.83
2000
.99
.95
.92
.89
.85
2250
.99
.96
.93
.90
.86
2500
.99
.96
.93
.91
.87
2. Irrigation project operation Rainfall will affect the day da y to day operation of an irrigation project. Timing and the amount of water applied to the field will affect irrigation efficiency. Irrigation intervals ma y vary from 4 to 15 days depending mainly on the soil type and crop grown. Scheduling of irrigation should account for the rainfall received and hence precise knowledge about effective rainfall within 24 to 48 hours of its receipt is required for planning the next irrigation application. The operation schedules of the canal system can be subsequently adjusted This will result in a better utilization of available water resources and improved crop production. The following data are needed: daily rainfall (duration, intensit y and amount); mean value of soil water storage capacity in the root zone in the area commanded by an irrigation outlet} ETa data of crops at different stages of growth; thorough knowledge of local agricultural and irrigation practices, including special water needs if any. A sample of a working sheet for irrigation programming follows. The example applies to conditions where the soil water storage capacity is 120 mm and irrigation is applied when 50% of the total available water has been used. All data dat a are given in mm. Dat e
Initial Irrigatio Total Et Net Soil Ineffectiv Decision Date of Actual soil n rainfal a change moistur e rain next irrigatio moistur l in soil e (surplus) irrigatio n date e moistur balance n contents e
1
2
3
4
5
6
7
8
9
10
11
1
60
60
0
8
-52
112
0
Next irrigation not before 7 days
8
0
2
112
0
0
8
-8
104
0
8
0
3
104
0
20
7
13
117
0
Irri. postponed by 2 days
10
0
4
117
0
30
7
23
120
20
Storage capacity limited to 60 mm. Irrigation postponed by 1 day
11
0
5
120
0
0
9
-9
111
0
11
0
6
111
0
0
10
-10
101
0
11
0
7
101
0
0
10
-10
91
0
11
0
8
91
0
0
10
-10
81
0
11
0
9
81
0
0
10
-10
71
0
Keep ready for irrigation on 11th
11
0
10
71
0
10
9
+1
72
0
Irrigation postponed by 1 day
12
0
11
72
0
0
9
-9
63
0
Irrigate on 12th
12
0
12
63
57
50
4
57
120
46
Rains received in the evening Hence became ineffectiv e
20
12
13
120
0
0
8
-8
112
0
20
0
14
112
0
0
8
-8
104
0
20
0
15
104
0
0
8
-8
96
0
20
0
16
96
0
0
8
-8
88
0
20
17
88
0
0
8
-8
80
0
20
18
80
0
0
9
-9
71
0
Keep ready for irrigation on 20th
20
19
71
0
0
8
-8
63
0
Irrigate on 20th
20
-
20
63
57
0
8
+49
112
0
Next irrigation not before 27th
27
20
21
112
0
8
-8
104
0
27
22
104
0
0
8
-8
96
0
27
23
96
0
0
7
-7
89
0
27
24
89
0
0
7
-7
82
0
27
25
82
0
0
8
-8
74
0
27
26
74
0
80
3
46
120
31
27
120
0
0
7
-7
113
0
4
28
113
0
0
8
-8
105
0
4
29
105
0
0
8
-8
97
0
4
30
97
0
0
8
-8
89
0
4
1
89
0
0
9
-9
80
0
Irrigation postponed by 8 days. Not before 4th of next month
Transfer to the next monthly sheet
4
4
Monthly Summary Total irrigations applied during June = 3 times = 174 mm ETa = 238 mm Total rainfall = 190 mm Mean ETa per day = 8.0 mm Ineffective rainfall = 97 mm Effective rainfall = (ETa - irrigations) + (Final balance - Initial balance) = (238 - 174) + (89 - 60) = 93 mm To increase the amount of effective rainfall and water economy personal judgment is often needed in scheduling water deliveries. As in the example exampl e given, on the 12th day irrigation water was applied in the morning and rain fell in the evening; however, if the sk y was cloudy and there was a likelihood of rain, it would have been worth while to take the risk of postponing irrigation by a day. Based on effective rainfall, swift s wift changes in the scheduling programme can be made from time to time, depending on practicability and flexibility allowed in the operation of the water distribution network.
3. Drainage projects
3.1 Drainage of Excess Water 3.2 Drainage for Leaching of Salts
Land drainage is essential for crop production in areas experiencing heavy rainfall. It is an inseparable part of irrigation systems s ystems to control salinity in arid and semi-arid semi- arid regions for permanent and sustained agriculture. The drains may not necessa rily flow throughout the year; the need may exist for just one or two months of the year, as is the case in many man y parts of the monsoon areas of Asia. Nevertheless, to save the crops from water-logging drainage becomes inevitable.
