Scenario-Based Optimization of Net Irrigation for Winter Wheat in the Huaihe Basin, China, Using AquaCrop-OSPy Constrained by Multi-Source Remote Sensing

Spatial heterogeneity and sparse management observations complicate regional irrigation planning. We constrained AquaCrop-OSPy with remotely sensed canopy cover (CC) and actual evapotranspiration (ETa), together with gridded profile soil moisture (SM), using Gaussian-process Bayesian optimization. Regional simulations covered 500 m cells with at least 70% winter wheat cover in the Huaihe Basin, China. Four stage-specific irrigation thresholds were optimized to maximize incremental irrigation water productivity while maintaining at least 95% of a model-defined reference yield. Three individually calibrated historical growing seasons represented dry, normal, and wet conditions. Separate evaluation against field observations from four growing seasons yielded normalized root mean square errors (NRMSE) of 18.29% for aboveground biomass and 4.54% for grain yield; county-level yield NRMSE ranged from 12.58% to 15.88%. Using annual winter wheat fractions as area weights, mean seasonal irrigation depths over irrigated wheat area were 154.98, 119.48, and 64.09 mm, with corresponding net irrigation volumes of 5.73 × 109, 3.81 × 109, and 1.16 × 109 m3. Excluding cells that violated the SM or ETa error criterion reduced these volumes to 5.01 × 109, 3.30 × 109, and 1.06 × 109 m3 over the remaining domain. Irrigation was concentrated during the sowing-emergence and flowering-grain filling stages under dry conditions, and during the sowing-emergence and jointing-heading stages under normal and wet conditions. Mean simulated yield reached 96.67% to 97.57% of the reference yield. These model-derived estimates describe optimized net irrigation under the specified objective and yield constraint, rather than minimum irrigation requirements or independently validated schedules. Scenario differences jointly reflect climate, initial conditions, calibrated parameters, and annual spatial coverage.