Integrated allocation of groundwater and surface water resources in the Shijin Irrigation District for groundwater exploitation reduction and stable grain yield

Water resource shortage and groundwater over-exploitation have been the primary limiting factors on sustainable agriculture in North China. It is often required for the balance between "Grain Yield Stabilization and Groundwater Extraction Reduction". This study aims to allocate groundwater and surface water resources for the groundwater exploitation reduction and stable grain yield. A case study was taken as the Shijin Irrigation District, Hebei Province, China. A process model was developed to strategically couple three models: SWAP, MODFLOW, and WEAP. A series of simulations were conducted to integrate with the water cycle. The framework was used to effectively capture the dynamic interactions between surface water, soil water, and groundwater. Four scenarios were designed for comparative analysis: the Current Scenario (S0), the Water-saving Irrigation Scenario (S1), the Planting Structure Adjustment Scenario (S2), and the Comprehensive Regulation Scenario (S3). The long-term simulations were then performed on the groundwater levels and crop yields from 2025 to 2036. An optimal allocation of multiple source water was formulated under these scenarios. The results indicated that the reliable performance was achieved to simulate the groundwater level, soil moisture content, and crop yield, with the determination coefficients (R²) of over 0.58. Compared with the Current Scenario (S0), the Water-saving Irrigation Scenario (S1) demonstrated the significant benefits: The annual water saving of the irrigation district reached 72 million m³, the amount of groundwater extraction reduction reached 46 million m³, while the groundwater level over the irrigation district raised cumulatively by approximately 3.5 m over the simulation period, and the total grain yield fluctuated within the range of 1.056 billion~1.126 billion kg. In the Planting Structure Adjustment Scenario (S2), the proportion of grain crops decreased from 70% to 60%, while the groundwater level exhibited a slow upward trend, and the grain yield remained stable at over 1 billion kg. Water-saving irrigation and adjustments were systematically integrated into various cropping patterns. An optimal mode was obtained suitable for the multiple source water allocation in the Shijin Irrigation District. Surface water was prioritized to supply from the Gangnan and Huangbizhuang reservoirs, whereas the groundwater served as a supplementary source. Compared with the current water allocation, the optimal mode raised the overall groundwater level of the irrigation district by about 4.68 m, and the fluctuation range of total grain yield was only 34 million kg, indicating both groundwater extraction reduction and grain yield stabilization. The research findings can provide the effective modeling on the water resources for the water-saving transformation and control of groundwater over-exploitation in similar multi-water-source irrigation districts in North China.