Research On Characteristics Of Non-point Source Pollution And Optimization Of Management Practices In Guishui River Basin Based On SWAT Model

At present,the control of agricultural non-point source pollution has become one of the most important issues facing the improvement of regional water environment quality.Guishui River Basin is not only an ecological gateway and an important water conservation area in northwest Beijing,but also one of the venues for the Winter Olympics in 2022.It has high requirements for the water environment,but in recent years,due to the impact of agricultural non-point source pollution,the water quality of the Guishui River Basin has been continuously deteriorating and facing severe challenges.This study took the Guishui River Basin as the research object,constructed a SWAT non-point source pollution model applicable to the watershed and simulated the non-point source pollutants(total nitrogen,total phosphorus)in the watershed.The temporal and spatial evolution characteristics of the non-point source pollution load in the river basin was analyzed,and the RUSLE model and the additive model was combined on this basis to identify the priority control area of non-point source pollution in the Guishui River Basin.Then,7scenarios of management measures were designed for the priority control areas of the river basin to evaluate and analyze the cost-effectiveness of the scenarios and optimize the allocation of measures.The main research conclusions are as follows:(1)Runoff and water quality calibration and verification of the constructed SWAT model are based on parameter sensitivity analysis.The results showed that the runoff and nutrient evaluation indicators of the regular rate and the verification period met the required range of simulation accuracy,indicating that the SWAT non-point source pollution model constructed in this study had good applicability and reliability in Guishui River Basin.(2)On the time scale,the annual pollution load of total phosphorus in the Guishui River Basin from2007 to 2016 was generally low and showed an insignificant downward trend,while the annual total nitrogen load was generally high and showed an insignificant increase trend.On a seasonal scale,the summer total phosphorus pollution load accounted for the largest proportion of the annual load in the four seasons,showing an upward trend as a whole,and the inter-annual fluctuation range was large.The total phosphorus load in autumn accounted for the smallest proportion of the total annual load,showing a decreasing trend.The total phosphorus load in spring and winter also showed an insignificant downward trend.In terms of total nitrogen load,winter was the season in which the total nitrogen pollution load accounts for the largest proportion of the four seasons.The total nitrogen load in summer and autumn showed an insignificant increase trend,while the spring and winter seasons showed a downward trend on the contrary.(3)On the spatial scale,the spatial distribution of total phosphorus and total nitrogen loadings in Guishui River Basin all showed the characteristics of "increasing first and then decreasing" from west to east.The sub-catchment in the middle of the basin produced a larger non-point source pollution load,while the mountain sub-catchment in the northern and southeastern edge of the basin had a smaller load relatively.The non-point source pollution load generated in the sub-watershed has a good correlation with the area of cultivated land,forest land,urban and rural construction land and the distribution of elevation in the sub-watershed.Regarding the spatial distribution of seasonal pollution load,the sub-basin located in the middle of the basin in the four seasons produced greater pollution load,and the load in the northern mountainous sub-basin was generally smaller.(4)The environmental benefit analysis of the optimal management measures scenario showed that engineering measures(including grass-planting waterways and vegetation filter belts)had overall better reduction effects on the total phosphorus and total nitrogen pollution load of the watershed than nonengineering measures(fertilizer reduction measures and no-tillage measures).And the cost-benefit value of interception measures in the engineering process was much higher than that of source control management measures,which was about 15 times that of management measures.In the six scenario simulations,the cost of planting a watercourse was the lowest,but its non-point source pollution load reduction was relatively high,making it the most cost-effective and optimal management measure.(5)The optimal allocation plan for non-point source pollution control and management measures in the Guishui River Basin was summarized.It was source control measures(domestic sewage treatment plus livestock with poultry bans and 20% reduction of fertilizer application measures)-process interception measures(vegetation filter belts and grass planting waterways)-end treatment measures(water source protection).