Uniform water application essential for irrigated plots and research

Assuring Irrigation system uniformity is an essential step in conducting irrigated crop research and demonstrations.

Much irrigation research is driven by the desire to take the lack of rainfall out of the production equation. This is based on the idea that irrigation applications are always going to be uniform. Unfortunately, experience has shown us that this is not always the case. Irrigation system uniformity is the ability of an irrigation system to apply even amounts of water to all areas within an irrigated field. In simple terms, if the producer’s goal is to apply one inch of irrigation water, the system will accurately deliver one inch of irrigation water across the entire area.

Water and fertigation applications can vary widely across a field if the system is not checked for uniformity. It is important to understand that the effect of uneven application rates are cumulative. Sprinklers that over and under apply water by 40 percent will provide 0.6 inches (if under) or 1.4 inches (if over) of irrigation water each time the system attempts to apply one inch of water. By the end of the season, areas requiring eight inches of irrigation water will receive 4.8 inches (if under) or 11.2 inches (if over) of irrigation water. Using the example discussed above, if 100 pounds per acres of nitrogen is applied through the irrigation system some areas will receive as little as 60 pounds and others as much as 140 pounds.

An irrigation system uniformity evaluation is conducted by sampling the output from a system and identifying areas of the sprinkler package that need improvement. Several procedures have been developed for test irrigation uniformity. All involve running the irrigation system over a set of uniform sized cups and measuring the amount of water collected. In a perfect world, each cup should catch an identical volume of water.

Many center pivot systems operate at about 75 percent uniformity, which leaves a 10-15 percent improvement in efficiency achievable. With proper adjustment of the end gun, a careful match between water supply volume/pressure and the sprinkler package requirements, almost all center pivot systems can achieve a 90 percent rate of uniformity of application or higher.

As an irrigation educator with Michigan State University Extension and Purdue Extension, I am often surprised about just how many irrigated field crop research plots are established each year without a thought given to how water application differences may impact yield variation as compared to the crop management factor or main treatment effect being investigated. Most producers have experienced the yield loss caused by drought, yet many assume that if data was collected from an irrigated trial, water related yield variation was eliminated. A simple irrigation system uniformity evaluation before a research plot is established can allow time for correction of the irrigation system or a plot design change that can minimize the effect of irrigation variability.

Center pivot irrigation systems often have a "sweet spot" for uniformity in their coverage area near the third of the machine furthest from the pivot point. This area represents the majority of the coverage area of the field and avoids the long duration of the application near the center point that may add an unwanted additional factor into the study. Avoid the end gun coverage area and its interface zone with the sprinkler package. These areas may have large amounts of variation as the uniformity can easily affect by wind. Also, crop damage caused by higher flow and pressure from the end gun may not represent the entire field.

Irrigation run-off issues can also cause a tremendous variation in water available in the plant root zone affecting potential yield. Avoid drop nozzles that have spray patterns affected by upper leaves of the crop that can disrupt the distribution pattern and increasing potential run-off issues. To determine if this is happening, walk just behind the application area, observe a typical application volume, starting at the end gun and moving toward the center point. Potential runoff will appear as sheen on soil surface, or in more obvious incidences, directly as runoff or ponding of water.

To learn more about avoiding Irrigation run-off, the irrigation system uniformity evaluation process, to view example results from system evaluation trainings or to download the Irrigation System Uniformity spreadsheets, visit the MSU Extension Irrigation Resource page.  

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