SWIP Manual Part 1

MANUAL FOR AGROHYDROLOGY AND ENGINEERING DESIGN FOR SMALL WATER IMPOUNDING PROJECT (SWIP)

Department of Agriculture BUREAU OF SOILS AND WATER MANAGEMENT Diliman, Quezon City March 1997

TABLE OF CONTENTS DESCRIPTION 1.

ESTIMATION OF RUN-OFF and DERIVATION OF INFLOW 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7

2.

Establishment of Project Data Estimation of Basin Lag Time and Time Concentration Computation for Rainfall Depth Rainfall Increments Determination Rearrangement of Rainfall Pattern Derivation of Synthetic Unit Hydrograph Convolution of Equation for Flood Hydrograph

PAGE NO. HYDROGRAPH 1 1 2 2 3 8 9

FIELD WATER BALANCE COMPUTATION

10

2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10

10 10 11 11 14 14 14 14 14 15

Establishment of Cropping Pattern and Cropping Calendar Computation of 80% Dependable Rainfall Crop Coefficient and Crop Rooting Depth Percolation Loss Soil Water Holding Capacity Actual Evapotranspiration Change in Storage Initial Storage Estimation of Water Storage at End of Decade Irrigation Efficiency

3. ESTIMATION OF 10-DAY RESERVOIR INFLOW 3.1 3.2

16

Estimation of 10-Day Inflow for Region I, II, & IV Estimation of 10-Day Inflow for Other Regions

16 16

Philippine Water Resources Region Climate Map of the Philippines Monthly Distribution of Potential Evapotranspiration of Selected Places in the Philippines Planting Calendar

24 25

4. ANNEXES A. B. C. D.

27 28

LIST OF TABLES TABLE NO.

PAGE NO.

Regression Coefficients of the Rainfall Intensity-Duration Frequency Curve for Different Locations

4

2

Soil Groups for Estimation of Watershed Index W.

6

3

Antecedent Moisture Condition for Estimation of Water Index W.

6

4

Values of Watershed Index W.

6

5

Adjustment of Watershed Index W for Antecedent Moisture 7

Condition

6

Recommended Retention Rate for Hydrologic Soil Groups

8

7

T/Tp versus q/qp for Dimensionless Hydrograph

9

8

Percolation for Different Soil Types

12

9

S W H C of Different Soil Textural Class

15

10

Regional Run-off Coefficient and % Monthly Baseflow Distribution

17

LIST OF FIGURES FIGURE NO.

TITLE

PAGE NO.

1

Rearrangement of Rainfall Increments

5

1

Water Management Scheme and Crop Depending Variables for Field Water Balance for Irrigated Wetland Rice.

12

2 4 5 6

Crop Depending Variables for Field Water balance of Irrigated Corn.

12

Crop Depending Variables for Field Water balance of Irrigated Mungo.

13

Crop Depending Variables for Field Water balance of Irrigated Tomato.

13

Crop Depending Variables for Field Water balance of Irrigated Peanut.

14

AGROHYDROLOGIC STUDIES AND ANALYSES There are 3 general computations to be considered in the study. These are as follows: 1. Estimation of Run-off and Derivation of Inflow Hydrograph (25 yrs.) 2. Field Water Balance Computation 3. Reservoir Inflow Computation 1.

ESTIMATION OF RUN-OFF AND DERIVATION OF INFLOW HYDROGRAPH This would require the following data and inputs to be taken from the project site. These are topographic map soil and land capability mp or report, land use/vegetation map or report and rainfall intensities. The following arranged procedures would be helpful in deriving the inflow hydrograph. 1.1

Establishment of the Project Data a. b. c. d. e. f. g.

1.2.

Drainage Area, A, in sq. km. Mainstream length from outlet to highest ridge, L. Mainstream from outlet to point nearest basin centroid, Lc. Total fall (elevation difference) from highest ridge to outlet, H, in meter. Watershed gradient, Soil type of watershed (dominant) to determine the soil group identified soil type in the watershed belong to. Watershed cover/land use.

Estimation of Basin Lag Time, TL and time of Concentration TC using Snyder’s Method (revised), Time to peak, Tp and peak runoff, qp. a. Compute for unadjusted TL (TL in hours) Where: L = mainstream length from outlet to highest ridge, in miles LC = mainstream length from outlet to the nearest basin centroid. Y = watershed gradient a = 0.38 Ct = coefficient with values (Linsley’s): 1.2 for mountatins drainage area 0.72 for foothill drainage area 0.35 for valley drainage area b. Adjust estimate of TL Adjusted TL (for ∆D = 0.4 ≠ ) Adjusted TL = TL + ¼ (∆D -

)

1 c. Compute time of concentration, TC, in hours.

the

Method, and

TC = TL / 0.70 d. Compute the time to peak, Tp using Tp = ½ ∆D + TL (adjusted) Where: ∆D = time duration of one inch of excess rainfall (USDA SCS) suggested values of ∆D as 0.5 hr. (or 0.40 hr.) where Tc < 3; 1 hr. where 3 allowable max storage Then DRAINAGE = STORi – allowable max storage STORi = allowable max storage IRRIGATION = Ø. Ø If STORi < allowable minimum storage Then IRRIGATION = Optimum Storage – STORi STORi = Optimum Storage Drainage = Ø. Ø ELSE IRRIGATION = Ø. Ø DRAINAGE = Ø. Ø 14

Note:

For upland crops, allowable min. soil moisture storage is usually assumed to 50% of soil water holding capacities in the root zone, that is 0.54 (WHC) (ROODEP). Do not irrigate during the last two decade of a given period.

2.10

Use an irrigation efficiency if 51% for paddy rice (lowland) and 54% for upland crops to the estimated net crop irrigation to get an estimate of system irrigation requirements.

TABLE 9

Soil Water Holding Capacities of Different Soil Textural Classes:

Soil Texture Sandy Sandy Loam Loam Clay Loam Silty Clay Clay

Total Available Moisture Pv =Pw As Volume% 8 (6-10) 12 (9-15) 17 (14-20) 19 (16-22) 21 (18-23) 23 (20-25)

15 3.

ESTIMATION OF 10 – DAY RESERVOIR INFLOW

3.1

For Regions I, III, IV, characterized by distinct wet and dry seasons, 10 – day reservoir inflow are estimated as follows:

a.

DQj = RCj . Pj Where: DQj = direct runoff in decade j (mm) RCj = runoff coefficient in decade j, equal to estimated mean monthly runoff coefficient Pj = 80% dependable rainfall

b.

10 – day Baseflow BFj = F .Qj – 1 Where: BFj = baseflow in decade j (mm) F = 10 – day reservoir factor = 0.002 + 0.026 (D.A.) where DA is drainage area in sq. km. (This regression equation analysis of several small watersheds