Research On Water Quality Simulation Of Medium Sized Reservoirs In Southwest Mountainous Area

China is a country with many lakes and rivers.From the full implementation of the river chief system to the formulation of the"three red lines"of water resources,it all means that the water environment of lakes and rivers has always been a topic of national concern.At the same time,with the rapid development of my country’s water conservancy industry,the number of reservoirs is also increasing,especially the emergence of medium-sized reservoirs,which can effectively help the region to better regulate water resources,which is of great significance and effect.The Youyi Reservoir in Sichuan Province has all the characteristics of the water environment faced by medium-sized reservoirs in the mountainous areas of southwestern China,and is typical.Research on the establishment of a water quality coupling model for the Youyi Reservoir has very important scientific value and practical significance for the pollution control of medium-sized reservoirs in the mountainous areas of Southwest China.This study adopts numerical simulation methods,combined with ArcGIS’s analysis and processing functions of the basin’s topography,hydrology,and pollution source databases,and MIKE21’s better simulation and prediction of water quality pollutants,and establishes a coupling model of ArcGIS and MIKE21 to analyze the hydrological characteristics of the Youyi Reservoir.Point and area pollution source loads,establish river basin hydrology and pollution source databases,conduct water quality simulations,evaluate the simulation results,and propose pollution control measures and pollution load reductions.The main research results of this paper are as follows:（1）Based on ArcGIS,use DEM,land use types,estimated population data,etc.to generalize and calculate the pollution load of Youyi Reservoir,and the calculation results are used to create an ArcGIS database;（2）Based on the ArcGIS database,use MIKE21 to establish the hydrodynamic and water quality models of the Youyi Reservoir,and verify the rationality of the parameters respectively to complete the establishment of the Youyi Reservoir coupled water quality model.（3）Focusing on the distribution and migration of pollutants in the water body of the reservoir,select various villages and water intakes in the catchment that discharge sewage as the research objects.The simulation results show that the main factors affecting water quality are flow velocity and pollutant load;the main pollutant in the water body of Youyi Reservoir is PO₄^3-.（4）In order to quantitatively analyze the relationship between pollutant concentration,flow rate and pollutant load,Tianshan Village is selected as a typical object,and PO₄^3-,the main pollutant in the reservoir area,is selected as the analysis index.Scenario analysis is used to simulate different runoff flows into the reservoir.,The change process of PO₄^3-concentration in the water near Tianshan Village after the proportion of different sewage into the reservoir and the pollution load reduction.And get the following conclusion:pollutant load is the primary factor that affects the concentration of pollutants in water bodies.The Youyi Reservoir is a typical medium-sized reservoir in the mountainous area of southwestern China.Considering other medium-sized reservoirs with water pollution problems,the main water quality influencing factor is the pollutant load.Finally,the corresponding water environment improvement measures are given.（5）In order to clarify the goal of water environment governance of Youyi Reservoir,a zero-dimensional steady-state model is used to calculate the PO₄^3-water environment capacity of Youyi Reservoir under different storage conditions.In order to meet the requirements of maintaining the water quality of the reservoir in the most unfavorable situation,it is calculated that the total amount of agricultural domestic sewage and agricultural non-point source pollution that needs to be reduced in the Youyi Reservoir should be no less than 0.163t/a.This study utilizes the powerful functions of ArcGIS in the dimension of spatial analysis,the analysis and processing functions of the topography,hydrology,and pollution source databases of the watershed,and the better prediction and simulation functions of the ECOlab module of MIKE21 on water quality pollutants,and the ArcGIS spatial analysis technology and the MIKE model Coupling is applied to the study of the water environment of reservoirs,studying the influencing factors of water pollution and the spatial diffusion of pollutants,promoting water environmental protection,planning and management,and can provide reference for the study of other medium-sized reservoirs in the southwest mountainous area.