Irrigation Water Management - Stretching Irrigation Water Supplies

by W.L. Trimmer (Washington State University, Pullman Extension Service) and James W. Bauder (Montana State University Extension Service)

There are many places to lose water in a typical irrigation system, and there are four basic ways to conserve that water. Schedule your irrigations, and apply only the water required. Apply water uniformly. Have a tight system. Change cultural practices. The following guidelines give practical ways to save water in your irrigation system.

Schedule Irrigations

Irrigation scheduling entails figuring out how much water crops use and how much the soil can store. Knowing this, you can scientifically determine when to irrigate and how much to apply. Most irrigation systems have the capacity to apply too much water early in a season. Scheduling helps you get the best use out of your irrigation water. Use the irrigation schedule to run the irrigation system only long enough to refill the root zone. When using sprinkler systems, apply the greatest depth of water that's possible without runoff. Odd set times may pose problems for systems and labor geared to12-hour set. To solve this problem, use a time clock to shut off systems automatically.

Use a Moisture Probe

Probe two or three times during an irrigation within a few feet of the lateral and halfway between sprinklers to confirm that the soil has the capacity to hold more water from center pivots, flood, and wheel line systems.

Try Different Crops

Cultural practices can affect the amount of water used. Substitute shorter-season crops into your rotation. Alfalfa and grasses need water all season, but wheat and barley need water only early in the season. Choose a crop variety with a shorter growing season.

Know Your Water Supply

Don't overextend your water supply. Plant only the land area that you can cover with your irrigation water supply. Employ drought-tolerant cover crops to protect fallowed land from wind erosion.

Be Flexible

Be ready to change. Adjust your fertilizing program when you expect drought. Set realistic yield goals. Use a soil test to avoid excess fertilizer.

Apply Water Uniformly

No system applies water uniformly to the soil; some areas will be overirrigated while others are underirrigated. Despite this, correct nozzle size, sprinkler spacing, and system pressure help achieve better uniformity. Check the pressure of the sprinkler at both the farthest and the highest points.

Watch the Weather

Keep evaporation losses to a minimum by avoiding irrigation during periods of high wind and warm temperatures. To do this, you need temporary storage if the water is delivered continuously. Sprinkler losses from a 5/32" nozzle will be about 9% in winds of 5 mph with a temperature of 80° F. These losses increase to 20% if the wind increases to 15 mph. With 15-mph wind and temperatures of 100° F, the losses reach 26%. High winds also distort the application uniformity.

Reuse water

Maybe the most important surface irrigation conservation practice is a tailwater reuse pit. You can reuse water by collecting runoff water at the end of the field and pumping it back to the top of a farm through an inexpensive pipeline. Reused tailwater is usually less expensive, including both equipment and operating costs, than purchasing additional water from an irrigation district.

*Excerpted from "Conserving Water in Agriculture: Stretching Irrigation Water Supplies", Pacific Northwest Extension Publication, PNW323.