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Irrigating Through a Dry, Hot Summer

Irrigating Through a Dry, Hot Summer

By Leah Fronk and Elsa Sanchez 

In dry, hot summers, uneven precipitation causes plant stress which affects both yield and quality of produce. Managing irrigation properly is critical to successful vegetable and fruit production.
 
If you have been fortunate enough to receive rain the past few weeks, conserving soil moisture is of utmost importance. Weed management plays a big role in conserving soil moisture. Weeds can be avid consumers of water. Michigan State University research showed thirsty weeds in asparagus used as much as ½” water each week in the summer. Some practices to rid fields of competing weeds include clean cultivation, timely herbicide use, and surface mulch.
 
Of course, irrigation is vital to vegetable and fruit farms when summer rains have not come. Supplemental irrigation can make the difference between bringing a crop to market or not. Irrigation may be roughly divided into two categories: high volume and low volume. High volume irrigation includes sprinkler or overhead irrigation, while low volume includes trickle or drip irrigation. Drip irrigation applies water only to the soil which means less water is evaporated or lost than with overhead irrigation. With proper operation, drip irrigation is 95% efficient compared to sprinklers at 70% efficiency.
 
The big question is: how long do I need to run the irrigation system for my crops? As a general rule, plants need 1–1.5 acre-inches of water each week. When plants are small, aim for the low end of this range and when they are large, the upper end. Sunlight, temperature, relative humidity, and wind all play a role in how much water plants use on a given day and may encourage you to water more or less depending on the environment. When using plastic mulch, the entire 1–1.5 inches will need to be applied weekly regardless of rain events. With bare ground or when using water permeable mulches (for example, straw or paper mulch), the amount of water applied by irrigation should be reduced by the amount of rainfall the field receives. For example, if it rains ¼ inch, reduce the amount of irrigation water applied by ¼ inch for that week.
 
The time it takes a drip system to apply 1 acre-inch of water to the soil depends on the drip tube flow rate and the width the roots extend, which is generally the same as the width of most beds: about 30 inches. The table below shows how long, in hours, to run a drip system to apply 1 inch of water to the 30-inch width based on the drip tube flow rate.
 
Emitter Spacing (inches)Drip tube flow rate @ 8 psi (gph/100 ft)Drip tube flow rate @ 8 psi (gpm/100 ft)Time to apply 1 inch of water (hours)
Low flow1213.20.2211.8
Low flow820.40.347.6
Medium flow1220.40.347.6
Medium flow830.60.515.1
High flow12270.455.8
High flow840.20.673.9
High flow480.41.341.9

If you have a drip tape with a flow rate of 0.45 gpm per 100 ft, it would take 5.8 hours to apply 1 acre-inch of water to the 30-inch beds. Typically, this is applied through several weekly applications.

Taking it one step further, proper irrigation means not over-applying and causing nutrient leaching and runoff. Soil texture and available water holding capacity determine the rate at which water moves through the soil and therefore how long to run the drip system per application based on soil texture. To determine how long to run the drip system at one time, first find the available water holding capacity of your soil using a table such as the one below.

Soil Texture

Available water holding capacity
(inch of water/inch depth of soil)

Coarse sand / compacted sands0.02–0.06
Fine sand0.04–0.09
Loamy sand0.06–0.12
Sandy loam0.11–0.15
Fine sandy loam / compacted loam0.14–0.18
Loam and silt loam0.17–0.23
Clay loam and silty clay loam0.14–0.21
Silty clay and clay0.13–0.18
For example, if you have a sandy loam, the available water holding capacity is 0.11 to 0.15 inch of water/inch depth of soil. Pick a number within the range, say 0.12.
 
Then, using the table below and the drip tube flow rate, find the maximum time in minutes to run the drip system at one time.
  • *TFR = Tubing flow rate (gpm per 100 ft)
  • **mmpa = maximum minutes per application—assumes 10-inch deep root zone and irrigation at 50% soil moisture depletion
 

Available water holding capacity
(inch of water/inch depth of soil)

*TFR*TFR*TFR*TFR*TFR*TFR
 0.20.30.40.450.50.6
 **mmpa**mmpa**mmpa**mmpa**mmpa**mmpa
0.02412720181614
0.03614131282420
0.04825441373327
0.051026851464134
0.061228261554941
0.071439571645748
0.0816310982746554
0.0918412292837361
0.1204136102928268
0.112241501121019075
0.122451631221109882
0.1325517713211910689
0.1426619014212811496
0.15276204153138123102
0.16287217163147131109
0.17297231173156139116

In a hot, dry summer such as this, it will be evident if you have good moisture uniformity in your soil. Use this to your advantage and when you observe sections with declining or wilting plants, be sure to fix leaks in your irrigation system and check for stopped-up emitters. Another issue that can be overlooked with drip irrigation is forgetting to turn the system off. This can be remedied by connecting a timer to the pump so that it shuts off when you want it to, therefore saving time and water.

Source : psu.edu