| It is an important premise and basis for the scientific implementation of comprehensive watershed management to quantitatively analyze the temporal and spatial distribution characteristics of various ecological environmental risks such as non-point source pollution,point source pollution,soil erosion and mountain disasters,and to divide the comprehensive risk level areas.This paper makes an example of the upper reaches of Beisha River,based on GIS technology and Python language,and introduces the potential non-point source pollution index(PNPI)model,the point source pollution estimation model,the model of Revised Universal Soil Loss equation(RUSLE)and sensitivity coefficient methods to quantitatively analyze the spatial distribution characteristics of non-point source pollution,point source pollution,soil erosion and mountain disasters risks in the basin,and constructs a multi-risk comprehensive simulation framework of the basin.In this study,the mean-square deviation decision method is employed to calculate the weights of the above risk factors,then determine the comprehensive risks of the basin,divide the comprehensive risks level of the basin and analyze spatial distribution characteristics of the comprehensive risks.The results show that:(1)Extremely low and low non-point source pollution risk regions are mainly distributed in the northwestern areas,while high and extremely high risk regions are mainly located in the southeastern areas in the upper reaches of the Beisha River.The extremely high-risk areas are mainly distributed near the densely populated villages and towns and river networks in the plain area,while the extremely low-risk areas are mainly distributed in the mountainous areas with sparse population.The risk of non-point source pollution in Beiliu village,Wangjiayuan plain,ximafang and Beixiaoying small watershed in the southeast plain area is significantly higher than that in other areas,such areas should be divided into key controlling areas.(2)Extremely low and low point source pollution risk regions are mainly distributed in the northwestern areas,while high and extremely high risk regions are mainly located in the southeastern areas in the upper reaches of the Beisha River,and the high-risk areas are concentrated in the Beixiaoying small watershed which belongs to the rural-urban continuum and formalization breeding farms area.(3)The weights of landslide,debris flow and collapse are 0.32,0.33 and 0.35 respectively.The high risk area and extremely high risk area of mountain disaster are mainly distributed in Xifeng mountain watershed and Wangjiayuan small watershed.The terrain of this area is steep,there is heiyukou-Liangxiang West fault passing through from here,and there are many dolomites in this area,which are sedimentary carbonate rocks with high hardness and brittleness,easy to cause disaster occurrence.The low risk areas and the extremely low risk areas are mainly distributed in small watershed of plain area without disaster occurrence conditions.(4)Extremely low and low soil erosion risk regions are mainly distributed in the southeastern areas,while high and extremely high risk regions are mainly located in the northwestern areas in the upper reaches of the Beisha River.The extremely low risk area of soil erosion is mainly distributed in the southwest plain area and the area near the river in the northwest mountainous areas of the study area,with an area of 57.86 km2,accounting for 50.56% of the total area of the study area;the extremely high-risk area is the smallest,mainly distributed the area near the river in the northwest mountainous areas of the study area,,with an area of 4.25 km2,accounting for 3.71% of the total area of the study area.(5)The weights of non-point source pollution risk,point source pollution risk,mountain disaster risk and soil erosion risk determined by the mean square deviation decision-making method are 29.85%,17.78%,34.37% and 18.00% respectively,which shows that the mountain disaster risk has the highest relative contribution rate to the comprehensive risk of the upper reaches of Beisha River,followed by the non-point source pollution risk.Generally speaking,high risk regions are mainly located in the northwestern mountainous areas and the area near the river in the southeast plain of the study area,low risk regions are mainly located in the north mountainous areas and the southeastern plain areas of the study area.(6)The weights of risk factors in different types of small watersheds are different.The weights of risk factors in small watershed are mountain disaster risk > non-point source pollution risk > soil erosion risk > point source pollution risk.The weight are 0.38,0.30,0.23,0.09 from large to small.The weights of risk factors in mountain-plain small watershed are point source risk > non-point source pollution risk > mountain disaster risk > soil erosion risk,and the weights are 0.40,0.27,0.22 and 0.11 from large to small.The weights of risk factors in plain small watershed are point source risk > non-point source pollution risk > mountain disaster risk > soil erosion risk is 0.47,0.28,0.18,0.07.(7)Based on the results of comprehensive risks assessment,different risk management and comprehensive management suggestions are proposed for different types and levels of risk areas. |
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