Exam: Section 7 - Farm water management

1. Define: (1) irrigation interval, (2) management allowed deficit, and (3) irrigation duration.

Answer:

Irrigation interval is the spacing between irrigations (days).

Management allowed deficit (MAD) is the allowed soil moisture depletion between irrigations.

Irrigation duration is the time required to irrigate a field.

2. What percent of the available water in the root zone (FC WP) can be extracted from a soil without stressing a lettuce or carrot crop during mid-season? A corn crop? A cotton crop?

Answer:

Lettuce - a sensitive crop - use 30% to 40%
Corn - semi-tolerant - use 50% to 60%
Cotton - tolerant - use 60% to 70%

3. Given a field with a loamy sand soil with AW = 100 mm/m, root depth = 60 cm, and an allowable deficit of 50%, how much water can be depleted below field capacity before irrigation (i.e., what is the management allowed deficit)?

Answer:

MAD = p' x Aw x D = 0.50 x 100 mm/m x 0.60 m = 30 mm

4. What is the required irrigation interval if the soil profile is filled with each irrigation, and daily crop water use ETc is 5 mm/day? (Assume rainfall and groundwater contributions are negligible.)

Answer:

I = MAD /Etc + Ts = 30 mm / 5 mm/day + 1 = 7 days

5. If a farmer irrigates every 7 days while crop water use is 5 mm/day with no precipitation, and overall efficiencies are 50%, what net and gross depth of application will be required?

Answer:

dn = I x ETc = 7 x 5 = 35 mm
dg = dn/Ea = 35/0.5 = 70 mm

6. A farmer waters a 1-hectare field with 10 liters/sec in 24 hours of operation. What is his gross application depth? Answer:

Using the Continuity Equation, QT = dA
d = OT / A

Q = 10 L/sec =.01 m3/sec = 36 m3/hr
T = 2 4 hours
A = 1 ha = 10, 000 m²
d = 36 m³/hr x 24 hours / 10, 000 m²=.086 m = 8.6 cm

7. Describe 5 characteristics of a good irrigation.

Answer:

A good irrigation
1) is uniform
2) is adequate
3) does not have excessive runoff
4) is not erosive
5) is on time
6) minimizes runoff
7) minimizes deep percolation

8. Name 5 factors which the farmer can control to improve his irrigation.

Answer:

1) flow rate
2) irrigation interval
3) duration
4) irrigation method
5) slope