Impacts of extreme climate on nitrogen loss in different forms and pollution risk with the copula model

Abstract

Climate change is a key factor that profoundly affects aquatic environments. Because of climate warming, the increase in the intensity and frequency of extreme climate events has aggravated the uncertainty of nitrogen pollution. However, the risk of nitrogen loss under different climatic conditions has not been well assessed, which is of great significance for controlling diffuse pollution. In this study, we used the upper and middle Wei River Basin (UMWB) as the study area, and selected organic nitrogen (Org-N) and nitrate (NO3-N) as the two forms of nitrogen pollution. Then, we quantified the contributions of 10 climate factors and combined the Soil and Water Assessment Tool (SWAT) and copula to analyze the risk of pollution when extreme weather occurs. Our results showed that during periods of high precipitation and temperature, Org-N loss accounted for 96% and 83% of the total loss, and nitrate loss accounted for 74% and 67%, respectively. Org-N loss responded more strongly to high precipitation than nitrate loss because Org-N was transported with soil particles. The attribution analysis indicated that high precipitation amount (R95P) contributed to the largest Org-N loss. As for the nitrate loss, R95P, normal precipitation amount, and consecutive days with no precipitation were the most important climatic drivers, accounting for 35%, 32%, and 13% of the watershed area, respectively. After selecting critical source areas by identification method, an optimized copula model for nitrogen loss and the main climatic factors was proposed. The risk of nitrogen pollution under the defined climate severity was then quantified. The probabilities of Org-N and nitrate loss exceeding the top 1%–20% were 0.2%–15% and 0.8%–10% when the precipitation exceeded the top 20%. The pollution risk caused by high temperatures is lower than that caused by precipitation. This study emphasized the dominant role of extreme climate in driving nitrogen loss and proposed a method for quantifying the risk of nitrogen pollution under specific climate conditions, which enabled managers to identify high-risk pollution areas and optimize management measures to prevent diffuse nitrogen pollution.