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!