Risk Assessment And Identification Of Critical Source Areas Of Phosphorus Nonpoint Source Pollution In A Subtropical Hilly Typical Small Watershed

In the process of water environment treatment,the nonpoint source pollution becomes more and more prominent after point source pollution is effectively controlled.The water system is densely distributed and agricultural production activities are strong in the subtropical hilly region.Affected by the nutrient loss of farmlands and the pollution of livestock and poultry breeding,a large number of nutrient elements enter rivers and lakes in the Dongting Lake basin,resulting in a serious trend of eutrophication of water environment.Pollution from agricultural nonpoint sources has become a prominent ecological and environmental problem.Phosphorus as one of the important nutrient elements,it is of great significance to strengthen the prevention and control of nonpoint source phosphorus pollution in watershed.In this study,Jinjing River Basin,a small watershed at the upstream of Dongting Lake,was selected to quantitatively analyze non-point source phosphorus pollution in the watershed and identify key source areas,and a prevention and control strategy with practical significance was proposed to provide scientific basis and technical support for the comprehensive treatment of agricultural non-point source pollution in the subtropical hilly watershed.Firstly,the output load of non-point source phosphorus was obtained by simulating the output load of dissolved phosphorus and granular phosphorus.Secondly,risk assessment was carried out for non-point source phosphorus pollution in the basin,and key source areas were identified by classification division combined with the intensity of non-point source phosphorus loss.Finally,scenarios of different control schemes were set up to evaluate the phosphorus reduction effect of non-point source combined with the phosphorus load estimation model.The main results and conclusions are as follows:(1)Nonpoint source phosphorus pollution load estimation:Model calibration and result verification showed that the constructed model had good simulation accuracy,in which R2 and NSE between simulated and measured values of dissolved load respectively was 0.84 and 0.82,with errors ranging from 3.9 to 24.7%.The R2 and NSE between the simulated and measured values of particulate phosphorus load respectively was 0.68 and 0.65,with errors ranging from 8.9 to 30.0%.The simulation effect of dissolved phosphorus was better than that of particulate phosphorus load.The simulation results showed that the average load of total phosphorus,dissolved phosphorus and particulate phosphorus was 64.3 kg km-2 a-1,29.6 kg km-2 a-1 and 34.7 kg km-2 a-1,respectively.The dissolved phosphorus load of farmlands and residential areas was significantly higher than that of forests,and the average export load was 40.5 kg km-2 a-1,31.1 kg km-2 a-1 and 13.4 kg km-2 a-1,respectively.The particle phosphorus in forests was significantly higher than that in farmlands and residential areas,and the average export load was 43.1 kg km-2 a-1,16.5 kg km-2 a-1,4.5 kg km-2 a-1,respectively.The spatial distribution of dissolved phosphorus and particulate phosphorus was siggnificantly different in the watershed.The high load area of dissolved phosphorus was concentrated in the southwest plain area,while the high load area of particulate phosphorus was mainly distributed in the northwest slope forest area.(2)Risk assessment of nonpoint source phosphorus pollution and identification of critical source areas:The dissolved phosphorus factor,particulate phosphorus factor and distance factor were selected to study the nonpoint source phosphorus pollution risk of Jinjing watershed through the improved multi-factor comprehensive risk assessment model.The Jinjing watershed was divided into 20 catchments,and the export load and risk index of nonpoint source phosphorus pollution in each catchment were studied respectively.The critical source areas of the watershed were determined with the watershed risk index as the main index.The results showed that the critical sources of nonpoint source phosphorus pollution in Jinjing watershed were catchment 4,catchment 18 and catchment 19,with a total area of 12.91 km2,accounting for 9.6%of the total watershed area.The critical source areas were distributed in the middle and lower reaches of Tuojia River and the middle reache of Guanjia River.The planting and breeding activities in the areas were strong,and the nonpoint source phosphorus pollution loss intensity was high,and the nearby migration into the river water causes nonpoint source phosphorus pollution.The areas with high risk of nonpoint source phosphorus pollution in Jinjing watershed were catchment 5,catchment 7,catchment 8,catchment 9 and catchment 13,with a total area of 28.87km2,accounting for 21.4%of the total watershed area.It was mainly distributed in the southwest part of the watershed,and was mostly distributed in the area with high distance factor value.Due to the short migration distance of nonpoint source phosphorus pollution within the region,less phosphorus pollutants were degraded and intercepted during the migration process,resulting in a high proportion of phosphorus pollutants entering the water environment.(3)Scenario simulation of nonpoint source phosphorus pollution control scheme:According to the loss and transport characteristics of dissolved phosphorus and particulate phosphorus,control scheme measures were set up,and scenario simulation of single measure and multi-measure was carried out to analyze the reduction effect of total phosphorus in Jinjing watershed.The simulation results showed that,in the single measure scenario,fertilization management in farmlands(Scheme 4)had the highest reduction rate of 10.5%on nonpoint source phosphorus pollution in the critical source areas,while increasing vegetation coverage in forests(Scheme 3)had the best reduction rate of 13.1%on nonpoint source phosphorus pollution in the watershed.In the multimeasure scenario simulation,the best reduction rate of nonpoint source phosphorus pollution in the critical source areas was 30.5%by adding soil and water conservation measures in farmlands and forests,fertilization management in farmlands,domestic sewage and livestock manure control(Scheme 8).Adding soil and water conservation measures in farmlands and forests and increasing vegetation coverage in forests(Scheme 6)had the best reduction rate of 26.9%on nonpoint source phosphorus pollution in watershed.Different control measures should be set according to spatial distribution characteristics and source classification of nonpoint source phosphorus pollution.