Optimizing Water Allocation and Simulating Groundwater Overexploitation Control Through a One-Way GWAS–GMS Coupled Framework: A Case Study of the Kuitun River Basin

The “Golden Triangle” area of the Kuitun River Basin is a typical arid irrigated region where limited surface-water supply and intensive groundwater abstraction have caused persistent groundwater decline and overexploitation. This study developed a one-way semi-loosely coupled GWAS–GMS/MODFLOW framework for allocation optimization, spatial projection, groundwater simulation, and effectiveness evaluation. An improved NSGA-II-S algorithm was used to optimize multi-source water allocation, and the resulting management-unit outputs were projected into MODFLOW source–sink terms using area-weighted transformation and mass conservation. GWAS satisfactorily reproduced monthly runoff at the Jiangjunmiao station, and GMS/MODFLOW reasonably matched observed groundwater heads at six national monitoring wells (R2 = 0.952–0.998; RMSE = 0.15–0.355 m; MAE = 0.12–0.315 m). Three 2030 scenarios were compared: baseline, optimized allocation, and combined pumping restriction with allocation adjustment. Under the baseline scenario, groundwater storage remained in deficit. The optimized allocation scenario expanded the quasi-stable zone (−0.1 to 0.1 m/a), whereas the combined scenario most effectively suppressed severe decline hotspots, reducing the proportion of the −3 to −2 m/a zone from 11.22% to 2.68%. These results indicate that recharge reallocation and pumping restriction play complementary roles in groundwater overexploitation control. The framework provides a quantitative basis for coordinated multi-source water management in arid irrigated basins.