Study And Application Of Numerical Model Of Water Environment For Yanghe Reservoir Basin System

With the sustainable development of the economy and society,the pollution of water environment in catchment area is becoming one of the major disasters for society.The eutrophication of lakes and reservoirs caused by the deterioration of ecological environment seriously affects the sustainable development of water resources and human health,especially for polluting the reservoir water as the source of urban water supply.In recent years,with the accelerating pace of the urbanization of Yanghe Reservoir and the adjustment of agricultural structure,the reservoir water quality rapidly worsened and the eutrophication of the water body gradually intensified,which seriously affected the local security of water supply system.Therefore,it is necessary to construct a water environment and water quantity mathematical model for the process of pollutant modelling,migration and diffusion under the river catchment system,and to research the water environment protection technology which focuses on simulate and forecast the spatiotemporal variation laws of pollutants prediction.Those methods provide scientific and technological support for protecting the water environment of Yanghe Reservoir,and contribute great significance to improve the water quality for current deteriorated phenomenon,and to maintain the safety of the urban water supply system in this region.This paper choose Yanghe Reservoir watershed as a research system,through identifying and analyzing the typical annual pollution sources in the upstream of Yanghe Reservoir,the output coefficient method is used to calculate the different types of pollution source and variables of load.GIS is used to carry out hydrological spatial discretization and establish the database for non-point source of pollution in the catchment.The laws of spatial distribution and process of evolutionfor non-point source of pollution in the catchment of Yanghe Reservoir was analyzed by using the distributed hydraulic model of Soil and Water Assessment Tool?SWAT?to simulate hydrology and water environment of the runoff-sediment-pollution in the catchment.And the quantitative relationship between load,output quantity and inputquantity of pollutants in the upstream of the catchment was obtained.The concentration various and distribution laws of total nitrogen?TN?,total phosphorus?TP?,and ammonia nitrogen?NH₃-N?under the special boundary and landform condition were calculated by using the SWAT model based on the input of runoff and pollutant,coupling with DHI MIKE21.The model analyzed internal and external factors for the change of water quality in reservoir,and then realized the joint simulation between hydrology and water quality in catchment system.Finally,the pollutant abatement research based on the water environment capacity for catchment area is carried out.In this paper,the following innovative results are obtained:?1?The research analysis the upstream source of pollution database of Yanghe Reservoir.The data structure shows that the population and village is positively correlated with the pollution load of discretized spatial element;the average annual TN,TP,COD and NH₃-N loading quantity in upstream of Yanghe Reservoirare 7983t,3759t,46527t and 2769t,respectively.Among them,livestock breeding is the largest pollution source in the catchment,followed by agricultural pollution.The spatial distribution of each pollution source basically decreases from west to east,which is basically consistent with the population distribution,topography and vegetation.?2?The SWAT model has a good effect on the runoff simulation.The rate on a regular basis R² and the verification period?Nash-Sutcliffe?NSE are 0.96 and 0.98respectively,and the error rate is only 2.87%.The simulation results of water quality TP and NH₃-N are better,R² and NSE are more than 0.85 and 0.8 respectively;TN model is less effective;on the whole,the model can simulate the hydrological process and water environment of the catchment area.The applicability of the model is strong.?3?The SWAT simulation result shows that annual average TN loss is 845.96t,drainage rate is 10.6%,TP is 240.56t,drainage rate is 6.4%;drainage loss per unit in catchment area is between 0.53kg/ha and 32.86kg/ha.The main component of N and P nutrients is the organic state?adsorption state?.The spatial distribution of non-point source pollution in Xiyang River Basin is the largest,followed by Dongyang River.Non-point source pollution loss in the inter-annual,year-to-year change significantly which have huge loss performance in the flood season and flood load,quite small loss phenomenon in dry years and non-flood season;increasing nutrient concentration due to large pollutant loading into the storage area,TN increased by 6.5 mg/L,TP increased by 0.45 mg/L and NH₃-N increased by 0.35mg/L,which was the external factor of reservoir eutrophication.?4?Based on the simulation of hydrodynamic and water quality in Yanghe reservoir area by using MIKE21,the diffusion and degradation laws of pollutants in the reservoirs on the flood season were studied.The result shows that the water carrying large nutrient in the flood season,with the increase of water amount,the flow velocity is increasing Which is beneficial for the diffusion of pollutant concentration,and the duration of the rising section is shorter and the peak time is beyond,leading to rapid mixing of pollutants with low concentration water,resulting in higher concentration of pollution in largewater area;however,as the lower flow after flood season,reservoir outflow is decreasing to meet the storage requirements and reduce the flow velocity.As large number of pollutants stored in the reservoir in the summer,high temperature and other factors under the influence could easily lead to"bloom"phenomenon.?5?Under the different condition of hydrological design,the management of water environment capacity for most river sections is seriously exceeding the standard.To meet the capacity requirements and achieve ecological restoration,it is need to process quantitative reductionfor different runoff guarantee rate of pollutants in catchment area.