Available-water adjustment improves monthly runoff simulation in cropland-dominated basins: A generalized water–energy complementarity model

Reliable monthly estimates of runoff and effective water availability are important for seasonal water assessment and agricultural water-resources planning. In cropland-dominated basins, storage, drainage, lateral water exchange, and water regulation may cause the effective water-energy constraints on evapotranspiration to deviate from conventional Budyko-type assumptions. We enhanced the generalized water–energy complementarity (GWEC) model by introducing separate adjustments to effective water and energy constraints and coupling them with dynamic basin storage for monthly runoff simulation. The model was evaluated using continuous records from 802 basins and compared with the Mezentsev–Choudhury–Yang equation (MCY), the proportionality-hypothesis formulation of Wang and Tang (PHWT), and the ABCD water-balance model. For each basin, the first 70% of the record was used for calibration and the remaining 30% for chronological validation. During validation, GWEC achieved median NSE values of 0.60 (CI: 0.57–0.63) and 0.77 (CI: 0.74–0.77) in two datasets, with significantly higher basin-wise NSE and KGE than the benchmarks. In cropland-dominated basins, GWEC achieved a median validation NSE of 0.65, exceeding the benchmarks by 0.02–0.11, and showed the smallest calibration-to-validation decline among the three dominant land-cover groups. Improvements were also more pronounced in catchments with higher baseflow contributions, consistent with better representation of storage and delayed water release. These results indicate that adjusting effective water availability improves temporal robustness under cropland-dominated conditions. Because irrigation withdrawals, return flows, and crop water demand are not explicitly represented, GWEC is best suited to regional screening and seasonal basin-scale water-availability assessment rather than operational irrigation decision-making.