SWAT Model-based Non-point Source Pollution In Upper Minjiang River Basin,China

As a key source of water security and a huge obstacle to global economic development,non-point source pollution has become an urgent problem to be solved in front of global economic construction and environmental governance workers.As a typical arid valley area in Western Sichuan,the upper reaches of Minjiang River are economically poor,culturally diverse and ecologically fragile and sensitive.The water safety problem caused by non-point source pollution not only seriously affects the production and life of the upstream people,but also indirectly affects the economic development and human life of the middle and lower reaches.Based on SWAT model,a watershed SWAT database is constructed,including digital elevation model,land use,soil,meteorology,hydrology,water quality and so on.On the basis of model calibration and validation,the characteristics of non-point source pollution load in watershed are analyzed from four dimensions of time,space,source and form to find source areas,main influencing factors and important sources.Based on climate change and land use to analyze the change characteristics and regulations of non-point source pollution load,and the best management measures are put forward;risk assessment of non-point source pollution hazards is carried out based on GIS grid technology,the results are as follows:(1)database in the upper reaches of Minjiang River has been constructed,and the basin is eventually divided into 31 sub-basins and 1369 HRUs.The correlation coefficients of runoff,sediment and water quality are all greater than 0.6,and the Nash coefficient NSE are greater than 0.5,which indicates that SWAT model has strong applicability in the upper reaches of Minjiang River,In terms of overall efficiency: runoff > sediment > water quality.(2)Using the calibration model simulates the output,variation,spatial distribution and source of the non-point source pollution load for 1981-2014.The results show that the annual average rainfall is 660.05 mm,the annual sediment load is 4724.23 million tons,and the annual total nitrogen and phosphorus load are 11212.52 tons and 2188.89 tons.The key period of non-point source pollution load control is flood season,and the key source areas are Songpan and Heishui.Soil nutrient loss is the main source of total nitrogen and total phosphorus pollution load.The contribution rates of different forms of nitrogen and phosphorus pollution load were as follows: ammonia nitrogen > organic nitrogenous >nitrate nitrogen > nitrite nitrogen and organic phosphorus > inorganic phosphorus.(3)Based on the GIS weighted stack analysis method,the risk area of non-point sourcepollution is found through raster-conversion,layer assignment and weight determination.The results show that the areas with risk of non-point source pollution are mainly distributed in the west,with proportion of total area is 10.48%,covering Heishui,Songpan and Wenchuan,etc.The protection of ecosystems should be strengthened to improve the stability of ecosystems in these areas.(4)Select land use data in 1980 and 2010 nd meteorological data in 1981-1997 and1998-2014.Through the intersection of land use and meteorological data in each period to explore the impact of climate change and land use on non-point source pollution loads based on scenario setting and variable control.The results show that when the forest land is reduced and the grassland and cultivated land increased,the non-point source pollution load increased,When the rainfall decreased and the average temperature increased,the non-point source pollution load decreased,The subduction of non-point source pollution brought by climate change is greater than the gain effect brought by land use.(5)Combine scenario analysis and cost-benefit assessment to propose the best management measures for the basin.The results show that the construction of the buffer zone is the best management measure for the upper reaches of the Minjiang River,which is suitable for large-scale promotion.Returning farmland to forest and grassland has the maximum subduction for pollution load,which is suitable for small-scale promotion because of the high cost.