Water Saving Strategies to Reduce Drainage from Flood Irrigation
The key to reducing tailwater drainage and deep percolation is to irrigate very fast and control how long the water is on the field. Good irrigation practices can result in overall irrigation efficiencies of 75 percent. These irrigation strategies were developed with the U.C. Cooperative Extension during the 1987 to 1992 drought and they still apply to today’s practices.
- Smaller or narrower checks irrigate better.
For example: on sandy soils use 1 acre checks 65 to 55 feet wide by 700 to 800 feet long; on loamy soils use 1.5 to 2 acre checks 80 to 90 feet wide by 800 to 1000 feet long.
- On existing large checks, install a berm in the center to make 2 narrow checks.
- A high uniformity of irrigation means all parts of the field have the same amount of water for the same time. This also reduces deep percolation losses.
- Laser level to insure cross slope is flat and check slope is uniform.
- A narrow check with normal slope of 1/10 percent irrigates more uniformly across the check than a wider check with a steeper slope of 2/10 percent.
This will reduce ponding, deep percolation losses, and tailwater volume. Cutback irrigation works because the water on the upper portion of the check will continue to flow down the check after the shutoff and irrigate the remainder of the check.
- Change shutoff times during season to match how soil seals between irrigations. Midseason cultivation will also change the shutoff time
- Shut off times will vary through the season. The first irrigation may require shutoff when the water reaches 90 percent of the distance down the check
- By the third irrigation this shutoff time may be reduced to only 70 percent of the distance down the check
- To reduce drainage volume, how long you irrigate is just as important as how often
- On sandy loam soils 1.25 inches of applied water fills 1 foot of soil; on sandy soil 1 inch of water fills 1 foot of soil
- The plants can only utilize 50 percent to 60 percent of the water applied to the soil
- Understanding the crop rooting depth which varies with the stage of growth, can save water
- The highest deep percolation losses occur early in the growth season for annual field crops when the roots are not well developed
- For example, the depths of corn roots are typically 3 feet during the peak stage of crop growth meaning each irrigation should require 3 inches on a sandy soil. But, since the irrigation efficiency is 75 percent, the total amount of water needed to reach the entire root zone is 4 inches, or 25 percent more than and be used by the crop.
- For winter oats the rooting depth in May is typically 18 inches while alfalfa roots can reach 5 feet
- Use crop water use or evapotranspiration (ET) information will help determine how much water is taken from the soil between irrigations
- A typical corn crop uses .27 inches of water per day at peak growth in early August
- On sandy soil with 3 foot roots, there is about 60 percent of the 3 inches of water in the soil or 1.8 inches of water available for the plant between irrigations
- To keep the soil moisture at the optimum level for best yields, irrigate every 7 days
- For the same crop and soil in June, the corn uses only .l3 inches per day and irrigations could be 13 days apart
- Because crop ET varies seasonally, use the current year ET values to schedule irrigations
- The local CIMIS site is Denair #168
- To obtain optimum yield, irrigation practices need to be tailored to specific situations
- Your local Farm Advisor can be of assistance in developing these practices