| Water is one of the most important natural resources.Yet non-point source pollution has been identified as the leading cause of surface water deterioration in the Three Gorges Reservoir catchment,China.More than 60% of large lakes have nitrogen and phosphorus eutrophication to different extents,and the contribution rate of non-point source(NPS)pollution to total nitrogen and total phosphorus can reach 81% and 93%,respectively.Since the impoundment of the Three Gorges Reservoir dam in 2003,a medium to severe eutrophication caused by overload of nutrients has developed over 40% sections of 38 main tributaries of the reservoir basin.Pengxi River is the largest tributary in the northern bank of the Three Gorges Reservoir area,with a medium eutrophication,great population density,apparent topographic relief of upstream and downstream,rich rainfalls and wide farmland area,as well as a typical basin for studying non-point source pollution.The study established spatial and attribute databases of hydrology,climate,topography,vegetation cover,land use,agricultural population,livestock and poultry farming,planting and fertilization management in Pengxi River Basin.Based on the improved the export coefficient model(ECM),introducing parameters of precipitation,terrain,vegetation coverage and distance to the river channel,which has been hardly any distance parameter introduced to modify the ECM in published papers,total nitrogen and total phosphorus loads exporting to the river in 2000 and 2015 in the Pengxi River basin was calculated using the GIS method,to compare total nitrogen and total phosphorus load change before and after the impoundment,identifying the crucial sources of non-point source pollution.According to the results,the influence factors of total nitrogen and total phosphorus pollution attribute to elevation,river distance,rainfall erosivity,slope and land use type.The classification and regression tree(CART)model was established by using Python programming language on Anaconda software to identify the crucial influence factors of total nitrogen and total phosphorus are determined.The spatial distribution of non-point source pollution in Pengxi River Basin was analyzed on two scales of pixel and sub-basin to identify the critical area of NPS pollution in Pengxi River Basin.According to the Press-State-Response framework,four response scenarios were proposed for the crucial pollution sources and areas in Pengxi River Basin.The total nitrogen and total phosphorus loads exporting to the river under the response scenarios were simulated and estimated,and a pollutant emission reduction plan that can meet the self-purification capacity standard of Pengxi River was proposed.The main results are as follows:In 2000,the total nitrogen and total phosphorus loads exporting to the river were 2466.39 t/a and 208.65 t/a,respectively.In 2015,the total nitrogen and total phosphorus loads exporting to the river were 5283.66 t/a and 466.72 t/a,respectively.The relative errors between the estimated total nitrogen and total phosphorus loads and the measured total nitrogen and total phosphorus concentrations in Pengxi River are 5.74% and 2.57%,respectively.The introduction of river distance parameter improved the simulation accuracy of the improved export coefficient model by 7.86% and 0.14%(total nitrogen and total phosphorus).The arable land(arid land and paddy field)was the crucial source of non-point source pollution in Pengxi River Basin,with the reason associated with excessive fertilization.In 2000,the contribution rates of total nitrogen and total phosphorus were 40.93%,52.00%,42.88% and 52.52% in 2015,respectively.Rural domestic pollution was the second pollution source,and its contribution rate to total nitrogen and total phosphorus in 2000 was 38.53%,26.18%,and in 2015 was 30.40% and 19.79%,respectively,which was correlated with the lack of centralized collection and treatment facilities for waste and sewage.Slope was the critical factor affecting total nitrogen loss in Pengxi River Basin,and rainfall is the critical factor affecting total phosphorus loss.In 2000 and 2015,the spatial distribution of total nitrogen and total phosphorus had large spatial heterogeneity.In 2015,the areas with serious pollution were more concentrated near the outlet of the basin,showing the shape of spreading from the outlet to the upstream.Yunyang County located in the downstream of the Pengxi River Basin and near the outlet of the basin,where urban land was more concentrated,had the largest non-point source pollutant load density in the six districts and counties of the Pengxi River.The phenomenon of " cultivation on steep slope land " increased slightly,and planting with slope greater than 25 degrees would aggravate soil erosion,and the risk of non-point source pollution would also increase.However,in 2000,the pollution level did not show a distinct centralized distribution pattern,and the heavily polluted areas were related to the distribution of arid land.In 2000,there was no significant difference in the total nitrogen and total phosphorus loads exporting to the river among the sub-basins,and the pollution was relatively light.In 2015,there were significant differences among the sub-basins and the pollution was relatively heavy.The highest density of total nitrogen and total phosphorus appeared in the C1 sub-basin,and the lowest density appeared in the C4 sub-basin.The difference of the density of total nitrogen and total phosphorus exporting to the river among the subbasins of Pengxi River was directly related to land use.There is a direct relationship between grassland reduction,arid land,urban land increase,and high pollution density.At the same time,the greater the slope and the smaller the elevation led to the more serious the non-point source pollution.Within the Press-State-Response framework,four response scenarios were put forward: reducing fertilization appropriately,returning arid land to grassland with slope larger than 25 degrees,establishing hedgerows near river channels and basin outlet to prevent nitrogen and phosphorus from exporting to the river,and establishing biogas digesters to collect and treat rural domestic wastes and livestock and poultry manure.The total nitrogen and total phosphorus can be reduced by 82.37% and 66.21% by "reducing fertilization","returning arid land to grassland" and "building hedgerows".It was calculated that 1.8% of households in the Pengxi River Basin need to build biogas digesters,and the four response schemes can ultimately reduce the total nitrogen and total phosphorus loads exporting to the river. |
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