3.1 Drainage of Excess Water Depth and frequency of rain and the peak drainage discharge are cl osely related. The rate at which excess water must be removed from the soil is termed 'drainage co-efficient', which is expressed in mm per day or in m³ /sec/ha. This value val ue is based on rainfall characteristics characteris tics and on the excess water tolerance of the crop. For a given situation consideration may need to be given to surface run-off, deep percolation losses from rainfall, seepage of irrigation irrigati on canals and underground flows from adjacent areas. The absolute as well as relative contributions from the different sources vary in time as well as in quantity. Often they are also interdependent. Only the rainfall component in drainage flows i s considered in the following section. In simple terms, the sum of the daily rainfall minus consumptive use rate plus or minus t he soil storage change, is the drainage need. In humid regions, the amount of precipitat ion will have a direct relationship to the quantity of water to be drained. In ar id and semi-arid regions, the annual surface run-off from rain may range from about 0 to 200 mm while the seepage, percolation and le aching in irrigation schemes may range from 200 to 2 000 mm. Losses from irrigation s ystems may be of great significance. Precipitation is of little li ttle consequence and can most often be ignored in computing drainage discharges. The ineffective rainfall for crop production, less the soil moisture left at harvest, amounts to effective rainfall in drainage, except in cases such as rice, leaching l eaching of salts, and. periods outside the growing season. Hence the methodology employed for assessing effective rainfall in crop production can also be used to assess it from the drainage point of view. Daily values of surplus rain water water can be worked out for the whole year by the soil moisture balance sheet method. Deep percolation losses can be measured with lysimeters. In case of high groundwater the fluctuation in groundwater tables should be measured. Drainage practices then, can be based on crop tolerance to high groundwater groundwater tables taking into account soil and topography and the natural drainage characteristics of the area. Several semi-empirical methods for estimating est imating run-off for drainage design have been developed; they are given in most standard handbooks on hydrology. A simple method is described below; the method is rather empirical and only provides first estimates on surface run-off for general planning purposes. The USDA, SCS (1969) has developed a procedure using charts and tables for estimati ng volume and peak rates of run-off. Apart from rainfall characteristics, important factors influencing rainfall run-off are the run-off potentiality of the area; ar ea; the antecedent moisture condition; the degree of vegetal cover; conservation practices followed. The peak flow rates are also strongly s trongly dependent on slope of the land and area of the watershed. water shed. The method includes the following steps: Processing of rainfall data: by processing records of the daily values of total rainfall, probability values at any frequency, for any given period, are obtained for the project concerned; Run-off potentiality: the soils are to be grouped into one of the four hydrological classes on the basis of their run-off potentiality which is closely scheduled to their infiltration rates. Class A (low run-off potential): deep sandy soils; B: shallow sandy soils and medium texture soils with above average infiltration rates; C: shallow soils of medium to heavy texture with below average infiltration rates; D: (high run-off potential) clay and shallow soils with hardpan, high groundwater table, etc.
Antecedent moisture condition: the moisture condition is selected from precipitation during the 5 days (or more) preceding the day in question; there are 3 classes, as follows: Precipitation during 5 days before the day in question (mm) Condition Growing season
Dormant season
Less than 35
Less than 12.5
Dry - I
35.0 to 52.5
12.5 to 27.5
Average - II
Greater than 52.5
Greater than 27.5
Wet -III
Run - off equation: the equation used for surface run-off is:
Q = run-off over the drainage area (mm) P = precipitation over the drainage area (mm) S = potential water retention by the soil over the drainage area at time of start of rainfall (mm) S values are expressed in the relation CN = 1 000/(10 + S). CN values for the different hydrological classes, A, B, C and D, the vegetal covers and the conservation treatment are shown in Table 12 for antecedent moisture condition II. Corrected values for moisture conditions I (dry) and III (wet) are shown in Table 13. Run-off values (Q) for different values of CN and rainfall rates can be obtained from Table 14. For example, if a row crop of maize is grown on 40 ha of loamy soil belonging to hydrological class D, without any conservation treatment and having a moderate slope ranging from 3 to 8 percent, the antecedent moisture condition is dry, and the rainfall i s 150 nun during 24 hours, evenly distributed, it follows that CN will amount to 91 for medium moisture condition as per Table 12 and 80 for dry condition as per Table 13. The depth of surface run-off flow will be 94.50 mm as per Table 14. Table 12: CN VALUES FOR WATERSHED CONDITION II FOR DIFFERENT TYPES OF LAND USE Land use or cover
Treatment or practice State Hydrological soil class A
B
C
D
