Enhancing nitrogen simulations in SWAT for paddy-dominated catchments through improved nitrogen balance module

Rice irrigation districts face the challenge of maintaining yields while achieving water conservation and emission reduction. The Soil and Water Assessment Tool (SWAT) model is widely used for hydrological and water quality assessments in irrigation districts. Still, it has notable limitations in representing nitrogen cycling processes in rice paddies due to improper description of ponded water nitrogen dynamics. Therefore, this study improved SWAT based on paddy nitrogen balance, introducing processes such as nitrogen fertilizer hydrolysis, ammonia volatilization, nitrification, denitrification, and leaching in ponded water. It also considered soil adsorption-desorption of ammonium nitrogen, nitrogen diffusion driven by concentration gradients, and nitrogen transport through irrigation and drainage and added ammonium nitrogen transport processes at the basin scale. Subsequently, the improved SWAT-Paddy Water and Nitrogen (SWAT-PWN) model was evaluated in the Yangshudang basin (55.7% paddy fields) in the Zhanghe Irrigation District, China. Results showed that SWAT-PWN accurately simulated field‑scale ponded water total nitrogen(TN) dynamics and ammonia volatilization fluxes of paddy rice, and performed well in simulating basin-scale runoff and TN loads. Scenario analysis under eight water and nitrogen management modes (WNMMs) from 2005 to 2022 indicated that, compared to the current WNMM (CFN180F1), adopting alternate wetting and drying irrigation with split fertilization (AWDN180F2) could reduce irrigation water use, ammonia volatilization, and outlet TN load by 10.93%, 21.44%, and 7.72%, respectively, without yield reduction. This study provides a validated modeling tool to support water‑saving and emission‑reducing management in rice‑based irrigation systems.