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(-)& sup1; . Following irrigation cutoff, ETc act declined from 6 to 9 mm d(-)& sup1; to sustained levels near 1-3 mm d(-)& sup1; , 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 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.