Fallow
Straight row
Poor 77
86
91
94
Row crops
Straight row
Poor 72
81
88
91
Straight row
Good 67
78
85
89
Contoured
Poor 70
79
84
88
Contoured
Good 65
75
82
86
Contoured and terraced Poor 66
74
80
82
Contoured and terraced Good 62
71
78
81
Straight row
Poor 65
76
84
88
Straight row
Good 63
75
83
87
Contoured
Poor 63
74
82
85
Small grain
Contoured
Good 61
73
81
84
Contoured and terraced Poor 61
72
79
82
Contoured and terraced Good 59
70
78
81
Closed-seeded legumes or rotation meadow Straight row
Poor 66
77
85
89
Straight row
Good 58
72
81
85
Contoured
Poor 64
75
83
85
Contoured
Good 55
69
78
83
Contoured and terraced Poor 63
73
80
83
Contoured and terraced Good 51
67
76
80
Pasture or
Poor 68
79
86
89
range
Pair 49
69
79
84
Good 39
61
74
80
Contoured
Poor 47
67
81
88
Contoured
Pair 25
59
75
83
Contoured
Good 6
35
70
79
Meadow
Good 30
58
71
78
(permanent)
Poor 45
66
77
83
Woodlands
Pair 36
60
73
79
(farm woodlots)
Good 25
55
70
77
Table 13: CN NUMBERS FOR DIFFERENT MOISTURE CONDITIONS AND S VALUES CN for condition II
CN I
S-values mm
III
CN for
CN S-values mm
condition II I III
100
100 100
0.0
58
38 76
181.0
98
94 99
5.1
56
36 75
196.5
96
89 99
10.4
54
34 73
213.0
94
85 98
15.9
52
32 71
230.7
92
81 97
21.7
50
31 70
250,0
90
78 96
27.7
48
29 68
270.0
88
75 95
34.0
46
27 66
292.5
86
72 94
40.7
44
25 64
317.5
84
68 93
47.5
42
24 62
345.0
82
66 92
55.0
40
22 60
375.0
80
63 . 91
62.5
38
21 58
407.5
78
60 90
70.5
36
19 56
445.0
76
58 89
79.0
34
18 54
485.0
74
55 88
87.7
32
16 52
530.0
72
53 86
97.2
30
15 50
582.5
Table 13 continued CN for condition II cm S-values mm CN for condition II CN S-values mm I III
I III
70
51 85
107.0
25
12 43
750.0
68
48 84
117.5
20
9 37
1000.0
66
46 82
128.7
15
6 30
1417.5
64
44 81
140.5
10
4 22
2250.0
62
42 79
153.2
5
2 13
4750.0
60
40 78
166.7
0
0 0
Infinity
Table 14: RUN-OFF UNDER DIFFERENT CN VALUES AND RATES OF RAINFALL IN mm Rainfall mm
CN values 70
65
70
75
80
85
90
25.0
0
0
0
0.75
2.0
4.25
8.0
30.0
0
0
7.0
11.5
35.0
0
0.75 1.75 3.75
0.50 1.50 3.25
9.75 15.2
40.0
0.25 1.25 2.75
45.0
0.75 2.25 4.25 7.25 11.0 16.2 23.2
50.0
1.50 3.50
62.5
4.25 7.50 11.5 16.2 22.2 29.5 38.2
75.0
8.25 12.7 18.0 24.0 31.2 39.7 49.5
100.0
19.0 25.7 33.2 41.7 51.0 61.5 73.0
125.0
32.5 41.2 51.0 61.2 72.2 84.2 97.0
150.0
48.0 58.7 70.0 82.0 94.5 107.7 121.2
175.0
65.0 77.5 90.5 103.7 117.2 131.5 145.5
200.0
83.2 97.5 111.7 126.0 140.5 155.5 170.2
225.0
102.5 118.0 133.5 148.7 164.2 179.7 194.7
250.0
122.5 139.2 155.7 172.0 188.0 204.0 219.5
275.0
143.0 161.0 178.2 195.5 212.0 228.5 244.2
300.0
164.0 183.0 201.2 219.0 236.2 253.0 269.0
6.0
5.0
6.0
8.50 13.0 19.0
9.50 14.0 20.0 27.2
Application of data Data collected over several years on quantity and peak rate of surface run-off ar e needed on which to base the value of the drainage co-efficient for the design of a drainage project. The value is determined taking into account level of probability and degree of protection to crops. Higher values will be needed for crops sensitive to water logging such as truck and most fruit crops, and generally lower values for field crops, pastures, meadows, nati ve ranges and forests. The mean rate of run-off for more than 24 hours for a t wo to five year frequency is generally generall y a good guide in selecting a value of drainage co-efficient. co-effic ient. Values may range from 8 to 25 and from 12 to 40 nun per day for general field crow and special high value crops respectively. The values will be about 1.5 times higher for organic or peat soils. Peak flow rates Peak flow rates can be determined by using the t he unit hydrograph method, Cook's method, or the rational formula. The last is very simple and hence popular, but also very empirical and should be used for first estimates only.
The relationship is Q = C I A/360 Q = peak rate of flow in m³/sec m³/ sec for a given frequency of rainfall C = a constant ranging from 0 to 1, according to watershed conditions I = maximum rainfall intensity in mm/hr A = drainage area in ha The values of C under different conditions are as f ollows. Soil type Cultivated lands Pastures
Sand
0.20
0.15
Loam
0.40
0.35
Clay
0.50
0.45
The maximum rainfall intensity, i, relates relat es to a rainfall duration equal to the concentration time, Tc, of the basin. To determine the maximum intensity, empirical formulas have been developed expressing precipitation for various durations as a function of frequency and which are of the form i = aT b/tc where T is the return period, t the duration in minutes and a, b, and c are regional constants. Values for the US show for rainfall of less than 60 mins a = 7 to 11, b = 0.17 to 0.23 and C = 0.38 to 0.47. Tc can can be be obt obtain ained ed from from the the empi empiri rica call form formul ulaa Tc Tc = 0.00 0.0019 195 5 (L/ (L/ )0.77 where Tc is in mins, L is the length of the mainstream and S = H/L where H is the difference in height between the highest and the lowest point in the field or basin. For example, if H is 9.5 m, L = 3070 m then Tc = 1 hr 27 mins. With precipitation intensity for various durations known, the value of i can then be determined. Such data can normally be obtained from the national meteorological service. To short cut the necessary calculations graphs are given to determine peak discharge in cus ecs for given water shed conditions and typical types of storm rainfall distribution (USDA, SCS, 1969). An example is given in Fig 9. The selection of the . co rrect graph for a given situation is somewhat s omewhat problematic.
