Research On The Best Management Measures For Non-point Source Pollution In The Ash River Basin Based On The SWAT Mode

The source of everything is water,and nowadays water resources are one of the important strategic resources,and water pollution has become an urgent problem.The study area of this paper,Ashe River Basin,located in Harbin,Heilongjiang Province,is one of the important rivers for producing water for grain production.This paper collects spatial and attribute data and constructs a SWAT model for the Ashe River basin to investigate the spatial and temporal distribution patterns of surface source pollution after the validation of the model.The key source areas of pollution in the basin are analyzed at both HRU and sub-basin scales,so that the model can be evaluated for different measures and cost-benefit values,and pollution control measures and recommendations are proposed for the basin.The main findings of the study are as follows:（1）Establishing a surface source pollution model for the Ashe River basin,the study area was divided into 36 sub-basins and 295 hydrological response units.Through the runoff and water quality data provided by the hydrological stations in the Ashe River basin and the environmental monitoring center,the time range is from 2015 to 2020,and after several adjustments of the parameters,the R² and E_ns of hydrology and water quality are above 0.7,which basically determines that the SWAT model has certain applicability in the Ashe River basin.（2）After simulation by the SWAT model,the simulation results found that the rainfall-runoff,sediment,and pollutants were mainly concentrated in the abundant water period（June to September）,and the simulated values were above 50%,reflecting the trend of increasing first and then decreasing,and reaching the peak in July.In terms of spatial distribution,rainfall is distributed more in the central and western parts of the river,and nitrogen and phosphorus pollutants are mainly accompanied by sediment transport,located in the middle reaches of the river as well as in the downstream part,where runoff affects the transport of pollutants to a lesser extent,and sediment has a greater impact on the distribution of pollutants in space.Through the study of load per unit area in the study area,the study area was graded,and the key source areas were determined according to the load loss grade evaluation index,and a total of 5 sub-basins,13,15,16,22 and 27,were identified as key source areas,with the basin area accounting for 16.52%of the overall,and the total nitrogen produced accounted for 53.28%of the study area and 56.74%of the total phosphorus produced in the whole area.（3）Through the SWAT model,which has been successfully validated,combined with a series of approach measures of the Harbin water pollution prevention and control program,12 different scenarios were set up for the Ashe River basin.In terms of assessing the effect of individual BMP loads,the terraced fields and the following project had good results at the HRU scale,with average nitrogen load reduction rates of more than 25%and total phosphorus reduction of more than 20%.The average N load reduction rate was more than 25%,and the total phosphorus reduction was more than 20%.The terracing project had the best reduction of 40.51%for total phosphorus.The reduction of grassed streams was slightly smaller than that of terraces,reaching 32.17%,and finally,the vegetated buffer zone reached 30.56%.For the combined best management practices,the pollution level of the more serious and heavily polluted areas decreased to moderate or even light,with a reduction rate of 20%,and for the best reduction scenario 12（stubble,terraces,and fallow measures）the nitrogen and phosphorus reduction rates were 37.45%and 34.78%,respectively.In terms of cost-effectiveness,grassed waterways had the highest benefit value,and contour hedges had the lowest benefit value,while areas with mild or moderate pollution could use small-scale measures such as reforestation and terracing,and areas with high pollution could use large-scale measures such as no-till and stubble mulching,and combined with scenarios 10-12.