Simulation Of Adsorbed Nitrogen And Phosphorus Load In Mountainous Watershed Based On Improved SWAT Model

Non-point source pollution is the main cause of current water pollution.Driven by rainfall runoff,the eroded soil and its nitrogen,phosphorus and other pollutants in each unit of space will be transported into the receiving water body,forming the adsorbed non-point source pollution load.Adsorbed non-point source pollution,especially adsorbed phosphorus pollution,is the main cause of eutrophication of many surface water bodies in China.Because of the large topographic height difference and obvious gradient characteristics,the mountain basin is not conducive to soil and water conservation.Therefore,in view of the special geographical situation of mountain basin,the study of slope(land)sediment delivery and adsorbed non-point source pollution is carried out.Taking the three Gorges Reservoir area,an important mountain basin in the upper reaches of the Yangtze River as an example,it is bound to have important theoretical and practical significance to carry out the research.In this paper,the temporal and spatial distribution of hillslope sediment delivery ratio is constructed from the point of view of sediment delivery on slope surface of mountainous watershed.On this basis,the sediment module in SWAT model is improved to form an improved slope sediment delivery model and adsorbed nitrogen and phosphorus load model.The model is applied to the Three Gorges Reservoir Area,the temporal and spatial distribution of adsorbed nitrogen and phosphorus load in the reservoir area is simulated,the key areas and main influencing factors of load formation are analyzed,and the load reduction effect under different pollution reduction measures is simulated.The main findings are as follows:(1)The temporal and spatial distribution of hillslope sediment delivery ratio is constructedThe hillslope sediment delivery ratio has obvious spatial heterogeneity and changes with time.In this paper,based on the analysis and research of the existing hillslope sediment delivery ratio formula,since the surface runoff formed by rainfall is the direct driving force of sediment delivery,the existing "Flux Connectivity" index is improved.The "Runoff Connectivity" index is put forward and the spatio-temporal distribution of hillslope sediment delivery ratio is constructed by this index.(2)Hillslope sediment delivery model and the load models of adsorbed nitrogen,adsorbed phosphorus were establishedThe spatial and temporal distribution of hillslope sediment delivery ratio was introduced into the revised universal soil loss equation(RUSLE)to form a model of hillslope sediment delivery in mountainous basin,which simulated the spatial and temporal distribution of slope sediment delivery.Furthermore,the adsorption nitrogen and phosphorus load model in mountainous basin is constructed and its rationality is verified.(3)Simulation of Spatio-temporal Distribution of adsorbed nitrogen and phosphorus in the Three Gorges Reservoir AreaIn order to apply the model to the Three Gorges Reservoir Area which is not a watershed,the reservoir area is divided into four districts according to the comprehensive conditions of hydrological environment,such as Yulin District,Xiaojiang District,Daning District and Xiangxi District.The model parameters of representative watersheds in each district were selected as the model parameters of the plot,and the sediment delivery distribution and adsorbed nitrogen and phosphorus load distribution in the Three Gorges Reservoir Area were simulated.(4)Analysis of influencing factors of pollution load in Three Gorges Reservoir Area and Simulation Evaluation of pollution reduction measuresThe slope sediment transport amount and adsorbed nitrogen and phosphorus load and load intensity of different elevation belts and slope ranges,various land use modes and soil types were analyzed to explore the main output sources of pollutants.After clarifying the main cause of pollution,simulate returning farmland to forests in dry land with slopes greater than 25°,20°,and 15° and simulate reduce fertilizer application by 10%,20% and 30% respectively,then the reduction effect of adsorbed nitrogen and phosphorus in these conditions was analyzed.