3.2 Drainage for Leaching of Salts Soils may contain large amounts of salts when they weather and decompose. Irrigation water may also carry salts in appreciable amounts; even the best quality is never as pure as rainfall. The electrical conductivity of rain water is some 30 to 50 micromhos/cm while the best quality irrigation water is never lees than 100 micromhos/cm, but frequently much higher. Irrigation water evaporates, leaving the salts behind. With the lapse of time, these salts accumulate in the root zone. The salt content is usually expressed in electrical conductivity (EC) in millimho/cm or in parts per million on a weight basis (ppm). . EC in millimhos/cm is roughly 1/640 x ppm. The presence of salts in the root zone beyond a certain concentration hampers crop growth. Therefore, it is necessary to provide additional water (Dd) to leach them. Part of the rain received during the non-growing season is also useful in removing salts from the root zone. The ext ent of effective rainfall may thus be lower in i n salt affected areas than in non-saline areas. On an annual basis, the net quantity of irrigation water (Di) needed to meet the consumptive use by the crop (Do) amounts to total water need (CD) le ss the annual effective rainfall (Dr). Di = Do = CU - Dr (1) When there is a salt problem, additional water is required (Dd) for leaching the salts and hence the relation becomes:
Di = Dc + Dd (2) Fig 9: PEAK RATES OF DISCHARGE FOR SMALL WATERSHEDS TYPE II STORM DISTRIBUTION SLOPE-MODERATE SLOPE-MODERATE / CURVE NUMBER 75 (USDA, SCS, 1969)
The leaching requirement (LR) is the fraction of t he total amount of irrigation water that must be passed through the soil in excess of crop water needs, to control salinity at a specified level. Different crops differ in salt tolerance. The salt content of drainage water should normally not exceed the tolerance level of a crop. cr op. Leaching requirement (LR) can be expressed in terms of ratio of t he depth of drainage water (Dd) to that of irrigation irri gation water (Di) or in terms of a ratio of the salt content of the irrigation water (ECi) to that of drainage water (ECd), or
(3) In case of rainfall, it is necessary to take into account its i ts amount and the electrical conductivity also and to work out its weighted average. (Di) adjusted = Di + Dr (4) ( 4)
and (ECi) adjusted =
(5)
When electrical conductivity of rain water is assumed to be zero for ill practical purposes, the equation 5 resolves to:
(ECi) adjusted =
(6)
In case of rainfall, therefore, theref ore, the equation 3 of leaching requirements resolves to:
(7) From equations 2 and 7 above, the quantity of irrigation water (Di) can be expressed in terms of EC values, annual leaching requirement (Dd) and consumptive water requirement as under.
and
(8)
Also from equation 3:
It follows that:
(9)
The amount of rain water entering the soil from the annual rainfall (Ra) is obtained by deducting from the latter the annual run-off (Sa) and evaporation from the soil surface in the t he non-growing season (En). Sa and En are not useful in the lea ching process and hence their values must be subtracted from the total, or Dr = Ra - (Sa + En) (10) ( 10) The values of evaporation from land surfaces can be determined loc ally. If these are not available, however, the broad monthly values can be read from a chart prepared b y the USDA, SCS (1967), shown in Pig 10. The chart has been prepared from relationships between rate s of rainfall, mean monthly temperature and evaporation. In applying the above method to determine annual gross leaching needs the following data should be available: - monthly and annual rainfall values (Rm and Ra); - rainfall during growing season (Rg); - growing season effective rainfall (Re);
- concumptive use of water by crop (CU); - salt content or electrical conductivity of irrigation water (ECi); - salt content (parts per million) or electrical conductivity of drainage water (ECd) or salt tolerance limit of crop; - annual surface run - off (Sa); - irrigation efficiency (Ef); - evaporation in non-growing season (En). Fig 10: EVAPORATION FROM LAND AREAS FOR VARIOUS TEMPERATURES AND RAINFALL (ASCE PRACTICE HYDROLOGY HANDBOOK)
A sample calculation for determining the effective effect ive rainfall and annual leaching needs of a crop of berseem is given below; in this example annual consumptive use (CU) is 750 mm, annual rainfall (Ra) is 400 mm, annual run-off (Sa) is 40 mm, evaporation in non-growing season (E) is 160 mm, effective rainfall during growing season (ER) is 150 mm, irri gation efficiency (Ef) is 70%, salt content of irrigation water (ECi) is 1 920 ppm, and salt tolerance limit of the crop (ECd) is 9.0 mmhos/cm.
Using equation (1), DC = CU - ER = 750 - 150 = 600 mm; and equation (10), Dr = Ra - (Sa + En) = 400 - (40 + 160) = 200 mm.
mmhos/cm, by
Assume first that no irrigation water is to be added. By using equations 7 and 8:
and
If this quantity of 200 mm is substituted in the above equations, then
The higher value of 221.75 mm is obtained meaning thereby that more than 200 mm of water will be required for leaching purposes. Next assume that 250 - of water are to be added. In this case:
Since 227.4 is lower than 250, the quantity of irrigation water to be added for leaching is lees than 227.4 mm. The required value lies between 221.75 mm and 227.4 mm. By interpolation:
Dd = 224.60 After substituting this value of 224.6 mm in the above equations:
Since the irrigation efficiency is 70%, the annual gross leaching need will be 224.60/0.70 - 320.8 mm. Another approach to estimating annual leaching needs was developed by Van der Molen and Boumans (1963). The relationship between different parameters was derived as follows:
= year total of indicated quantities in dm P = deep percolation below the root zone required to maintain soi l salinity at the selected average avera ge level ET = Evapotranspiration in din/month N = Precipitation less interception and surface run-off in din/month k = leaching co-efficient ECe = electrical conductivity of saturation extract in mmhos/cm or tolerated average salt level ECi = electrical conductivity of the irrigation water in mmhos/cm A calculation example of the average annual drainage needs according to the equation given above is presented below. Soil Type
Leaching Co-efficient (k)
Sand Loam Clay
0.8
0.6
0.3
Evapotranspiration (ET) dm
9
9
9
Precipitation (N) dm
3
3
3
Tolerated average salt level (ECe)
4
4
4
Conductivity irrigation water (ECi)
2
2
2
Required leaching (£P)
2.5
3.3
6.6
Required irrigation G[i3?)
8.5
9.3
12.6
0
+2
-3
Total drainage dm
2.5
3.3
6.6
Artificial drainage dm
2.5
1.3
9.6
Natural drainage (+) or seepage supply (-)
The average annual requirement is, however, inadequate as a desi gn criterion. Information on peak drainage needs can be obtained by studying irrigation - Bait - drai nage relationships for each monthly period. To derive the necessary monthly data, reference is made to Chapter 11 edited by Tan den Berg in FAO/UNESCO International Sourcebook on Ir rigation Drainage and Salinity (1973). ( 1973).
