Sprinkler-Irrigated Corn Fields Show Higher Realization of a Reference-Field Evapotranspiration Benchmark: A Six-Season Satellite Analysis of Colorado’s South Platte Corridor

In appropriated basins, reduced diversions do not necessarily conserve water because return flows can sustain downstream supply. This study compared the crop water use of sprinkler- and flood-irrigated corn using Landsat-based AgroET retrievals for 12,430 corn-majority field-years on 5091 fields in Colorado’s South Platte corridor (2020–2025). Realization was AgroET-estimated evapotranspiration divided by a benchmark fitted to reference fields on a self-calibrated alfalfa-reference basis. Irrigation method and water source came from a 2020 parcel layer and were not verified annually. Mean realization was 96.2% for sprinkler and 91.0% for flood field-years (unadjusted difference, 5.2 percentage points; 95% CI, 4.6–5.8), despite similar benchmark evapotranspiration. The primary adjusted estimate, the geometry-adjusted average marginal contrast from the water-source by irrigation-method interaction model, was 4.10% (95% CI, 3.22–4.99); additive, matched, and field-level sensitivity estimates were 3.97%, 3.8 percentage points, and 4.06 percentage points. Mean realization was 93.3% in the archived pipeline and approximately 94.0% after correcting a reference-field selection gate; it rose to 97.0% among field-years with at least thirteen overpasses. The year 2021 remained a year-specific anomaly of unresolved cause. These observational associations do not identify irrigation-conversion effects or quantify return-flow changes; consumptive-use changes should be verified before crediting diversion reductions as conservation.