Research On Non-point Source Pollution Management Measures Of Danjiang River Basin Based On SWAT Model

Non-point source pollution has become one of the important factors affecting the water environment and water ecological safety of river basins.The Danjiang River Basin belongs to the water source area safety guarantee area of the South-to-North Water Diversion Project.To ensure water quality safety,it is necessary to solve the problem of water environment pollution,control the agricultural non-point source pollution in the river basin,and ensure that "one Hongqing water is sent to the north continuously".Best management practices(BMPs)are a series of measures to reduce or prevent water environment pollution,and have become one of the most effective non-point source pollution prevention measures.This paper simulates the characteristics of non-point source pollution in Danjiang River Basin based on SWAT model.Analyze the law of spatiotemporal changes,identify key source areas according to the intensity of pollution loss,evaluate the best management measures at the HRU scale and sub-watershed scale,and reduce the effect of source pollution load reduction.At the same time,the multi-attribute decision-making method based on information entropy evaluates the best management measures Cost-effectiveness,and put forward countermeasures for the prevention and control of non-point source pollution in river basins.The main research conclusions are as follows:(1)Analyze the water quality characteristics of typical sections of the Danjiang River Basin from 2013 to 2018,and calculate the Nemero pollution index and pollution sharing rate.The results show that:the interannual variation is small,the water quality of the basin is relatively stable,and the regional changes along the distribution are obvious.The Zhangcun section and the Danfeng lower section had a light pollution state,and ammonia nitrogen and total phosphorus pollutants had a greater impact on water quality.Simultaneous monitoring of water volume and water quality in the flood and non-flood periods of the Danfeng section in 2019 revealed that the total phosphorus,positive phosphorus,ammonia nitrogen,and SS index concentrations during the flood period were much greater than the average concentration during the non-flood period,and the total nitrogen and COD were less than the average concentration during the non-flood period.The change trend of the concentration of each pollutant with the flow rate is basically the same,first increases and then decreases;the average concentration method and the runoff segmentation method are used to estimate the non-point source pollution load of the Danfeng section at different levels,the overall pollution load is a rich year>normal year>In the dry season,the total phosphorus,ammonia nitrogen and COD non-point source pollution load account for a large proportion,and the impact on water quality cannot be ignored.(2)Establish a non-point source pollution model of the Danjiang River Basin,divided into 21 sub-basins and 614 HRUs.Based on the flow,sediment and water quality data of the Jingziguan section for many years,select certainty coefficients,Nash coefficients and relative errors for evaluation indicators.The monthly verification results all meet the accuracy requirements of R2>0.6,Ens>0.5,and RE<±20%.It is considered that the SWAT model-based simulation of non-point source pollution in the Danjiang River Basin is reasonable.(3)The simulation results of the SWAT model show that the annual precipitation,runoff depth,sediment volume and non-point source pollution load are mainly concentrated in the wet season(June to October),reaching more than 65%,and the correlation shows that the non-point source pollution is affected Among the load factors,the overall performance is runoff>sediment>rainfall;in terms of spatial distribution,the nitrogen and phosphorus load mainly migrates with the sediment and is located in the upstream and downstream of the basin;correlation studies show that runoff has little effect on the nitrogen and phosphorus non-point source pollution load.Silt has a greater spatial impact on the nitrogen and phosphorus non-point source pollution load.Among the non-point source pollution contribution rates of land use types,the non-point source pollution load of cultivated land is the largest,and the non-point source pollution load of various types accounts for more than 57.28%,followed by grassland and woodland.Use the unit area load index method and the natural crack point classification method to divide the nitrogen and phosphorus loss intensity into five levels,identify the key source areas in the watershed,and determine the sub-basins 1,2,3,6,7,21 as the key source area.The sub-basin area accounts for 27.76%of the entire basin,the total nitrogen load produced accounts for 49.39%of the research basin,and the total phosphorus load accounts for 45.49%of the research basin.(4)Simulate the management measures of setting 12 scenarios.The evaluation of the reduction effect of single BMP non-point source pollution load shows that the total nitrogen reduction effect of returning farmland to forest and terrace projects on the HRU scale is better,with average reduction rates reaching 58.60%and 25.85%.Terrace projects,grass-planting rivers and vegetation buffer zones reduce the total phosphorus efficiency by more than 40%;the combined scenario scheme has a higher BMPs reduction rate,of which stubble coverage+terrace projects+returning farmland to forests to reduce the pollution load on the exit section of the basin is the best.More than 30%.The information-entropy-based multi-attribute decision-making method to evaluate the cost-benefit value of management measures shows that control measures preferentially use combined BMPs,and the comprehensive cost-benefit attribute value reaches above 0.8,and the effect of controlling non-point source pollution is the best.When using a single BMP,the comprehensive attribute value of returning farmland to forest is high,reaching 0.62,which is suitable for non-point source pollution control in a small area;for large-scale non-point source pollution control,terraced planting is a project that has continuous benefits and can maintain a long-term high Benefits of soil and water conservation,supplemented by stubble mulching and grass planting river measures for large-scale promotion.