4. Rice cultivation To assess the effective rainfall in rice culture, the water balance should preferably be calculated from daily data. The use of the drum culture technique described earlier permits the measurement measurem ent of actual evapotranspiration (ETa), deep percolation (Dp) and effective rainfall (ER). If this degree of accuracy accurac y is not needed, however, the effective rainfall can be meas ured using values of estimated ETp, deep percolation and permissible water depth for land submergence to obtain a daily account value, X X = (Water stock on hand - Water losses) - (Permissible water depth) (for land submergence) If the value of X is positive, it indicates the amount of water surplus or rainfall is ineffective. If the value of X is 0, it means that all the rainfall received on that day has been effective effecti ve and the field has attained its maximum allowable flooding capacity. There is no room to store any more water. Also, there is no water surplus on that day. If the value of X is negative, it means that all the rainfall received on that day is effective and some more (equivalent to X value) can be accommodated in the field without it being ineffective. ',"' As already stated, stock on hand includes the previous da y's balance plus any addition due to rain or irrigation. Losses are those due to evapotranspiration and deep percolation, The permissible water depth for land submergence depends on the stage of crop growth and field bund height, whichever is lower of the two. The illustration of three cases cas es is given below, and daily water balance can thus be maintained to derive the monthly values. Parameters
Case A
Case B
Case C
Previous 'balance (ram depth)
30
30
30
Rainfall received (mm)
40
45
50
Irrigation (mm)
0
0
0
Evapotranspiration (mm)
8
8
8
Deep percolation (mm)
7
7
7
Crop stage allowance (mm)
60
60
60
Field, bund height allowance (mm)
125
125
125
X value (mm)
-5
0
5
Balance to be carried forward
55
60
60
Conclusion ,
All rain effective Space for All effective No space for additional 5 mm storing more rain
5 mm ineffective 45 mm effective
Data requirements for this method are: total rainfall from rain gauge (daily value); evapotranspiration from open pan (daily value); percolation, mean value for the field (daily value); crop stage allowance (weekly value); field bund allowance from direct measurement of bund height (seasonal value). A summary table can be used as given for the following example, where at a given period the crop stage allowance is 75 mm and the field bund allowance is 150 mm. All data are in mm. Data Rainfall Irrigation Percolation plus ET
Change in water stored
Depth of stored water
Ineffective rain
1
2
3
4
5
6
7
-
0
0
0
0
75
0
1
20
0
15
5
75
5
2
40
0
18
22
75
22
3
100
0
15
85
75
85
4
15
0
15
0
75
0
5
0
0
18
-18
57
0
6
0
0
19
-19
38
0
7
20
0
17
3
41
0
8
60
0
16
44
75
10
9
70
0
15
-55
75
55
10
0
0
18
-18
57
0
11
0
0
18
-18
39
0
12
0
0
17
-17
22
0
13
0
0
15
-15
7
0
14
0
75
17
58
65
0
15
30
0
15
15
75
5
Total
355
-
-
-
-
182
5. The effect of groundwater High groundwater tables may be due to ample rain at t he site or in the surrounding area or due to seepage and percolation losses from irrigation canals. They may also be due to underground water flow from other areas. The contributions from different sourc es may vary from year to year. With groundwater tables within 1-1.5m from the root zone, there is on most soil types some contribution by capillary movement of water towards the water needs of crops. When the water depth is beyond this limit, the contribution by capillary flow for most s oils is negligible. A distinct feature of irrigation systems based on pumped groundwater is that the command areas are frequently nearer the source of water, as compared to the canal systems where long distanc es may be involved. The area supplied by one well is small in size, easy to manage and has a high flexibility. The cultivation and irrigation practices can be adapted quickly to the available water supply. Kith deep water table conditions, water is pumped and used for irrigation in a similar method to that used for canal irrigation systems. Effective rainfall can be calculated by b y the same method as that used in surface irrigation project designs or project operations. From the frequency distribution of effective rainfall and by b y selecting the proper percent chance of occurrence of total rainfall, and of effective rainfall, the net irrigation requirements can be calculated.
