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Abstract SWAT model is one of the primary tools for assessing irrigation district water management and water‐saving measures. However, its incapacity to consider the diverse growth and water requirements of paddy during various growth stages, as well as the insufficient availability of external water sources. This study introduces the Penman‐Monteith equation and Jensen model into the SWAT framework, setting crop coefficients, crop base coefficients, and growth stage sensitivity indices based on the different growth stage. Additionally, modifications are made to the external water source available for irrigation and paddy field leakage modules, establishing a distributed agricultural hydrological model suitable for accurately simulating water balance elements and paddy yield in multi‐source irrigation districts. The Yangshudang watershed in the Zhanghe irrigation district is chosen for the evaluation of the modified model's simulation performance, with a quantitative assessment of water‐saving and yield‐increasing effects. The results demonstrate that the modified model effectively meets the requirements for simulating paddy evapotranspiration of various growth stages, yield, agricultural irrigation water consumptions, and runoff, exhibiting a notable enhancement in performance. As two common water‐saving measures in irrigation areas, intermittent irrigation and irrigation district renovation were used as two water‐saving scenarios in the simulation of the modified SWAT model. Under intermittent irrigation, the watershed experiences a 6.58% reduction in net irrigation water use. In the scenario with irrigation district renovation, the water resources in the watershed are utilized more efficiently. The modified model from this study can be applied for assessing the synergistic effects of irrigation district water‐saving and yield‐increasing measures, providing crucial insights for the formulation of irrigation district water‐saving strategies and water resource optimization plans.

A experiment was conducted at the Jiangxi Province Center Station of Irrigation Experiment from 2019 to 2021 to study the water and nitrogen balance under water and fertilizer regulation modes. The study aimed to propose a recommended mode for paddy fields that could save water, control pollution, reduce gas emission, and improve fertilizer use efficiency. This study examined the impact of different irrigation methods and nitrogen application levels on water saving and emission reduction in paddy fields. The experiment included six treatments, which involved two irrigation methods (intermittent irrigation and flooding irrigation, referred to as W1 and W0, respectively) and three application rates of nitrogen fertilizer (0 kg/ha, 135 kg/ha, 180 kg/ha, referred to as N0, N1 and N2, respectively). The study found that irrigation methods had a significant effect on the amount of irrigation, drainage, leakage, nitrogen load from drainage, soil nitrification potential, and ammonia volatilization. The results showed that compared to flooding irrigation, intermittent irrigation reduced the amount of irrigation, drainage, leakage and nitrogen load from drainage by an average of 25.98%, 16.03%, 8.43% and 10.86%, respectively. However, the study also found that the nitrification potential and ammonia volatilization increased by an average of 6.45% and 4.32%, respectively. Fertilization levels had a significant effect on drainage nitrogen load, early soil nitrification potential and ammonia volatilization. Compared with the treatment of N2 (180 kg N/ha), the drainage nitrogen load under the treatment of N1 (135 kg N/ha) decreased by 10.86% on average, while nitrification potential and ammonia volatilization increased by 38.74% and 3.33%, respectively. In terms of nitrogen output, the amount of nitrogen absorbed by crops was the largest, followed by the nitrogen load from field drainage, then ammonia volatilization, and then denitrification. Considering the goals of water saving, emission reduction, and the efficient utilization of water and fertilizer in paddy fields, the optimal water and fertilizer regulation mode was the W1N1 mode (intermittent irrigation combined with reduced nitrogen fertilizer application rate, 135 kg N/ha).