Coupling Hydrological Scenarios and Water Footprints for Multi-Objective Cropping-Structure Optimization in the Middle and Lower Shiyang River Basin

Hydrological fluctuations affect cropping-structure optimization in arid inland river basins by altering agricultural water-supply boundaries and crop yields. This study focused on five counties and districts in the middle and lower reaches of the Shiyang River Basin. Based on the 1995–2024 runoff series, four hydrological scenarios—wet, normal, dry, and extremely dry—were constructed. Crop yield responses were incorporated into blue, green, and gray water footprint accounting, and NSGA-III coupled with entropy-weighted TOPSIS was applied for multi-objective optimization. The results showed that (1) under the current cropping structure, unit water footprints generally increased as hydrological conditions shifted from wet to extremely dry, with gray water footprints showing greater sensitivity to yield reductions; (2) the overall directions of cropping-structure adjustment were broadly consistent across scenarios, with the planted areas of wheat, maize, and tubers decreasing by nearly 30% and melon area increasing by nearly 30%, while vegetable and oilseed areas exhibited clear scenario-dependent adjustments; (3) relative to the corresponding 2024 cropping structure evaluated under the same hydrological conditions, the selected compromise schemes increased net economic return by 14.58–16.23 × 108 CNY across the four scenarios. However, the attainable net economic return declined from 73.74 × 108 CNY under the wet scenario to 17.95 × 108 CNY under the extremely dry scenario. The blue water footprint decreased by 18.18–19.07%, whereas the gray water footprint decreased by approximately 17–18% across scenarios. Deteriorating hydrological conditions reduced the attainable economic return and highlighted clear county-level differences in adjustment pathways and economic vulnerability. These findings provide a quantitative basis for scenario- and county-specific cropping-structure optimization and water-resource regulation in arid inland river basins.