With shallow water table conditions, there can be a significant moisture recharge by capillary action to the root zone. Effective rainfall can decrease by an amount equal to the contribution from groundwater storage. There have been few studies on the relationship between depth of groundwater and effective rainfall. Hater will rise by capillary action above the water table, its height and rate depending on the type of soil and its hydraulic properties, the soil moisture content and consequently the type of crop and level of evaporative demand. Both distance and rate of water movement are important; for heavy textured soil the distance is great but the rate slow, whilst for sandy soils the distance is small but the rate is high. In the absence of impervious layers, the contribution of water to the root zone of less than 1 mm/day may be approximatel y taken when the upper soil layer is moist at 50 to 90 cm for coarse and heavy textured soils and some 120 to 200 cm for most medium textured soils. The contribution from the groundwater table can be assessed with the methods reported by Doering (1963), Dastane (1972) and White and Troxell, quoted by Chow (1964). In the White and Troxell method, it is assumed that evapotranspiration is negligible between midnight and 4 a.m. and that the level of the water table during this interval int erval approximates the daily mean using Fig 11. Fig 11: DIURNAL FLUCTUATION OF A WATER TABLE AS A RESULT OF TRANSPIRATION TRANSPIRATION (AFTER TROXELL)
If h is the hourly rise from midnight to 4 a.m. and S = net fall or rise of water table in one day, then the daily volume discharge = V ET = Sy A (24 h ± S) , where S y is specific yield and A is the area of vegetation. The specific yield is defined as the ratio of volume of water which, after the soil is saturated, will drain out by gravity to the total volume of the soil. It is non-capillary non-capillar y porosity of the soil. If the groundwater reservoir has a surface surf ace area of 10 km² n and an average specific speci fic yield is 7.5
percent, each 1 metre rise or fall in the water level over the area represents 75 hm of water. This method assumes that there is no but or in i n flow of water in the area under consideration and a change in the water table is a result of ET only. Recently, Allison (1964) derived a formula for estimating capillar y rise of water as follows:
Dgw = depth of ground water evaporated or removed ECe = electrical conductivity of saturation extract in mmhos per dm at 25°C. ds = soil density (gm/cc) Ds = depth of soil, where salt accumulation takes place Sp = saturation percentage of soil dw == density of water (gm/cc) ECgw = electrical conductivity of groundwater in mmhos/cm The daily soil water budget method can also be used for determining the va lue of effective rainfall under shallow water table conditions; an additional column on ground-water contribution on the income side, similar to that of irrigation is needed. If no information is available on the extent of groundwater contribution, the value of effective rainfall can be estimated by obtaining first the values of uncorrected effective rainf all under deep groundwater conditions from Table 8 and then correcting them by applying the multiplication factors as shown in Table 15. For example, if the total rai nfall is 100 mm, the depth of water application is 50 mm and consumptive use is 150 mm, then the effective rainfall under the deep water table conditions is 2.97 x 0.93 x 25» 69 mm. Under the conditions of a shallow water table, say at 140 cm, the effective rainfall will be 69.05 x 0.6 = 41.5 mm Table 15: MULTIPLICATION FACTORS TO BE APPLIED TO THE VALUES OF UNCORRECTED EFFECTIVE RAINFALL FROM TABLE 8 TO OBTAIN EFFECTIVE RAINFALL UNDER SHALLOW WATER TABLE CONDITIONS Depth to water table from soil surface (cm)
Soil Type Sand Loam Clay
50
0
0
0
60
0.25
0
0
70
0.50 0.30
80
0.75 0.50 0.22
90
0.95 0.66 0.40
100
1.00 0.80 0.57
110
0.90 0.75
120
0.95 0.86
130
1.00 0.95
140
0.97
150
1.00
Over 150
0
1.00 1.00 1.00
6. Effective rainfall in unirrigated and low rainfall, areas
In low rainfall, unirrigated areas, knowledge of the eff ective rainfall is most essential e ssential in planning crop production with the correct selection of crop species and agronomic practices. Two situations can be distinguished; rainfall may be received, predominantly in the crop growing season, and rainfall may be received during the non-growing season. In the f irst situation, it is necessary to know the extent of rain which is or can be actually used and that which is utilizable utili zable but lost through surface run-off and deep percolation, and whether suitable me asures can be adopted to reduce these losses. In the second case it is necessary to know the extent of rainwater which can be conserved economically and carried over to the next se ason for crop use. In other words, is it possible to conserve moisture and, if so, to what extent. These two situations are dealt dealt with below and illustrations are given for the best utilization of the often limited water. The essential data needed include: layer-by-layer values of field capacity, capacit y, wilting point, bulk density and depth of the soil in order to compute the moisture storage capacity and the moisture balance; daily rainfall records for several years from available records; data on the consumptive use of crops, class A pan records or other ET data; equipment needed includes soil augar, soil moisture boxes, weighing balance, drying oven, rain gauge. The following steps are required: - determine the moisture storage capacity of the soil; - maintain daily records of gains due to rains rai ns and losses due to evapotranspiration; if records ar e available, they must be tabulated; - prepare a daily soil moisture moist ure balance sheet, as shown for the Thornthwaite's method, using rainfall and consumptive use values. If data on actual consumptive use values are not available, these may ma y be computed by using any of the formulae, their selection depending on the climatological data available. To obtain the first approximate value of evaporation from the soil surface, Fig 9, showing broadly the relationship between temperature, rainfall and evaporation, may be be used; - check the balance sheet value against that determined by periodic soil moisture sampling and oven drying. Samples should be taken immediately after rains, as early as possible. Moisture lost during the period from the cessation of rainfall until the soil is sampled should be estimated by multiplying the class A pan evaporation value for the corres ponding period by 0.4 to 0.8, depending on on the siting of the pan. (see FAO Irrigation and Drainage paper No. 24); - determine the total and effective effecti ve rainfall for each week, month and season, and the year; - plot a frequency distribution diagram of effective rainfall during the growing season using available records; - select the value of the effective rainfall at the desired percent chance of occurence for planning purposes; - interpret the data from the t he economic and crop production points of view. A sample calculation is given below which uses data from moisture studies after a rain shower to find the moisture storage capacity, effective rainfall and soil moisture balance when total rainfall is
