Name_________________________
Lesson 6: Mulches and Drip Irrigation - Scheduling Irrigation Practice Problems
Shallow
6 – 12 inches
Moderate
18 – 24 inches
Deep
More than 36
inches
Broccoli
Greens
Onion
Snap beans
Pepper
Cabbage
Cucumber
Muskmelon
Eggplant
Potato
Tomato
Asparagus
Lima Bean
Watermelon
(Seedless)
Table 1. Rooting depth of various crops. The bolded crop names means plastic
mulch generally is used with this crop and the rooting depth is shallow.
Soil Texture
Inches/Foot
Sands
0.5 – 1.0
Sandy Loam
1.0 – 1.5
Loams
2.0 – 2.5
Silt Loams
2.5
Clay Loams
2.0 – 2.5
Table 2. Water-holding capacity for various soil types.
Sandy loam
Field
Capacity1
5 – 10
25 Percent
Depletion2
10 – 15
Loams
10 – 15
20 – 30
Silt Loams
15 – 20
25 – 35
Clay loams
25 – 40
40 – 50
Soil Texture
Table 3. Useful set points for tesniometers.
1
Field capacity values indicate no irrigation required.
2
25 percent depletion of the available water (AWC) in the crop effective root zone.
1. You are growing a small crop of tomatoes in your high tunnel.
Soil type = Silt loam (Holds 2.5 inches available water per foot per acre – Table 2)
Rooting depth = 19.5 in. or 1.625 ft (Table 1)
Bed or row spacing = 2 ft between plants; 4 ft between rows
2 rows 36 feet long each
Wetted radius = 15 inches (30 in diameter; 2.5 ft.)
Crop wetted volume = Given formula of 1 acre-inch of water = 27,000 gallons
Square feet in an acre = 43,560
Our trickle system delivers .53 gallons per emitter per hour
How long will you need to run the pump to adequately run the pump to replace the
needed amount of water? ____3.5 to 4 hours ___
SHOW ALL OF YOUR WORK!
2 rows x 36 feet = 72 linear feet of bed
2.5 ft wetted diameter x 72 linear feet = 182 square feet/43,560 = .00413 acres under
trickle system
Rooting depth is 1.625 feet x 2.5 inches of water per foot = 4.0625 inches of water per
foot per acre at field capacity. We have .00413 acres so .00413 x 4.0625 = 0.0168 inches
x 27,000 gallons per inch = 453 gallons available to crop at field capacity. If we allow
25% depletion, the tensiometer should read 30 cbar (Table 3 – selected middle of correct
range), we would have lost 136 gallons of water (453 x .30 = 136).
We will apply about 136 gallons of water. Our trickle irrigation system delivers .53
gallons per emitter per hour. 72 linear feet of row equals 72 emitters, 0.53 gallons per
hour per emitter = 38.16 gallons per hour for the system. If we need to replace 136
gallons of water, then 136/38.16 = 3.5 or 4 hours to run the pump.
2. You are growing a cucumber crop in your high tunnel.
Soil type = Clay loam (Holds 2.4 inches available water per foot per acre – Table 2)
Rooting depth = 22 inches or 1.83 feet (Table 1)
Bed or row spacing = 24 in. between plants; 48 inches between rows
4 rows 30 feet long each
Wetted radius = 16 inches (32 in. diameter or 2.67 feet)
Crop wetted volume = Given formula of 1 acre-inch of water = 27,000 gallons
Square feet in an acre = 43,560
Our trickle system delivers .53 gallons per emitter per hour
a) Linear feet of bed = _120 linear feet _
4 rows x 30 feet = 120 linear feet of bed
b) Acres under plastic/trickle system = ___.00736___
2.67 feet of wetted diameter x 120 linear feet = 320.4 square feet/43,560 = .00736
c) Gallons available to crop at field capacity = __872.78__
1.83 ft. rooting depth x 2.4 inches of water per foot = 4.392 inches of water per foot
per acre at field capacity. .00736 acres x 4.392 = .032325 inches x 27,000 = 872.78
gallons available to the crop at field capacity.
d) Reading on tensiometer at 25% depletion* = __45 cbar__
*Table 5 – Pick the middle of the correct range.
e) How many gallons of water have been lost = __392.75 gal__
872.78 x .45 = 392.75 gallons of water lost
f) Pump Run Time = _6 to 6.5 hours_
120 linear feet = 120 emitters x .53 gallons per emitter per hour = 63.6 gallons per
hour from the system. To replace 392.75 gallons of water/63.6 = 6 to 6.5 hours
SHOW ALL OF YOUR WORK!