Abstract: This study aimed to evaluate more accurately the characteristics of non-point source pollution in the Danjiangkou Reservoir basin following the water supply of the middle route of the South-to-North Water Transfer Project. The aim was also to improve the accuracy of SWAT model in simulating non-point source pollution. To accomplish this, we coupled the improved output coefficient method and SWAT model to conduct a comprehensive simulation on the process of multi-type non-point source pollution entering the river. First, the improved output coefficient method estimated the load of TP, TN, and other pollutant indicators related to livestock and poultry, rural life, and aquaculture pollution in three stages from generation, loss, to entering the river. Second, the SWAT hydrological model comprehensively simulated and analyzed the temporal and spatial distribution characteristics of non-point source pollution in the Danjiangkou Reservoir basin in 2019. The pollution from livestock and poultry breeding, rural life, and aquaculture industries was generalized at the county level and added to the SWAT Model in the form of point sources. The model's hydrological parameters were calibrated and verified for accuracy with Hanjiang River, Danjiang River and Laoguan River basins as reference basins. Results reveal that, in the Danjiangkou Reservoir basin in 2019, the loss of TN and TP was 275 600 tons and 18 600 tons, respectively. The inflow of TN and TP into the river was 33 100 tons and 2 400 tons, respectively. The planting industry was the primary contributor to both TN and TP inflow, accounting for 72.45% and 64.97%, respectively. The amount of TN and TP entering the river was greatest in Hanbin District, Xunyang County, Yunyang District, Yangxian County, and Xixiang County, while the smallest was recorded in Shangzhou District, Fengxian County, Zhangwan District, Taibai County, Lueyang County, and Liuba County. The contribution rate was the highest in May and the lowest in October. The research findings provide scientific basis for the comprehensive management and control of non-point source pollution in the Danjiangkou Reservoir basin.

Key words: agricultural non-point source pollution, improved output coefficient method, SWAT, generation-loss-inflow, Danjiangkou Reservoir basin