Definition and control strategy of the key source areas of non-point source pollution based on SWAT model in Wuqiang River Basin, Zhejiang of China

With the improvement of point source pollution control and treatment technology, non-point source pollution has become an important source of water pollution. As the second largest tributary flowing into Qiandao Lake, it is important to quantify the non-point source pollution load in the Wuqiang River Basin, analyze the spatial and temporal distribution characteristics of non-point source pollution, and propose the best management practices (BMPs) suitable for reducing pollutants in the Wuqiang River Basin. Based on the Soil and Water Assessment Tool (SWAT), this study analyzed the temporal and spatial distribution characteristics of runoff and total nitrogen output load in Wuqiang River Basin, explored the reduction effects of different management measures and combinations, and proposed targeted treatment measures for non-point source pollution in Wuqiang River basin. The results showed that: 1) The SWAT model had good applicability for the simulation of runoff and total nitrogen pollution load in Wuqiang River Basin, and the coefficient of determination (R²) of runoff calibration period and verification period were 0.86 and 0.97, respectively. The Nash-sutcliffe coefficient (NSE) were 0.83 and 0.96, and the percent bias (PBIAS) were 15.8% and -6.3%, respectively. The coefficient of determination of total nitrogen calibration period and verification period were 0.87 and 0.74, respectively. The Nash coefficient was 0.63 and 0.66, and the PBIAS were 31.6% and 21.2%, respectively. 2) The runoff and total nitrogen load were mainly concentrated from March to July, accounting for 71.67% and 75.76% of the annual load, respectively. Considering the source and loss path of nitrogen, subbasins with large proportion of cultivated land and forest land and steep slope were set as the key pollution source areas of total nitrogen, and measures to reduce non-point source pollution such as adjusting fertilizer application rate, changing farming methods and setting vegetation buffer belts were taken into account to simulate the reduction efficiency of total nitrogen pollution load. The results showed that the 10-meter vegetation buffer zone was the best single control strategy to reduce the total nitrogen pollution load, and the total nitrogen reduction rate could reach 69.90%. The effect of integrated management measures on the reduction of total nitrogen pollution was better, and the reduction rate of total nitrogen can reach 74.79% when the 10-meter vegetation buffer zone and fertilizer amount were reduced by 20%. The results of this study can provide a theoretical basis for water quality management and control of Qiandao Lake.