Research On Grading Risk Assessment Method For Non-point Source Pollution Based On GIS Area

In recent years,as point source pollution has been effectively treated,non-point source pollution has become a major source of pollution in the water environment.For decades,China s research on non-point source pollution control has focused on the southern region.For example,Taihu Lake,the Yangtze River delta,etc.However,there are few studies on non-point source pollution in the western region,and there are fewer studies on non-point source pollution zoning,which cannot provide direct conclusions and visual distribution to managers.This study establishes a regional non-point source pollution classification method based on GIS:Firstly,a soil erosion type non-point source pollution model based on general soil erosion equation and nitrogen and phosphorus pollution load is established,and the distribution of regional non-point source pollution load is calculated.Secondly,using the current non-point source pollution risk assessment method,selecting non-point source pollution assessment indicators and data standardization treatment,and using the potential non-point source pollution index to determine the calculation method of non-point source pollution risk assessment;Using the data obtained from the source intensity distribution and risk assessment of non-point source pollution combined with the sub-basin division of the SWAT model,the non-point source pollution load and the non-point source risk assessment of the study area were graded by the natural discontinuous point grading method(Jenks).Division of partitions;Finally,using the established non-point source pollution grading and zoning method to analyze the Anding District of Dingxi City,the main conclusions of the study are:(1)The simulation results show that the soil erosion degree in Anding District of Dingxi City is slightly erosion and mild erosion.The total nitrogen loss load in the non-point source pollution in Anding District of Dingxi City is 77856.25t/a,and the total phosphorus loss load is 45265.72t./a.(2)The average risk value of non-point source pollution risk in Anding District of Dingxi City is 0.772903;From the spatial distribution map of the non-point source pollution risk index in Anding District of Dingxi City,the highest non-point source pollution risk value is 0.890386.The spatial distribution of non-point source pollution risk values generally shows that the northwest is higher than the southeast.Mainly because the distance between the northwestern region and the pollutants in the southeast is small,the degradation of the pollutants by the biological and physical(sedimentation,interception)of the plants is not high,resulting in high risk of non-point source pollution in the northwest.(3)Studying the Dingxi City Anding District into 46 sub-basins in the SWAT model according to the hydrological response unit.Secondly,the distribution map of non-point source pollution nitrogen and phosphorus loss in Anding District of Dingxi City and the distribution of non-point source pollution risk assessment in Anding District of Dingxi City are used.Dividing non-point source pollution zones in Anding District of Dingxi City into high-risk areas,medium-risk areas,and low-risk areas;Among them,the high-risk areas in Anding District of Dingxi City are distributed in sub-basins 2?3?4?5?6?7?8?9?10?11?12?17?27?28?30;where soil erosion is severe,mainly Due to the high slope of the area,This study defines the non-point source pollution nitrogen and phosphorus loss and risk assessment index series in Anding District of Dingxi City as the high-risk area of regional non-point source pollution control in the 4 and 5 levels,and its main distribution is Dingxi City,Anding District in white Luxiang,Shixiawan Township,Xinji Township;The low-risk areas in Anding District of Dingxi City are mainly distributed in sub-basins 32?33?35?36?40?44?45?46;and the slope in the inner Guanying Basin is low.In this study,the non-point source pollution nitrogen and phosphorus loss and risk assessment index series in Anding District of Dingxi City are both 1?2,which is defined as the low-risk area of non-point source pollution in Anding District,Dingxi City.lt is mainly distributed in Ningyuan Town,Xingyuan Township and Neiguanying Town of Dingxi City.In addition to the low-risk areas and high-risk areas in Anding District of Dingxi City,the other sub-basins are divided into medium-risk areas,which are mainly distributed in Shiquan Township,Qinglanshan Township,and Luqi Town.(4)According to the distribution of non-point source pollution in the Anding District of Dingxi City,the corresponding management suggestions are put forward;in the case of high-risk areas,the engineering methods and management measures such as slope conversion and reforestation are adopted.Source pollution is controlled;the medium-risk area should obtain the main influencing factors of non-point source pollution loss through field analysis,such as sub-basin 20,the low slope of this area and the close proximity of water body lead to low distribution of non-point source pollution nitrogen and phosphorus loss.However,the value of the non-point source risk assessment is high,and it is necessary to formulate corresponding management measures for the region;the measures adopted in the low-risk zone are to achieve the management of non-point source pollution by strengthening people's awareness of prevention and control.