Impact of irrigation technologies on withdrawals and consumptive use of water

Developments in irrigation technologies have subsequently led to increases in irrigation application efficiency, including conversion from surface to sprinkler irrigation, variable rate irrigation (VRI), and low energy precision application. However, while these increases in application efficiency may reduce water withdrawals and/or diversions (from groundwater or surface water), they may not necessarily translate to additional water being available for future or downstream use. Consumptive use of water, water consumed and not returned to the local or sub-regional watershed, is not necessarily impacted by increased application efficiency, but is an important metric in conserving water for other uses. One recent irrigation technology is VRI; while conventional irrigation scheduling methods generally do not consider terrain or root zone available water variability across a particular field site, VRI can account for such spatial variability. This research evaluated several site-years of field VRI research data in an attempt to quantify potential reductions in pumping and consumptive use from irrigation technology. The previously developed consumptive use ratio (change in consumptive use / change in irrigation water applied) provides a useful metric to estimate potential consumptive use reductions for irrigation technologies compared to conventional irrigation. The consumptive use ratio was calculated for different irrigation management scenarios, including VRI (utilizing zone control prescriptions), across four years and two field sites in Eastern and Western Nebraska. Comparing well-managed conventional irrigation scheduling to VRI scheduling, the consumptive use ratio was found to range between 63% and 77%. When VRI does result in reduced water withdrawals, it likely includes some reduction in both consumptive use and deep percolation of irrigation water. Finally, a theoretical approach was presented to model the marginal consumptive use ratio based on both Cobbs-Douglas and polynomial functions. The marginal consumptive use ratio depends on whether seasonal irrigation is in the deficit irrigation, full irrigation, or over irrigation range.