Exam: Section 5 - Calculating water requirements

1. List 5 factors that you must quantify in order to determine the design capacity of an irrigation system.

Answer:

Crop water use
Effective rainfall
Irrigation efficiencies
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?

Answer:

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 Td^1/2 x (T° C + 17.8) = 0.0023 x 13.6 x 13^1/2 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?

Answer:

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?

Answer:

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?

Answer:

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?

Answer:

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.

Answer:

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%?

Answer:

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?

Answer:

Q = 2.8 x A x Irq / t= 2,8 x 2 x 5.0 / 12= 2.3 L/sec