Temporal And Spatial Distribution Characteristics Of Surface Water Pollution And Identification Of Key Source Areas In The Yunnan Section Of The Yangtze River Basin Based On SWAT Model

The Yunnan section of the Yangtze River Basin is located in the upper reaches of the Yangtze River system,determining the surface water environment in the basin by its unique geographical location,which not only affects the national economic and social development of Yunnan Province,but also is of great significance to the water quality and safety in the middle and lower reaches of the Yangtze River.The Yangtze River Basin has highest level of urbanization in Yunnan Province,industrial point source and agricultural non-point source pollution has brought serious challenges to the water of the basin.Quantitative identifying the temporal and spatial distribution and key source areas of surface water pollution in the Yunnan section of the Yangtze River Basin is of great significance to solving the problem of surface water pollution in the basin,building a strong ecological security barrier in the upper reaches of the Yangtze River and the country’s southwest,and help Yunnan Province to become a vanguard of national ecological civilization construction,which provides theoretical basis and data support for scientific pollution control and precise pollution control.To this end,this paper builds a simulation model of surface water pollution in the Yunnan section of the Yangtze River Basin based on the SWAT model,and uses the SWAT-CUP calibration and verification model to analyze the spatial and temporal distribution of pollutant loads in the study area and identify key sources of pollution in the basin.The research conclusions are as follows:（1）The continuous improvement of water quality monitoring data from 2015 to2018 indicates the water quality of the basin.Based on the second pollution source census in Yunnan Province,this paper constructs an accurate pollution point source database.According to the results,the total nitrogen point source emissions in the basin are mainly concentrated pollution control facilities,relatively small emissions from industrial,agricultural and domestic sources;the total phosphorus point source emissions are mainly from centralized pollution control facilities and agricultural sources,and the proportion of industrial and living point sources is relatively low.（2）A large-scale,whole-basin,and refined SWAT surface water pollution simulation model is constructed for the Yunnan section of the Yangtze River Basin.The runoff and water quality of Longxing Village,Wenquan Bridge,and Fengzhu Section were used to calibrate and verify the upper,middle,and lower reaches of the watershed,respectively.The visual method and the function index method were used to evaluate the fitting results of simulated values and measured data.The objective function evaluation results are relatively good.It is found by visual method that the simulated values of the SWAT model are in good agreement with the measured values in terms of rising and falling trends and the month of the peak.（3）The total nitrogen load of the basin is 2 ～ 3 times that of total phosphorus.From 2015 to 2018,the pollutant load of total nitrogen and total phosphorus decreased continuously.The pollutant load intensity in rainy season is much higher than that in dry season,with the peak value in August and the lowest value in December or February.The season with more severe pollution is summer,and winter is less polluted.The spatial distribution of total nitrogen and total phosphorus load intensity is similar,and the pollution load in the middle and downstream of the basin is relatively heavy.（4）From 2015 to 2018,cultivated land and forest land use types contributed significantly to pollutants,with cultivated land contributing more than 84.30% to total nitrogen load and 87.54% ～ 91.46% to total phosphorus load;The contribution of forest land to total nitrogen load is 5.18% ～ 6.50%,and the contribution to total phosphorus load is 2.77% ～ 5.31%.（5）This paper identifies four indicators of key source areas based on four indicators: rainfall,total nitrogen load intensity,total phosphorus load intensity and land use type.They are divided into four levels: A,B,C,and D according to the potential pollution risk.From 2015 to 2018,the number of key source areas for total nitrogen was 45,31,37,and 23,and the number of key source areas for total phosphorus was 40,26,47,and 18,respectively;level A total nitrogen and total phosphorus pollutants are mainly located in the middle and lower reaches of the basin,level B is mainly located in the upper and lower reaches,level C is mainly distributed in the upper and middle reaches,and level D has fewer key source areas,most of which are located in the lower reaches of the basin.