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Field scale quantification indicates potential for variability in return flows from flood irrigation in the high altitude western US

Gordon, Beatrice L., Paige, Ginger B., Miller, Scott N., Claes, Niels, Parsekian, Andrew D.
Agricultural water management 2020 v.232 pp. 106062
altitude, aquifers, drainage water, ecosystem services, flood irrigation, geophysics, groundwater, groundwater recharge, irrigated farming, irrigation management, issues and policy, riparian vegetation, streams, uncertainty, watersheds, wildlife habitats, Wyoming
The assessment of hydrologic ecosystem services associated with flood irrigation in the western United States is particularly limited by a lack of data about return flows. Return flows, the portion of applied water that returns to adjacent surface and groundwater hydrologic systems during flood irrigation, may provide wildlife habitat, recharge aquifers, and supplement late season baseflows. To improve understanding about variability in the timing and volume of return flows to improve management and policy decisions associated with different irrigation management techniques, our study used an agricultural water balance that combined hydrologic, geophysical, and meteorological tools. We directly measure key components of the water balance and quantify error and uncertainty on a ∼100 ha portion of the Bear Creek watershed in the Upper Wind River Basin of Wyoming. Over the course of a single irrigation season with average precipitation, we found that of the 1,133,000 m³ of water applied to irrigated lands, 328,000 m³ (29 % of applied water) returned to Bear Creek and 487,000 m³ (43 % of applied water) was consumptively used by vegetation. Our data also suggest that 59,000 m³ (18 % of total return flows) occurred more than two weeks after irrigation had ceased. The percent of return flow for this study site is slightly lower than the median of 43 % for all reviewed studies and the percent of delayed return flow—return flow after irrigation has ceased—is slightly higher than the median finding of 14 % across available studies. We found that return flows from flood irrigation at this site are lower than generalized regional estimates and those found by other studies. Thus, our study indicates that the amount and timing of return flows are not necessarily coupled (i.e. lower overall volume did not correspond to lowered late season contribution at this site). In addition, we found that these return flows support hydrologic ecosystem services including, riparian vegetation (wildlife habitat), groundwater storage, and late season baseflows.