Field-scale evaluation of OpenET for quantifying consumptive water savings under deficit irrigation in alfalfa

Deficit irrigation is increasingly used to reduce agricultural water use in arid regions, yet reliable field-scale quantification of consumptive water savings under commercial conditions remains challenging. Most satellite-based evapotranspiration (ET) studies focus on well-watered systems, whereas deficit irrigation imposes distinct soil–canopy controls on ET response. This study evaluated OpenET satellite-derived actual crop ET (ETc act) for quantifying summer deficit-irrigation impacts in alfalfa systems of California’s Imperial Valley using a two-year field analysis (2024–2025) across 20 grower-managed fields. OpenET ETc act estimates were benchmarked under full irrigation using eddy covariance and under deficit irrigation using SM-RZD. Under full irrigation, OpenET products reproduced daily ETc act dynamics with good agreement, with mean bias errors generally below 0.5 mm d⁻¹ . Following irrigation cutoff, ETc act declined from 6 to 9 mm d⁻¹ to sustained levels near 1–3 mm d⁻¹ , driven primarily by soil-water depletion and reduced transpiration rather than atmospheric demand, supported by declines in NDVI and Kc act. Deficit irrigation reduced cumulative ETc act by 150–200 mm per deficit period, corresponding to 40–50% lower consumptive water use relative to full irrigation (p < 0.001). Despite rainfall variability during the wetter season, deficit-irrigated fields exhibited significantly lower cumulative ET, and the OpenET ensemble provided the most stable representation across conditions. These results indicate that satellite-derived ETc act, when evaluated under water-limited conditions and interpreted using soil-moisture and canopy indicators, provides a credible and operational basis for verifying consumptive water savings from deficit irrigation in arid agricultural systems.