Study On Risk Assessment Of Non-Point Source Pollution In Fuxian Lake Watershed

The influx of a large number of pollutants into the water body caused by non-point source pollution which is one of the important reasons that directly leads to water quality deterioration.At present,non-point source pollution has become an urgent environmental and livelihood issue that needs to be solved.Fuxian Lake has a large number of high-quality water resources,which plays an important role in Yunnan province and even the whole country.With the development of the region around the lake,the problem of non-point source pollution in Fuxian Lake becomes more and more serious,and the water body is at risk of beingpolluted.Therefore,it is of great significance to study the risk of non-point source pollution in Fuxian Lake watershed for the protection and treatment of its water environment.In this paper,the improved output coefficient model considering the influence factors of rainfall and terrainis used to estimate the load value of the non-point source nitrogen and phosphorus pollution in the watershed,and the spatial distribution characteristics of the load intensity are analyzed;The revised general soil erosion equation is used to estimate the soil erosion in the watershed,and then the sediment loss is estimated based on the relationship between soil erosion and sediment loss;Finally,based on the estimation of nitrogen and phosphorus pollution load and sediment loss from non-point source,combined with the natural factors affecting the risk of non-point source pollution,and selected the multiple factor to construct the comprehensive evaluation index of nonpoint source pollution risk to evaluate the risk of non-point source pollution in Fuxian Lake watershed.The comprehensive evaluation results of non-point source pollution are classified according to the sub-watershed division.The results of this study are as follows:(1)In 2016,a load of non-point source nitrogen and phosphorus in Fuxian Lake watershed was 1058.536t and 100.845t respectively.The contribution rates of different sources in non-point source nitrogen pollution are in order from large to small:land use>rural life>livestock and poultry breeding>atmospheric deposition.The contribution rates of different sources in non-point source phosphorus pollution are in order from large to small:livestock and poultry breeding>land use>rural life>atmospheric subsidence.The main source of phosphorus pollution load is livestock and poultry breeding.The reason was that there were more pigs in the drainage area and the content of phosphorus in their excreta was higher.A load of non-point source nitrogen and phosphorus pollution was the highest in Longjie town,followed by Yousuo town and Luju town.The average load intensity of non-point source nitrogen and phosphorus pollution in Fuxian Lake watershed was 1.940 t*km’2 and 0.186 t·km-2 respectively,and the load intensity was relatively low.In the north,west,and east of the watershed,the loss of non-point source nitrogen and phosphorus is the key control area.Agricultural land and rural production and living are important sources of non-point source nitrogen and phosphorus pollution load.In order to reduce the water pollution in the watershed,control the pollution discharge from agriculture and animal husbandry,improve the sewage treatment rate of rural residents and other measures play an important role.(2)In 2016,the total soil erosion in Fuxian Lake watershed was 803,150 tons/year,the sediment loss was 22,167 tons/year,and the average erosion modulus of the watershed was 1,158 t·km-2·a-1,which was of mild erosion intensity.The areas with low sediment loss intensity are mainly distributed in the north bank of the lake and along the lake.The terrain of the region is relatively flat and mostly residential and urban.The areas with high sediment loss intensity are mainly distributed in the north,west and southeast.The slope of this area is relatively high,and the grassland and bare land are the main types.Topography is one of the important factors affecting soil erosion in the river watershed.The sediment loss intensity increases with the increase of slope.The order of sediment loss intensity of different land use types is:bare land>grassland>farm land>woodland>urban.Therefore,it is necessary to carry out rational land use planning,strengthen ecological protection and restoration in alpine areas of the watershed,and effectively improve soil and water conservation capacity.(3)The highest risk value of non-point source pollution in Fuxian Lake watershed is 0.916,and the average value is 0.407,which is a medium risk level.In terms of space,the whole watershed shows a high degree of risk in the plain area in the north of the lake,the vicinity of Haikou town in the east coast and part of the southern area.The spatial distribution characteristics of risks are similar to the overall characteristics of nitrogen and phosphorus loss intensity.The main controlling factors of non-point source pollution risk are a loss of nitrogen and phosphorus.At the same time,it is also controlled by the distance of the river to a certain extent.The plain area on the north bank of the lake is the key source area of nitrogen and phosphorus pollutants,which are mainly caused by the loss of agricultural pollutants.The spatial distribution characteristics of sediment loss intensity are basically consistent with the topographic characteristics.The implementation of measures to prevent sediment loss should focus on barren mountain areas with steep slopes.Through the identification of non-point source pollution key source areas and high-risk areas,non-point source pollution in the watershed should be targeted for prevention and control,pollution control planning in different regions should be done,control and management of non-point source pollution source areas and migration process should be strengthened.