200 mm; class A pan evaporation value for the three days after cessation of rains until the soil is sampled is 8, 10 and 12 mm. The soil moisture data on oven - dr y weight basis are as follows: Soil Layer Field Capacity % Wilting point % Moisture % Bulk Density (gm/co) (cm) Before rains After rains
0-30
24.0
12.0
14.0
22.0
1.40
30-60
26.0
13.0
15.0 15.0
24.0
1.45
60-00
28.0
14.0
16.0 16.0
25.0
1.50
90-120
30.0
15.0
22.0
22.0
1.50
Available moisture storage capacity capacit y of soil on a volume basis:
or
Total = 23.7 cm Similarly, the initial available soil moisture on a volume basis:
or
Total =5.8 cm The final available soil moisture balance on a volume basis
Total =17.1 cm The evapotranspiration during the interval between the cessation of rains and soil moisture sampling is:
The effective rainfall is equal to the increase in soil s oil moisture plus that evapotranspired during the interval between the cessation of the rain rai n and the soil moisture sampling, or (17.1 - 5.8) + 2.4 = 13.7 cm. Thus of 200 mm of rainfall only 137 mm was effective. The rest, or 63 mm, was lost by surface runoff. The moisture did not reach the fourth layer because it had too little time for infiltration. Suitable measures should therefore be taken to avoid this run-off and to s tore the moisture for use by the crop, such as ploughing, mulching or terracing. terraci ng. A sample calculation is also given on moisture studies during the crop growing season. Weekly data on rainfall and consumptive use are given in columns 2 and 3 in the water balance sheet. The available soil water storage capacity is assumed to be 75 mm. Period week
Rainfall mm
Actual evapotranspiration evapotranspiration ETa
Water storage change in soil
Water storage balance in soil
Water surplus
1
150
10
140
75
65
2
40
15
25
75
25
3
50
20
30
75
30
4
130
25
105
75
105
5
0
30
-30
45
0
6
0
30
-30
15
0
7
0
15
-15
0
0
8
10
10
0
0
0
9
5
5
0
0
0
10
10
10
0
0
0
11
100
35
65
65
0
12
60
35
25
75
15
13
70
30
40
75
40
14
0
20
-20
55
0
15
20
10
10
65
0
16
0
5
-5
60
0
Total
645
305
-
60
280
In the latter example, during the first six weeks, the rainfall exceeds the consumptive use and moisture storage capacity of the soil, and so there will be substantial losses loss es amounting to 225 mm by surface run-off and deep percolation. After six weeks, the crop will suffer through lack of moisture until the end of the tenth week. From the eleventh week onwards, there will again be adequate moisture until the crop matures. The water surplus is 55 mm, whilst considerable moisture (60 mm) is left at the end of the growing period.
A rainfall frequency distribution on a weekly basis should be prepared from available rainfall records. If this shows a trend like li ke that shown above, measures for more efficient use of the soil moisture through judicious crop planning should be taken. The possibility of conserving surplus water during the first four weeks by the constructi on of ponds should be investigated, and whether the water stored can be used to irrigate the crop during the seventh to the tenth week. Also, a crop with a longer growing period may be selected since considerable moisture is left unused at the end of the crop season. In rainfed agriculture it is advisable to choose different types t ypes of crop, with the length of growing seasons varying. Mixed cropping is a frequently applied method of using the available soil moisture efficiently efficie ntly and reducing the risk of total crop failure. Studies on the efficiency of fallowing should be made where applicable. Moisture conservati on through fallowing the land has been an ancient practice in many parts of the world. Quantitative and comprehensive studies on soil moisture have been conducted and have been reported by Kanitkar (1944), Jenkins (1951), Evans and Lemon (1957), Duley and Coyle (1955). Literature shows that the efficiency of fallowing depends upon the water holding properties of the soil, the rainfall pattern during the fallowing period, the evaporative demand during the fallowing period, the length of the fallowing period and the management practices. The efficiency of s ummer fallowing may range from 15 to 30 percent and that of winter fallowing from 25 to 60 percent. The value of effective rainfall cannot exceed the moisture storage capacity of the soil and hence moisture storage or fallowing efficiency should be regarded as 100% if the soil water is wetted to its maximum capacity. Dastane and Joshi (1961) suggested the following formula for the evaluation of moisture conservation efficiency during the fallowing period. Soil moisture available at sowing time for use by the crop amounts to the effective rainfall in the proceeding seasons.
E = efficiency of moisture conservation in percentage M1 = available moisture at the beginning of the fallowing period M2 = available moisture at the end of the period R = rainfall received Sc = available moisture storage capacity of the soil If the available moisture in the root zone before and after fallowing is 50 and 150 mm respectivel y, the rainfall during the fallowing period is 250 mm and the available water storage capacity of the soil is 200 mm, then the efficiency of fallowing will be 75 percent. The effective rainfall is 150 mm only out of the total received. Due to the enormous variations in soil conditions, it is emphasised that, in determining the soil moisture properties, it is necessary necessa ry to provide an adequate number of replications of soil samples in time and space. With all care taken, errors may still be in the order to 5 to 20 percent, or larger. Reference is made to the monograph by Black et al (1965) for a detailed discussion on the errors involved in the determination of soil moisture properties.
Chapter IV. Increasing the proportion of effective rainfall its effectiveness and further lines of work 1. Increasing effective rainfall 2. Increasing the effectiveness of rainfall 3. Further lines of work needed in the field of effective rainfall in agriculture
Rainfall is a free source of water and its proper utilization utili zation will lead to increased crop production. Farmers and researchers, planners and administrators, have been engaged from early times in the tas k of increasing the effective proportion of the total rainfall and also its effectiveness for increased crop production. he main practices in this respect can be listed as follows:
1. Increasing effective rainfall
1.1 Reducing Surface Run-off 1.2 Increasing Infiltration (except in rice culture) 1.3 Building Water Storage Structures 1.4 Minimizing Peep Percolation Losses
The approach to increasing effective rainfall in agriculture includes: 1.1 Reducing Surface Run-off
This is achieved by altering the topography of land, erecting barriers to the flow of water and by increasing the opportunity for infiltration. The practices followed include: - land grading and levelling; - bunding, terracing, basin listing, criss-cross ridging; - retaining crop r esidues after harvest; - reducing depth as well as frequency of irrigation.
