Hydrological and Pollutant Export Responses to Rainfall Intensity in Paddy Fields

Agricultural non-point-source pollution poses a significant threat to water quality and sustainable water resource management, a challenge intensified by climate change-induced increases in rainfall intensity. In this study, we quantified how rainfall intensity controls runoff and the export of key pollutants (COD, TN, and TP) from paddy fields. Controlled simulation experiments with two rainfall intensities (40 mm/h, S40; 120 mm/h, S120) were conducted in the Yangtze River Basin. The results showed that high-intensity rainfall (S120) nearly doubled the surface runoff volume and coefficient compared to S40. A notable finding was the observed asymmetric response between pollutant concentration and export load. Despite a marked dilution effect under high-intensity rainfall that sometimes led to lower concentrations of COD and TP, the total export loads of all pollutants increased sharply due to the overwhelming increase in runoff volume. Specifically, COD export rose from 2.21 kg/ha (S40) to 4.90 kg/ha (S120), and TN export increased from 211.71 to 585.16 g/ha. In May, TP export under S120 was 2.5 times greater than that under S40. These results provide critical evidence and a mechanistic basis for developing climate-adaptive, sustainable nutrient management strategies aimed at mitigating water pollution and enhancing the environmental sustainability of rice production systems in the Yangtze River Basin and similar monsoon-affected regions.