Simulation Of Non-point Source Nitrogen And Phosphorus Pollution Effects And Pollution Loads In A Typical Plain River Network Area Watershed

In recent years,serious non-point source pollution in the plain river network area has exacerbated the water crisis,and the water environment and water ecology of the basin are in urgent need of improvement.Changzhou is located in the lower reaches of the Yangtze River basin,and the water pollution problem caused by non-point source pollution in the basin is more serious.During the"Thirteenth Five-Year Plan"period,only five of the eight cross-sections included in the national"Water Ten"assessment met or exceeded the requirements of Class III water quality,and the compliance rate of Class III water was only 62.5%,failing to meet the water quality assessment requirements.At the same time,the water quality of the section is prone to decline or even deteriorate during the flood season.In order to improve the water quality of the basin and its river network,it is necessary to quantitatively analyze the production and discharge and water pollution characteristics of a typical plain river network,and construct a corresponding model for a series of calculations to provide scientific support for further pollution reduction,water environment management and its sustainable development in the region.This paper proposes a multi-scale and hierarchical method for watershed delineation in plain river network areas based on the collection and processing of basic data on meteorology,hydrology,water quality,soil,land use,dikes and gates and dams.The HSPF land non-point source pollution and HEC-RAS river network hydrodynamic water quality coupling model was further constructed by selecting a typical plain river network area in Changzhou with more serious water quality exceedance,higher environmental risk and higher environmental concern.The quantitative solution analyzes the spatial and temporal distribution of nitrogen and phosphorus non-point source pollution in the region,changes in water quality concentrations at major cross-sections and the production and transport effects of nitrogen and phosphorus pollution sources in typical river sections.The main findings of this study are as follows:（1）The multi-scale and hierarchical method of watershed delineation in the plain river network area based on the HSPF model is proven to be feasible,accurate and reasonable,as the delineated sub-basins can fully reflect the spatial topographic and geomorphological differences in the basin,realistically represent the hydrological units of the basin and accurately simulate the hydrological processes in the plain river network area.（2）In response to the problems of simplifying the river network and retaining only the main river channels when the existing watershed model,water quality model and other coupled models are applied in the plain river network area,it is impossible to simulate the reciprocal flow of the plain river network and completely simulate the non-point source pollution process in the watershed.In this study,a coupled HSPF land non-point source pollution and HEC-RAS river network hydrodynamic water quality model was constructed and selected for application in a typical plain river network area.The model simulation results show that the constructed coupled model solves the problem that the watershed model only retains the main river channels and cannot simulate the reciprocal flow and cross-loop river network when applied to the plain river network.The coupled model completely simulates the non-point source pollution and water pollutant migration processes in the watershed,improves the spatial accuracy of the model,and enhances the applicability and scientificity of the coupled model for application in the plain river network area.（3）The simulation results of non-point source pollution from the coupled model show that non-point source pollution is the main source of nitrogen and phosphorus pollution in the watershed,accounting for more than 70%of the pollution in the watershed,while non-point source pollution in the watershed mainly comes from aquaculture and domestic sewag.From the time point of view,nitrogen and phosphorus pollution is mainly concentrated in the rainfall time period,non-point source pollution and rainfall correlation is strong,the flood season is the main time period of non-point source pollution generation.Spatially,NH₃-N,TN and TP are mainly distributed in the southeast and southwest of the two lakes innovation area,while the central part is less polluted.（4）The water quality impact of nitrogen and phosphorus pollution under different scenarios of non-point source pollution ratio was chosen as the most serious flood season.The study shows that when the existing point and non-point source conditions remain unchanged（scenario 1）,the NH₃-N concentration at the Huan Hu Xi Road Bridge section and the TP concentration at the Buqiao section meet the Class III water standard situation most of the time,while the water quality situation at the rest of the sections have more time to exceed the water quality standard.Scenario 2 and scenario 3 of the non-point source nitrogen and phosphorus pollution conditions,although the water quality concentration of each section fluctuates,but some sections of the water quality changes are small.In scenario 4（50%reduction of non-point source pollution）,the concentration of NH₃-N,TN and TP at each section are reduced substantially,and the water quality condition of each section is optimal at this time,but the water quality of Beqiao section is less affected by land-based non-point source pollution due to its own larger flow,and more affected by the water quality from upstream.Therefore,in order to improve the water quality of the basin,in addition to increasing the control of non-point source pollution in the basin,it is also necessary to control the water quality condition of the upstream water from the cross section.The methods and conclusions of this study have important application value for accurate understanding of pollution source production and discharge load distribution,identification of pollution exceedance process in river network water system and environmental refinement management in Changzhou City.