Using stable water isotopes to assess the influence of irrigation structural configurations on evaporation losses in semiarid agricultural systems

The arid areas east of the Cascade Mountains in the Pacific Northwest have high irrigation water use and changing the elevation of sprinkler nozzles in these systems has been proposed as a strategy to reduce evaporative losses in order to diminish the total irrigation water demand. Validation of any structural modifications requires techniques able to identify water losses from these systems that were non-productive, particularly soil evaporation. In this study, we quantified evaporation losses from soils under Mid Elevation Spray Application (MESA) and Low Elevation Spray Application (LESA) irrigation systems by using stable isotope tracers to estimate the fraction of non-productive water losses (i.e. the ratio of soil water evaporation to irrigation water applied, E/I). During mid-summer 2017 in eastern Oregon and southeast Washington, we collected soil samples at multiple positions and depths around the central irrigation tower from four different agricultural fields, each with both LESA and MESA configurations. We measured the hydrogen and oxygen stable isotope ratio of soil water at MESA and LESA field locations using H2O liquid - H2O vapor equilibration laser spectroscopy. Though soil moisture contents were similar, the average isotope ratio of soil water under LESA irrigation (delta D = -114.5 parts per thousand, delta O-18 = -14.5 parts per thousand) had lower values than under MESA irrigation (delta D = -108.2 parts per thousand, delta O-18 = -13.1 parts per thousand). Calculated E/I values demonstrated higher sprinkler and soil water evaporation occurring at the MESA irrigated locations (E/I = 16.1%) compared to the LESA irrigated locations (E/I = 9.0%). We find that LESA systems have lower non-productive water losses than MESA systems and are thereby more efficient users of applied water. Our results suggest that stable water isotopes provide a technique for improving the management of water resources through the assessment of irrigation efficiency.