Optimal Allocation of Water Resources Under the “Four Waters and Four Determinations” Policy: A Case Study of the Kuye River Basin in Inner Mongolia, China

To alleviate the severe supply–demand imbalance and promote coordinated socioeconomic and ecological development in the Kuye River Basin of Inner Mongolia, this study constructed an optimal water resources allocation model based on the General Water Allocation and Simulation (GWAS) model. With 2023 as the base year, monthly-scale optimal allocation of water resources was conducted for both the short-term planning year (2030) and the long-term planning year (2035), and appropriate development scales were proposed in accordance with the “Four Waters and Four Determinations” principle. The results indicated that the total volume of optimally allocated water was 195.25 × 106 m3 in 2030 and 207.41 × 106 m3 in 2035, with overall deficit rates of 3.63% and 5.02%, respectively. Deficits occurred during the agricultural irrigation season from March to November and were mainly concentrated from May to September. The water supply structure was markedly optimized in both planning years. The surface water share rose from 13.28% to 14.03%; the unconventional source share rose from 47.50% to 52.13%; and the groundwater share dropped from 39.22% to 33.84%. In terms of water allocation, industrial allocation rose steadily while agricultural allocation decreased from 13.52% in 2030 to 10.22% in 2035, as water consumption shifted toward high-benefit, water-efficient sectors such as industry and domestic use. Under the “Four Waters and Four Determinations” policy, the agricultural irrigation area that could support sustainable development was calculated at 15.93 × 104 mu in 2030 and 15.07 × 104 mu in 2035. The development scales of urban construction, population agglomeration, industry, and services were also estimated. These results provide a quantitative basis for green, high-quality development under rigid water constraints in the Kuye River Basin. They also offer an important reference for other arid and semi-arid regions seeking to optimize water allocation and implement the “Four Waters and Four Determinations” policy.