Identification Of Critical Source Areas Of Nonpoint Source Pollution And Priority Control Measures Research In A Typical Mountainous Watershed Of The Three Gorges Reservoir Area

With the technology of point source pollution control becoming mature,non-point source(NPS)pollution has become the main source of water pollution.The use of hydrological models to simulate NPS pollution is one of the main trends of NPS research in the world.As a typical semi-distributed hydrological water quality model,HSPF(Hydrological Simulation Program-Fortran)model is suitable for the simulation of hydrological water quality process and scene response.It is widely used in the research of NPS in the world,but it is still in its infancy in China.Accurately simulating the temporal and spatial distribution of NPS pollution is an important prerequisite for identifying the critical periods(CPs)and critical source areas(CSAs).CPs and CSAs respectively refer to the high-risk periods and areas of NPS pollution in the watershed.In the control of NPS pollution,the identification of CPs and CSAs is directly related to the efficiency and effect of prevention and control measures.Most of the international studies on the identification of CPs and CSAs were carried out separately,and it was difficult to unify the periods and areas in the formulation of pollution control measures to achieve the best governance benefits.Therefore,it is of great significance to establish an effective and objective collaborative analysis method for CPs and CSAs.Based on HSPF simulation results,this paper proposed a collaborative analysis method of CPs and CSAs.Based on the cumulative load-time/area curve fitting,this method could effectively and quantitatively identify the CSAs in CPs of NPS pollution in a small mountain watershed,and provided a reference for the control of water environment pollution in mountainous areas.Daning River is one of the important tributaries in the Three Gorges Reservoir Area(TGRA).The research object is upstream Daning River Basin.Based on the basic data from 2004-2008 and 2016,the HSPF model was used to simulate the spatial and temporal distribution of phosphorus loss in the study region.The CPs of phosphorus loss in the basin was identified by the load-time curve fitting,then,and then the CSAs in CPs was identified by load-area curve fitting.Furthermore,the multi-level priority control measures for NPS pollution in the basin were proposed.The main contents and conclusions are as follows:(1)Calibration and validation of HSPF model.The R~2 of the HSPF model for flow and sediment simulation were all greater than 0.85 in the calibration and validation periods,and the R~2 of the total phosphorus(TP)load were all around 0.7,which specified that HSPF model was suitable for NPS pollution simulation in the upstream Daning River Basin and the similar mountain basins.(2)Simulation of NPS phosphorus pollution in the upstream Daning River Basin based on the HSPF model.The maximum TP loads appeared in July,and the total TP loads was 819.05t,accounting for 27.21%of the annual TP loads;the minimum value appeared in October,and the total TP loads was 48.73t,accounting for 1.62%of the annual TP load.The areas with high TP loads were mainly distributed in the northwest and south-central part of the basin;the areas with low TP loads were mainly distributed in the north and middle of the basin.(3)Identification of CPs and CSAs of NPS phosphorus pollution in the upstream Daning River Basin based on cumulative load-period/area curve.The CPs of NPS pollution were months 5,6 and 7,which accounted for 53.67%of the TP loads;sub-CPs were months 1,3,4 and 8,which accounted for 29.22%of the TP loads;and non-CPs were months 2,9,10,11 and 12,which accounted for 17.11%of the TP loads.In CSAs,49.51%of the TP loads were from 26.76%of the area,and the CSAs were mainly located along the Daning,Dongxi and Houxi rivers;in non-CSAs,50.49%of the TP loads were from 73.24%of the area.(4)Multi-level priority control measures for NPS phosphorus pollution in the upstream Daning River Basin.The following multi-level priority control measures were recommend:CSAs in CPs,CSAs in sub-CPs,CSAs in non-CPs,non-CSAs in CPs,non-CSAs in sub-CPs and non-CSAs in non-CPs.In the CSAs in CPs,agricultural land should be taken as the priority control area of NPS pollution.In the CSAs in sub-CPs,the agricultural land was still a critical area of NPS pollution control,and measures such as afforestation and conversion of farmland to forest should be strengthened to control the NPS pollution in forest.In the CSAs in non-CPs,rural living pollution was the focus of control.Specific measures for controlling NPS pollution in the basin were put forward from three aspects:source control,process reduction and end treatment.