Study On The Identification And Scenario Simulation Of Key Source Areas Of Non-point Source Pollution In Small Watersheds

Non-point source pollution is the main source of water quality deterioration and water environment pollution in river basins at present.The effective control of non-point source pollution is the key task of water pollution control in river basins.Binan River has been comprehensively remediated since 2010.The results have been remarkable.Point source pollution control is appropriate,and non-point source pollution has become the leading cause of unstable water environmental quality.The Binan River is a first-class tributary on the left bank of the Yangtze River.Its water environment safety is of great significance to the water ecological environment of the main urban section of the Yangtze River and Chongqing and the entire Three Gorges Reservoir area.In view of this,this study takes the Binan River basin of Chongqing as an example to build a SWAT model of the Binan River Basin,use the model to calculate the non-point source pollution load in the basin,analyze its time and spatial distribution characteristics,analyze the pollution load and contribution of different land use modes in the river basin,and quantify the pollution load to divide the non-point source.Pollution source area level,identify key source areas.Through the design scenario scheme,simulate different management measures,and quantify the effectiveness of non-point source pollution reduction of each management measure in combination with the watershed SWAT model,it provides reasonable non-point source pollution prevention and control suggestions.The main findings are as follows:(1)According to the historical measured runoff and water quality monitoring data of the Binan River Basin,the sensitivity analysis and rate determination of the parameters of the SWAT model in the Binan River basin are carried out in combination with the SWAT-CUP software.The results show that the determinant coefficients R~2 and the NS efficiency coefficients are greater than 0.65(the average value of the runoff rate R~2 coefficient is 0.78,and the NS coefficient is 0.71;the nitrogen rate is constant R~2 coefficient is 0.71,and the NS coefficient is 0.68;the ammonia-ridity R~2 coefficient is 0.81,and the NS coefficient is 0.78),indicating that the model has good accuracy after rate determination,which can be used for the simulation of non-point source pollution in the Binan River basin,which is reasonable.(2)Based on the simulation results of the SWAT model in the Binan River basin,the annual change of nitrogen and phosphorus pollution loads generally shows a downward trend in time distribution.The overall trend of inter-month variation of nitrogen and phosphorus pollution loads is first increased and then decreased.There are more pollution loads in April and September,accounting for more than 85%of the total annual total.In terms of spatial distribution,the overall rainfall spatial difference in most of the basin is small,slightly higher in the north and lower in the southeast corner.The spatial distribution of sand-producing intensity varies greatly,and the higher sediment output load is in the Longjixi basin in the southwest of the basin.Regions with high pollution output loads such as organic nitrogen,nitrate nitrogen,organophosphorus,soluble phosphorus and mineral phosphorus are generally concentrated in urban built-up areas,Longjixi and the lower reaches of the Meijiang River.(3)Among different land use types,total nitrogen load:agricultural land>construction land>grassland>forest land,total phosphorus load:agricultural land>grassland>construction land>forest land.The output contribution of agricultural land to nitrogen and phosphorus pollution in river basins is absolutely in place,accounting for more than 65%.Organic nitrogen,organophosphorus and inorganic phosphorus have a significant positive correlation with inorganic nitrogen.The amount of sediment is significantly positively correlated with organic nitrogen,and there is a significant positive correlation with organophosphorus and inorganic phosphorus.It shows that rainfall is an important cause of nitrogen and phosphorus entering the river in the Binan River basin.From the perspective of loss morphology,organic nitrogen,organophosphorus and inorganic phosphorus are heavily adsorbed in sediment and lost in particulate matter.(4)The research takes the pollution output load per unit area(kg/hm~2)of total nitrogen and phosphorus as the index factor.Combined with the actual situation in the Binan River basin,each sub-basin basin of the Binan River is divided into three pollution source areas according to the output load intensity of total nitrogen and total phosphorus.Total nitrogen load>12.5 kg/hm~2 in the primary source area;total phosphorus load>4 kg/hm~2.Total nitrogen load in the secondary source area<12.5 kg/hm~2,>5kg/hm~2;total phosphorus load<4kg/hm~2,>2 kg/hm~2.Total nitrogen load<5 kg/hm~2 in tertiary source area;total phosphorus load<2 kg/hm~2.According to the results of the classification of nitrogen and phosphorus output loads and output load levels in each sub-basin,the large output volume and high output load per unit area are listed in the key source areas for the prevention and control of non-point source pollution in the Binan River basin.(5)Using the SWAT model of the Binan River basin,six single-measure scenarios and three multi-measures scenarios are set from the aspects of"source reduction"and"interception"to analyze the non-point source pollution load reduction capacity in key source areas of the basin.According to the simulation results,in the single-measures scenario scheme,the 10 m measure load reduction capacity of the vegetation buffer zone is the strongest,and the comprehensive capacity of the grass-planting channel measures is the best.In the multi-measures scenario,the combination of source reduction and interception measures,that is,no-tillage+reduction of 20%of fertilizer+vegetation buffer zone 10 m and no-tillage+20%reduction of fertilizer+grassland and river courses perform excellent in pollution reduction and water quality improvement.The average reduction rate of total nitrogen and phosphorus load is higher than 40%,and the Ammonia nitrogen and total phosphorus concentrations decreased by more than 10%.The application process needs to select the best management measures according to the pollution control objectives of different river sections,analyze the capital budget,land reserves and other actual conditions.The SWAT model can simulate the migration of runoff,sediment and nutrients in the basin.Before simulating,the model should be built.This process needs a lot of meteorological data in addition to watershed topography,soil type and crop management.This research area is a smaller river basin.The degree of data shm~2ring of township-scale meteorological stations in the region is low,and it is difficult to obtain it.Supported by the relatively macroscopic data of the national basic meteorological stations in the basin and surrounding counties,it can complete the simulation work well,but the impact of microclimate is not taken into account.In addition,more scenario simulation schemes need to be explored to broaden the idea of basin water environmental management measures and improve the database of water environmental management measures in watersheds.