Study on Optimal Allocation of Agricultural Water Under Total Water Use Constraints in Grain-Economic-Forage Intercropping Areas with Severe Groundwater Overdraft and Intensified Water Competition Among Crops Taking a typical mixed farming area as the study subject, an optimal agricultural water allocation model based on the CPSP water balance model was constructed, integrating data on evapotranspiration , water resources, and land use from 2023 to 2024. Eight water allocation scenarios were established by incorporating management measures such as planting structure adjustment, a 5% reduction in total water volume, and irrigation schedule optimization. The agricultural water distribution and water-saving effects under different scenarios were comparatively analyzed. The results demonstrate that the CPSP model exhibits high simulation accuracy in the mixed farming area. The Nash efficiency coefficients for calibration and validation are 0.99 and 0.93, respectively, with small average errors, and the crop coefficient is identified as a highly sensitive parameter dominating ET variations. Compared to other scenarios, the comprehensive optimization scenario SC8 reduces the total allocated water to 1107.21×104m3, achieving a water saving of 212.75×104m3. Specifically, the water allocations for grain, cash, and forage crops are reduced by 141.65×104m3, 10.75×104m3, and 60.35×104m3, respectively. Furthermore, this optimal configuration accurately matches the spatio-temporal water requirement patterns of the crops. By implementing differentiated management across various irrigation methods, agricultural water resources are rationally optimized, providing a scientific reference for the efficient utilization of water resources in grain-cash-forage mixed farming areas.