Refinement Modelling Of The Non-point Source Pollution In The Interval Basins Of The Three Gorges Reservoir

The hydrological conditions of original river channel and intrinsic ecological balance between nature-people-water systems have been modified,as well as the transformation of regional landscape patterns and impacts on eco-environment have been greatly altered since the Three Gorges Project started operation.This led to the significant increases in the complexity of non-point source(NPS)pollution with regard to spatial heterogeneity and dynamic variability in the,Three Gorges Reservoir(TGR).In the context of the refinement management of water environment,promoting refinement modelling of NPS pollution in the TGR will provide important implications and lessons in term of water quality assessment and water environment security for the TGR region or even nation-wide.In this paper,based on the thorough review of the existing NPS pollution models and techniques,a refinement modelling theoretical framework of NPS pollution was proposed,and then was calibrated and validated in the interval basins of the TGR.The main research contents and findings include:(1)Taking into account the requirements for NPS pollution refinement model,the existing refinement-related models were reviewed,including NPS pollution model,model fundamental databases selection,refinement level of model computing units,and accuracies of model parameters.At present a more detailed NPS pollution refinement modelling theoretical framework was needed.(2)From the ' model structure-model inputs-model calculation-parameter optimization-statistical analysis' whole-processes NPS modelling perspectives,this study proposed that the integration of distributed physical model,high resolution,space representation and localized database,multilevel spatial discretization and overall parameter optimization were essential for refinement modelling.This refinement model was efficiently validated and applied in the interval basins of the TGR,and results outputted from the macroscopic scale,computing microscopic scale,and township-level mesoscopic scale;comprehensively elucidating the NPS pollution characteristics,and identifying crucial periods and areas for NPS pollution generation in the interval basins of the TGR.(3)Based on the spatial heterogeneity characters,macroscopic scale analysis of the spatial distribution of runoff and pollution loadings in the interval basins of the TGR were conducted.The results of runoff indicated that distinct differences between flood and non-flood seasons,showing a seasonal trend of summer>autumn>spring>winter,spatially downstream>upstream and left bank>right bank,linear correlation between precipitation and runoff generation;whilst pollution loadings generation presented a seasonal trend of summer>spring>autumn>winter,spatially higher pollution loadings generated in the upstream and hinterland,higher pollution generated per unit area from the Chongqing part than Hubei part,and relatively evident linear correlation between runoff and NPS pollution loads.(4)Based on the TGR macroscopic scale analysis findings,Pengxi River and Xiangxi River basins were selected for HUR-level and township-level NPS pollution statistical analysis respectively,because of the special geological and climatic characteristics and prominent environmental problems after the TGR impoundment.Microscopic scale analysis of NPS pollution under different underlying surfaces was carried out.The results showed surfaces under agricultural and forest lands,purple soil and paddy soil,0?20°land produced higher nitrogen and phosphorus loadings per unit area.Based on area-weight allocation method,NPS pollution generation at township level was analysed,identifying sensitive areas and partitioning multi-gradient prevention and control areas for NPS pollution.A pattern of'locality concentrated,uneven distributed,and near water' distribution of NPS pollution loadings was presented.The highest pollution loading was produced from townships dominated by cultivation lands(>40°)on yellow loam soils,forest land(0-20°)on calcareous soils,and forest lands(20?40°)on purple soils.Mainly cultivation lands(20?40°)contributed most to the generation of NPS pollution loadings in the second-gradient townships.Less NPS pollution loadings were generated from townships where mainly covered by forest and water areas.