Effects of urban stormwater pollution on watershed diffuse loads under extreme precipitation conditions

High-quality stormwater management is crucial for sustainable urban development and the protection of aquatic ecosystems, particularly because urbanization and climate-induced extreme events intensify runoff and pollution risks. Although urban stormwater pollution and its potential impacts have been addressed, its impacts on diffuse pollutant loads under compound climate have not been well evaluated. Our study focused on the Bahe River Basin and two river sections at the inlet and outlet of Xi’an city and quantified the impacts of extreme climate and high load risk. The contributions of different precipitation events to pollutant losses were explored using the Soil and Water Assessment tool (SWAT). Cropland was the main source of total nitrogen (TN) and total phosphorus (TP), accounting for 94 % and 91 % of the total pollution during single extreme precipitation events, respectively. However, during compound drought and extreme precipitation events, more than 50 % of the TP pollution originated from urban land. Attribution analysis showed that the heavy precipitation amount (R95P) and frequency (R20MM) were the main factors affecting TN and TP loads, accounting for 65–78 % and 83–88 % of the total impact. In addition, these factors have a greater impact downstream of the city, indicating that urban stormwater pollution increases the response of diffuse loads to climate extremes. The SWAT and copula function were combined to quantify the high-load probabilities under different climatic conditions. The probabilities of the TN and TP load exceeding the top 5 %–30 % were 4 %–19 % and 3 %–21 %, respectively, when the precipitation exceeded the top 20 %, and the probabilities of the TP load downstream of the city were more susceptible to high precipitation. Our study highlights the role of urban stormwater pollution on watershed diffuse loads in addition to point source pollution, with urban development potentially leading to more pollutants being transported to the receiving rivers.