|Irrigation Training Manual: Planning, Design, Operation and Management of Small-Scale Irrigation Systems (Peace Corps, 1994, 151 p.)|
|Section 5: Assessing irrigation water requirements|
1. List 5 factors that you must quantify in order to determine the design capacity of an irrigation system.
Crop water use
Hours per day of operation
Area to be irrigated
2. In February, at a station with a latitude of 14°N, daily maximum average temperatures are 28°C and average daily minimum temperatures are at 15°C. What ETo might you expect?
Ra = 13.6 mm/day (from Table 4.1, Irrigation Reference Manual),
Td = 28 - 15 = 13°C
T° C = 1/2(28 + 15) = 21.5
ETo = 0.0023 x Ra x Td1/2 x (T° C + 17.8) = 0.0023 x 13.6 x 131/2 x (21.5 + 17.8) = 4.43 mm/day
3. With low wind and high humidity conditions, what maximum Kc might you expect for a lettuce crop at mid-season?
Kc is approximately that of cabbage and equals
0.95 for low advective conditions (from Table 4.3, Irrigation Reference Manual).
4. What ETc might you expect at mid-season for the conditions of Problems 2 and 3?
ETc = Kc x ETo = 0.95 x 4.43 = 4.2
5. What ETc might you expect initially in the season for the lettuce crop if ETo is 5 mm per day?
ETc = 0.4 x 5.0 = 2.0 mm/day
6. If your mean rainfall is 50 mm for February and is 80% effective, what is the average monthly effective rainfall (Pe) in mm/month? mm/day?
Pe = Pmean x effectiveness
= 50 mm x 0.8 = 40 mm/month
= 40/28 = 1.4 mm/day
7. What is the net daily water requirement (Ire) with the ETc of Problem 4 and the Pe of Problem 6? Assume Wb and Gw = 0.
Irn = ETc - Pe = 4.2 - 1.4 = 2.8 mm/day
8. What is the gross daily irrigation requirement (Irg) for the example of Problem 7 if the efficiency of application is 70% and the conveyance efficiency is 80%?
Irg = Irn / Ea x Ec = 2.8 / 0.7 x 0.8= 5.0 mm/day
9. What is the design capacity, using Irg of Problem 8, of a system that will operate 12 hours per day every day on a 2-hectare plot?
Q = 2.8 x A x Irq / t= 2,8 x 2 x 5.0 / 12= 2.3 L/sec