Location: Soil and Water Management ResearchTitle: Assessing application uniformity of a variable rate irrigation system in a windy location) Author
Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2013
Publication Date: 8/22/2013
Citation: Oshaughnessy, S.A., Evett, S.R., Colaizzi, P.D., Howell, T.A. 2013. Assessing application uniformity of a variable rate irrigation system in a windy location. Applied Engineering in Agriculture. 29(4):497-510. Interpretive Summary: Variable rate irrigation (VRI) systems provide flexibility in applying irrigation water along the lateral of a moving sprinkler system. This capability can allow treatments of inconsistent issues within a field, such as changes in elevation, soil texture, and disease or pest infestation. It is common to divide a field into management zones with the goal that each management zone be as homogeneous as possible. Although variable rate irrigation implies non-uniform irrigation on a field scale level, uniformity of application within a management zone is very important. We performed multiple catch can trials at Bushland, Texas on a three- and six-span center pivot sprinkler system to evaluate the performance of a commercial VRI system. Catch cans were placed in transect, radial, and grid patterns to test the uniformity in multiple zones along the pivot lateral at different watering rates. Results showed that the Heerman and Hein coefficient of uniformity was in the range of 87% to 95%. These values compared well to uniformity results reported for other VRI systems and non-VRI irrigation systems in the literature. Unexpectedly, there were no significant differences in uniformity for the higher watering levels or among span locations. Some irrigation water was lost to evaporation and drift during the tests, which was expected due to the high but typical wind speeds under which the tests were conducted.
Technical Abstract: 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 low drift nozzles and different pulsing rates (100%, 80%, 70%, 50%, and 30%). Catch cans were arranged in grid patterns to test the application uniformity within irrigation zones, in arc-wise patterns to test the circumferential uniformity of the system when irrigation rates were changed in the direction of travel, and in transect patterns to investigate the change in uniformity along the pivot lateral between irrigation zones. Results showed that the Heermann and Hein coefficient of uniformity (CUHH) and the lower-quarter distribution uniformity (DUlq) within irrigation zones (grid pattern) averaged 92.5% and 90.5%, while along the pivot lateral (transect pattern), mean CUHH = 87.6% and mean DUlq =80.0%. In the direction of pivot travel, the mean CUHH and DUlq were 87.6% and 81.8%, respectively. Average mean absolute error of the measured depth was < 3% of expected application depth. Application uniformity was impacted at the border of adjacent irrigation zones along the pivot lateral when zones where irrigating at different rates, but wind speed and direction did not appreciably decrease uniformity of application. The mean evaporation and drift losses were 9.2%. Overall, application uniformity from these trials for both center pivots was similar or better than results reported from other studies using VRI equipment.