The Non-point Source Pollution And The Response To The Hydrological Processes Of Upper Reaches Of The Yangtze River

With the effective control of industrial and municipal wastewater,how to control non-point source pollution has become the key issue of water pollution prevention and control.Non-point source pollutant output is driven mainly by the hydrological process,so it’s of great significance to explore the impact of hydrological processes on non-point source pollution output.The Upper Reaches of the Yangtze River(URYR)Basin,which controls the lifeline of the whole basin’s water resources,faces difficult task of water pollution prevention and control,especially after the completion of the Three Gorges reservoir.Therefore,this paper studies the characteristics of and the effect of hydrological processes on the nitrogen and phosphorus output in the URYR Basin based on the SWAT model,and some advice also given on the control of nitrogen(N)and phosphorus(P)pollution in the watershed.SWAT model is constructed in the URYR Basin using the calculated point source and non-point source pollution emissions based on statistical data,DEM,land use,soil and meteorological data.The model parameters are calibrated and verified using the measured runoff,sediment and N,P load,and the results show that: SWAT model can simulate hydrological process,sediment and N,P migration very well,and the simulation results can be further analyzed and applied.The simulation results shows that the average annual non-point source TN,TP load in URYR Basin were 13.94kg/ha and 2.92kg/ha respectively,and shows a significant upward trend over the last forty years.In view of the inter-annual change,the load intensity of TN and TP in flood season is much higher than non-flood season,and the TN,TP load in May-October accounts 74% and 85% of the total annual load respectively.Sub-basin TN,TP load intensity are highly correlated with the proportion of arable land,Tuo River Basin’s load intensity is the maximum,and Jialing River Basin’s load is the largest.The annual average TN,TP output of the Yichang site are369,000 tons and 103,100 tons respectively,and there is an upward trend.The TN,TP concentrations of the reach in Sichuan Basin are generally high,part of them has exceeded Grade V index limits.The amount of TN source and rainfall-runoff process are two control factorsaffecting the TN output.When the amount of TN source is relatively sufficient,it can’t restrict the erosion driven by the rainfall-runoff process;but when it’s relatively insufficient,the restriction will result in limited amount of TN output.In the agricultural watershed where the TN source is sufficient,controlling rainfall-runoff erosion is a more effective way to reduce TN output.The N output in different runoff components is affected by land use type,soil properties.In the source-sufficient agricultural watershed,the control function of rainfall-runoff process is more prominent,and therefore N concentration is higher in flood season than non-flood season;while in the non-agricultural watershed,N concentration may be lower in flood season than non-flood season because of the insufficient source.In the URYR Basin,soil permeability of agricultural areas are generally not large,and the N load comes mainly from surface runoff,thus reducing nitrogen load in the surface runoff in flood season in agricultural areas should be the focus of watershed non-point source pollution prevention and control.Taken together,avoiding fertilizing before rain and in surface,and taking soil-water conservation measures is the more effective way to reduce TN output in the URYR Basin.