A Study Of The Dynamic Water Environment Capacity Of The Fen River Based On Numerical Hydrodynamic Water Quality Model

In recent years,China has attached great importance to water environment issues.Although the water quality of the main stream of the Yellow River has improved significantly overall,there are still some pollution problems in certain tributaries in the middle reaches of the Yellow River.The Fen River is the second largest tributary of the Yellow River and the largest river in Shanxi Province,water pollution caused by the extensive development of industrial enterprises has become the main bottleneck problem for local green development.Accurately calculating the water environment capacity of rivers（i.e.environmental capacity）is the core scientific basis for watershed water ecological environment management and key planning,and the water environment capacity of water bodies has always been a key planning and treatment method for watershed water ecological environment.However,previous traditional research on the calculation of the water environment capacity of a watershed was a static and fixed value.A single total amount often could not truly transform to meet the needs of actual refined management,leading to insufficient accuracy and difficult implementation.This article proposes a dynamic calculation method for water environment capacity based on the MIKE（hydrodynamic water quality coupling）model,studying and analyzing the water quality evolution trend of the Fen River in the middle reaches,revealing the changing response mechanism of the dynamic water environment capacity at different time scales for the main pollutants in the river.The main research content includes a comprehensive investigation of the Fen River basin,collecting basic water quality data and natural condition information such as geographic location,topographic features,climate characteristics,and hydrological characteristics of the Fen River basin.At the same time,basic social and economic information such as population profile,industrial status,and economic overview in the research area is also collected and analyzed briefly,analyze and study the current water quality status of the water functional zones in the Fen River Basin,evaluate the water environmental quality;based on the results of water quality evaluation in the Fen River Basin combined with actual data,analyze the temporal and spatial characteristics of pollutants to identify the main pollution factors in the research area,and further determine the regional scope for the study of the dynamic water environment capacity model.The research aims to establish different hydrological scenario models,calculate the dynamic water environment capacity of various water functional zones in the middle reaches of the Fen River at different scales of wet and dry periods and monthly scales,and analyze the differences in water environment capacity under different water periods and monthly scales.The main conclusions of the study are as follows:（1）Overall,the main influencing factors affecting the water quality in the Fen River basin,in addition to TN,are NH₃-N.Spatially,the order of water quality from good to bad in the Fen River is Wangzhuangqiaonan,Wennanshe,and Shanglan section.Temporally,water quality from February to August is better than that in other months,with significant differences in monthly changes for all indicators.The main pollutants TN,NH₃-N,and COD are still severe in all sections,and water quality in flood periods is significantly better than that in dry periods.（2）Based on the results of regression analysis and principal component analysis,the study area can be divided into urban and rural domestic pollution sources,agricultural non-point source pollution（Land Use Type）,livestock breeding,and industrial point source pollution.The annual riverine input of TN in the study area is5935.45 t/a,followed by COD at 3774.84 t/a,NH₃-N at 1900.29 t/a,and TP at 168.8t/a.The main sources of NH₃-N,TN,and TP loads are urban domestic pollution sources,with contribution rates exceeding 50%of the total riverine load.The main source of COD cannot be determined from the given information.（3）Collected and organize hydrological and water quality data as well as river section information in the study area.Using MIKE11 software,we established a hydrodynamic-water quality coupled model for the study area,determined boundary conditions,and calibrated relevant parameters.We conducted simulation calculations on water quality in the river sections of the study area and established the relationship between pollution sources and water quality sections.The simulation results show that the simulated flow coefficient of the Second Dam in the Fen River is R²=0.8219,Ens=0.69>0.5;the relative errors of COD,NH₃-N,TN,and TP at the Wennanshe section are 2.58,0.34,0.63,and 0.024,with absolute errors of 10%,20%,8%,and9%,respectively;the correlation coefficients are R²=0.7861,0.8527,0.8667,and0.8491.The acceptable difference between the simulated values and the measured values indicates that the model can reflect the actual variation trend of pollutant concentrations,as the general trend remains consistent.（4）NH₃-N,COD,and TP were chosen as calculation factors to calculate the dynamic water environment capacity values of various water functional zones in the middle reaches of the Fen River at different scales of high and low water levels and on a monthly scale.The results showed that the water environment capacity during the flood season was much greater than that during the dry season,while the water environment capacity during the normal water level season was only slightly different from that during the dry season,overall reflecting dynamic changes.The results showed that the water environment capacity during the flood season was much greater than that during the dry season,while the water environment capacity during the normal water level season was only slightly different from that during the dry season,overall reflecting dynamic changes.The dynamic water environment capacity of each functional zone at a monthly scale also varies with changes in hydrological conditions.The monthly water environment capacity values for COD and TP show similar trends,with the greatest change occurring in March due to local supplementary water projects increasing flow rates.The dynamic water environment capacity of functional zone C is higher than that of other river sections,while the dynamic water environment capacity of functional zone D is the lowest among the four functional zones.The changes in pollutant reduction are similar to the changes in dynamic water environment capacity.For pollution control measures in the middle reaches of the Fen River,supplementary water projects can be implemented to ensure the flow of the river,and efforts should be made to strengthen the management of agricultural non-point source and livestock and poultry breeding pollution,as well as industrial pollution sources supervision.