Study On Non-point Source Nitrogen And Phosphorus Loads Simulation In Chongqing For Water Quality Management

In recent years,with the control and treatment of point source pollution,the problem of non-point source pollution has become increasingly serious.Compared with point source pollution,non-point source pollution has a more complicated occurrence mechanism and influence process,and it is much more difficult to control and harness than point source pollution.Chongqing is located in the Three Gorges Reservoir area,where the main stream of the Yangtze River traverses the whole territory from west to east.The special ecological location plays an important role in the entire Yangtze River Basin and the ecological security of China.The agricultural industry in Chongqing is relatively developed,with the crop planting area accounting for 42.24% of the city’s total area.Chongqing’s landforms are mainly mountainous,and the special topography has increased the output of non-point source pollution from crop cultivation.Therefore,this study takes Chongqing as the research area,and adopts the SWAT(Soil and Water Assessment Tool)model to simulate the non-point source pollution in Chongqing.The after-determination and verification showed that the constructed model meets the applicability.Based on the simulation results of the model,the distribution characteristics of non-point source total nitrogen and total phosphorus pollution load in Chongqing were analyzed from the time and space.According to the water quality assessment of Nemerow,this study evaluates the water quality of non-point source pollution in each district and county and second water resources districts of the area,and simulates the pollution load and water quality change of non-point source total nitrogen and total phosphorus in Chongqing under three different fertilizer reduction scenarios.The result shows:1)The spatial and temporal changes of non-point source total nitrogen and non-point source total phosphorus in Chongqing have a certain similarity,and there is basically a positive correlation with rainfall,while the total nitrogen and total phosphorus differ in 2012.From a spatial perspective,the total nitrogen and total phosphorus is mainly distributed in the west of Chongqing,the main city of Chongqing,Qijiang,Wanzhou,Kaixian and Wuxi.2)The average annual water quality status of all districts and counties in Chongqing is poor.The city’s average comprehensive water quality index is 2.66,which indicates moderate pollution.The number of districts and counties with mild or higher water pollution is 8,accounting for 21.05%,while the moderately polluted and with severely polluted are up to 30 districts and counties,accounting for 78.95%.The comprehensive water quality index in the main city area is generally high and the pollution is relatively serious.The water quality of the Dongting Lake system,the Han River,and the Wujiang River is slightly polluted,and the main stream of the upper Yangtze River is moderately polluted.The Jialing River and the Mintuo River were seriously polluted.3)Under the condition of 10% fertilizer reduction,the total nitrogen reduction rate in different years is between 5.69% and 7.82%,the total phosphorus reduction rate is between 3.26% and 4.61%,and the city’s average comprehensive water quality index is 2.54,which is moderately polluted.Nanchuan District has been reduced from severe pollution to moderate pollution.Under the condition of 30% fertilizer reduction,the total nitrogen load reduction rate is 14.22% ～ 19.54%,the total phosphorus is 6.52% ～ 9.23%,and the city’s water quality index is 2.4,which is medium polluted,and Yongchuan District has been reduced from severe pollution to moderate pollution.Under the condition of 50% chemical fertilizer reduction,20.48%to 28.14% of total nitrogen and 11.17% to 15.82% of total phosphorus are reduced.The city’s water quality index is 2.2,which is moderately polluted,and close to light pollution.Reducing the amount of fertilizer application has a positive effect on abating the non-point pollution load in Chongqing.The reduction rate of total nitrogen is greater than that of total phosphorus.