Best Management Practices Of Watershed Non-Point Source Pollution Based On SWAT Model

Agricultural non-point source pollution,as a global environmental pollution problem,is one of the main factors affecting the regional water environment and is also a research hotspot in the field of environmental science.Gongjiang Basin is the main source of Ganjiang River,and its water ecological environment plays an important role in the ecological security of the whole Ganjiang River Basin.Therefore,it is of great significance to determine the best management measures of non-point source pollution（BMPs）for water environmental protection.Taking Gongjiang Basin as the study area,SWAT（Soil&Water Assessment Tool）model was used to simulate the characteristics of non-point source pollution in the basin,and the spatial and temporal distribution characteristics of non-point source pollution were analyzed.The intensity of nitrogen and phosphorus loss and population density in the basin were classified by natural fissure classification method,and the risk level of non-point source pollution was evaluated according to the water quality level of the basin,and the key source areas of pollution were identified.To evaluate the reduction effect and cost-effectiveness of different management measures for non-point source pollution in key source areas,and to establish the best management measures for non-point source pollution in the basin.The results are as follows:（1）The watershed was divided into 39 sub-basins.Based on the hydrological and water quality data from 2013 to 2018,SWAT-CUP software was used to calibrate and verify the model.The R2 of runoff,sediment,ammonia nitrogen and total phosphorus reached more than 0.68,and Ens was more than 0.58,which met the operation requirements of the model（R2>0.6,Ens>0.5）.The optimal parameters of the model were determined.（2）From 2013 to 2018,the annual variation of non-point source pollution load was large,with the maximum in wet year 2016（total nitrogen 6852.62 t,total phosphorus 1184.06 t）,and the minimum in dry year 2018（total nitrogen 1905.57 t,total phosphorus 517.07 t）.The total nitrogen loss intensity in each sub-basin was54.77–510.03 kg/km²(y^-1),and the total phosphorus loss intensity was 5.9–94.63kg/km²(y^-1).The pollution load showed fluctuation characteristics in the year,which was mainly closely related to fertilization and rainfall runoff.From the spatial distribution characteristics of pollution,the spatial heterogeneity is large,and the intensity of nitrogen and phosphorus loss is generally higher in the south than in the north,and the sub-basin located in the main channel is higher than that in the tributaries.（3）The critical source areas of non-point source pollution are mainly located in Hefeng Town,Gongjiang Town and Zishan Town of Yudu County,and the junction area of Yudu County,Ruijin City and Huichang County.The regional population is relatively dense,and the planting industry in the middle and lower reaches is developed,which is a serious pollution area.In particular,Hefeng Town has the highest pollution load,with total nitrogen intensity of 510 kg/km² and total phosphorus intensity of 94.63kg/km².（4）According to the results of scenario simulation,the reduction effect of combined measures is much stronger than that of single measures.Among them,the reduction effect of chemical fertilizer 20%+stubble mulching+vegetation buffer zone is the best,and the reduction rates of total nitrogen and total phosphorus are 42.71%and 50.40%,respectively.From a single point of view,the effect of engineering measures on total nitrogen reduction is generally greater than that of non-engineering measures,and the reduction of total phosphorus is better than that of engineering measures.In terms of BMPs cost-effectiveness,the cost-effectiveness of engineering measures is much higher than that of non-engineering measures,of which the cost-effectiveness of grass-planting waterways is the best and that of terrace engineering is the worst.Based on the analysis of the results of the above SWAT model,considering the cost-benefit,regional natural characteristics and economic and social development,this paper determines the optimal management measures of non-point source pollution in the basin,which provides theoretical support and scientific basis for the effective prevention and control of non-point source pollution and water environment management in the Gongjiang Basin.