The Soil and Water Assessment Tool (SWAT) simplified the hydrological processes in large patches of paddy fields. Based on the original SWAT code, the main equations are modified and validated with the long-term field observations in freezing-thawing watershed. The original SWAT and SWAT with paddy module (SWAT-P) both perform well in the simulation of stream flow at the basin scale (NSE ≥ 0.7). SWAT-P performs better in the soil water simulation (RMSE = 24.22 mm, smaller than 104.63 mm of SWAT). The monthly diffuse nitrogen loadings simulated by SWAT-P are within a reasonable range and close to the monitored nitrate loads in 2011 and 2012. SWAT-P represents the ponding water depth dynamics during the rice growing period. The original SWAT overestimates the diffuse pollution loads in the freeze-thaw period. SWAT-P has better performance for water cycles and diffuse pollution load simulations in watersheds with large rice paddy fields.

土壤和水评估工具（SWAT）简化了大片稻田中的水文过程。在原始SWAT代码的基础上，通过对冻融流域的长期现场观察，对主要方程进行了修改和验证。原始的SWAT和带有稻田模块的SWAT（SWAT-P）在流域规模（NSE≥0.7）的水流模拟中均表现良好。SWAT-P在土壤水分模拟中表现更好（RMSE = 24.22 mm，小于SWAT的104.63 mm）。SWAT-P模拟的每月弥散氮负荷在合理范围内，接近于2011年和2012年监测的硝酸盐负荷。SWAT-P代表水稻生长期的池塘水深动态。原始的SWAT高估了冻融期的扩散污染负荷。