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Variable rate irrigation (VRI) systems are commercially available and can easily be retrofitted onto moving sprinkler systems. However, there are few reports on the application performance of such equipment. In this study, application uniformity of two center pivots equipped with a commercial VRI system were tested using fixed-plate sprinklers and a range of irrigation rates (100%, 80%, 70%, 50%, and 30%). Catch cans were arranged in transect, arc- wise, and grid patterns to test the accuracy of application depth, and the uniformity of application in the direction of pivot travel and to investigate changes in uniformity along the pivot lateral between irrigation zones. The mean Heermann and Hein coefficient of uniformity (CUHH) and the mean lower-quarter distribution uniformity (DUlq) in the direction of pivot travel and along the pivot lateral were approximately 88% and 80%, respectively. Application uniformity was impacted at the border of adjacent irrigation zones along the pivot lateral when zones were applying different irrigation depths. While wind speed and direction did not appreciably decrease uniformity of application, it did impact absolute catch measurements. The mean evaporation and drift loss for all trials was 9.3%, and ranged from 1% to 19%. Applying variable rate irrigation significantly impacted uniformity of application for a distance 3 m wide between irrigation zones of different irrigation depths in spans 1-3 of the three-span, and a distance 6 to 9 m wide between irrigation zones in spans 5 and 6 of the six-span center pivot. Overall, the uniformity of application in the direction of pivot travel and within each irrigation zone was similar to values reported for other VRI sprinkler systems. The root mean square error between the prescribed and actual applied depth was <3.0 mm for catch collected in all spans along the pivot lateral and in the direction of pivot travel. This indicates that this commercial VRI is well suited for site-specific irrigation management. However, uniformity of application could be affected by the width of the irrigation management zones as well as sprinkler design and environmental factors.

Assessing Application Uniformity of a Variable Rate Irrigation System in a Windy Location