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 (ET c 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 ET c act estimates were benchmarked under full irrigation using eddy covariance and under deficit irrigation using SM-RZD. Under full irrigation, OpenET products reproduced daily ET c act dynamics with good agreement, with mean bias errors generally below 0.5 mm d⁻¹ . Following irrigation cutoff, ET c 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 K c act . Deficit irrigation reduced cumulative ET c 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 ET c 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.