1.2 Increasing Infiltration (except in rice culture)
Increased infiltration is achieved by improving the soil structure or condition at the surface, as well as in the subsurface. The practices include: - deep ploughing, sub-soiling or breaking hard pans;
- adding organic matter and soil conditioners to improve the texture of the growing pasture or grass legume mixtures;
- using mulches to prevent breaking of aggregates which seal the soil surface; 1.3 Building Water Storage Structures
Farm ponds at suitable places are very v ery useful to collect run-off water for f or use during periods of low rainfall at critical stages of crop growth. 1.4 Minimizing Peep Percolation Losses
Deep percolation losses can be minimized by practices such as; - increasing the water holding capacity of light textured soils by the addition of clays or organic matter, if this is economically feasible;
- indirectly by extending the root zone though selecting deep roted crops; - reducing permeability beyond the root zone, especially in rice, using soil conditioners (krylium) and placing plastic sheets and bituminous layers at some depth in the soil. The losses due to deep percolation can be very high in rice fields and may amount to 50 to 75 percent of the water applied. It is here that a great impact can be made in economising on irrigation water or in increasing the efficiency of rainwater by adopting the various measured listed (Dastane et al (1970)). - select for the rice growing heavy soils or those with hard pans and shallow depths;
- grow crops in large blocks instead of isolated fields, since the seepage can be inversely proportional to the perimeter of the area; - scrupulously level the land to avoid excess water applications; - puddle and destroy structures in the surface soil; - compact soil, embed polythene sheets and apply chemicals such as bitumen and asphalt, or a cement-concrete layer; - construct sub-soil dams to arrest horizontal flows of water; - apply light instead of heavy irrigation in irri gated areas to accommodate rainfall; carefully car efully follow a water budgeting procedure.
2. Increasing the effectiveness of rainfall The object of measuring the effectiveness of rainfall is -so -s o increase production per unit of water, per unit of land and per unit of time. This can be a chieved by planning suitable crops and carrying out farm operations consistent with the amount, intensity and frequency of rainfall. These include; - planning cropping patterns consistent with rainfall patterns;
- selecting high value crops for maximizing the income;
- adjusting sowing times according to the probable rainfall so that drought sensitive stages of growth do not synchronize with periods of inadequate rainfall; - timing other farm operations such as tillage, fertilizer application, applying insecticides, fungicides and herbicides etcetera, in relation to rainfall probability periods; - following mixed cropping of shallow and deep rooted crops for full exploitation of stored soil moisture; - adopting 'multi-storied' cropping practices, water needed for a single crop and for several crops together is more or less the same, but total production can be much. higher in the latter system than in the former; - improving weather forecasting techniques and using the predictions in planning farm operations.
3. Further lines of work needed in the field of effective rainfall in agriculture
3.1 Collecting Data by Setting Up Crop Lysimeters 3.2 Verification of Empirical Methods 3.3 Development of Empirical Methods in Different Areas 3.4 Utilizing Information of Effective Rainfall in Agricultural Practic es 3.5 Increasing Effective Rainfall Under Field Conditions 3.6 Increasing the Effectiveness of Rainfall
3.1 Collecting Data by Setting Up Crop Lysimeters Ly simeters
Very little experimental information is available for planning purposes on the region-by-region values of effective rainfall in agriculture. This data should, therefore, he collected. The setting up of crop lysimeters at suitable places would go a long way towards achieving this. Agricultural research stations, agro-meteorological stations and irrigation scheme headquarters may be selected for such studies. The staff can work out the corrections which should be applied to the value of total rainfall for the different conditions, such as for leaching le aching requirements, for shallow water tables, and different crops. Effective rainfall is an entity governed by site and situation, hence systematic collecti on of data and judicious interpretation should be done on a small region-wide basis. Additional field studies should be made to verify the collected information under field conditions. In this respect, several techniques are available to study water balance parameters. One of them, lysimetry is rather costly and requires competent personnel. However, it is the most reliable technique. Alternatively, water balance studies can also be conducted in small watersheds in representative irrigated areas. 3.2 Verification of Empirical Methods
Empirical methods for estimating effective rainfall r ainfall are simple and practical, practic al, and have a high extension value. However, these methods should be verified in individual areas against the standard methods and improved, if necessary, by applying suitable corrections*
3.3 Development of Empirical Methods in Different Areas
Rapid methods must be developed for estimating effective rainf all in all irrigated areas. Empirical methods with suitable assumptions would be the best initial approach because of their inexpensiveness and reliability when corrected for local conditions. 3.4 Utilizing Information of Effective Rainfall in Agricultural Practices
Studies on effective rainfall in rainfed farming areas will reveal to what extent water is used at the site and to what extent it is wasted. These will facilitate the drawing up of plans for more efficient utilization of water resources. Information can be used productively and profitably in all irrigation projects. In new schemes, wate r deliveries, both in time and depth, should be planned on the basis of short period values of effective rainfall. Often either no values or generalized values are taken for the entire season. Planning, management and operation of irrigation schemes on the basis of carefully computed effective rainfall data would increase agricultural productivity and economize on irrigation water usa ge. 3.5 Increasing Effective Rainfall Under Field Conditions
Effective rainfall can be increased i ncreased by reducing surface run-off, increasing infilt ration and reducing deep percolation losses. Several practices are listed, but they are often not followed f ollowed because their usefulness is under-estimated or their cost is too high. It is therefore necessary to popularize them through extension agencies. What can be done at the individual, community and district levels needs to be defined and demonstrated. Rice culture offers an enormous scope for incre asing effective rainfall by reducing percolation loss es ranging between 50 and 75 percent. Studies using techniques of puddling, compacting, adding chemicals, lining the sub-soil with impervious materials and constructing sub-soil dams, would go a long way towards reducing the crops' water needs. 3.6 Increasing the Effectiveness of Rainfall
For lack of studies on the subject, very little littl e progress has been made in developing practices for increasing the effectiveness of rainfall. Economic studies on the value of rainfall in terms of the production and benefits should be made made for its proper appraisal and further augmentation.
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