Characteristics Of Water Quality Variation And Pollutant Flux Analysis Of Si River Using MIKE21 Water Quality Model Coupled With Hydrodynamics

In recent years the Si river ecosystem has been damaged,which has threatened the sustainable development of cities along the Si River and the water quality safety of Nansi Lake.In this paper,the main tributaries of Si River including Shilai River,Shilou River,Ji River,Xian River and Xiaoyi River are the study area.Based on the investigation results of Si River,Mike21 hydrodynamic water quality coupling model was established.TN,TP,NH3-N and COD were taken as the research objects to simulate the hydrodynamic and water quality change process under different scenarios.Finally,according to the simulation results and the measured data,the temporal and spatial variation characteristics of water quality and pollutant flux of Sihe River are summarized.The main research results are as follows:(1)This study used three methods to evaluate the water quality of the main stream of Si river.The overall water quality of Si River from good to bad in different hydrological periods was in the order of dry season,normal season,wet season,and the spatial distribution of water quality from upstream to downstream is from good to bad.Based on the above water quality evaluation results,the standard reaching rate of secondary water function area of Si river was evaluated.The results showed that the rate of reaching the standard was 0%in wet season and 100%in normal and dry season.(2)The seasonal Kendall test was used to analyze the trend of water quality.The results show that during 2012-2017,the concentrations of TN and NH3-N have a downward trend,the concentration of TP has an obvious upward trend,while the COD has no obvious trend.(3)The hydrodynamic and water quality model set up by MIKE21 in the study area was verified by the measured data from Si River hydrological stations.The simulation results are as follows:1)the variation trend of water quality concentration in the simulation period is basically consistent with that of flow;2)in the same hydrological year,the rate of water environment function zones that meet the standard of Si River in the initial and late simulation period is higher than that in the middle simulation period,and that rate is higher than that of class V water when the inflow water is class IV water.When the inflow water quality is the same,the water function area has the highest compliance rate in dry year,followed by normal year,and the lowest compliance rate is in wet year;3)vertically,after the tributaries flow into the main stream,the water quality concentration on the side of the river near the tributaries first increases,and then gradually increases with the dilution and diffusion of the flow In the horizontal direction,the water quality concentration in the middle channel is lower than that on both sides before the tributaries join,and after the tributaries join,the water quality concentration distribution changes as follows:one side near the tributaries>the other side of the river>the middle channel.(4)The pollutant flux estimation results based on the measured data show that the variation trend of annual pollutant flux is basically consistent with that of annual runoff.Based on this,the linear equation of annual flux and annual runoff is established to predict the flux change.The annual variation characteristics of pollutant flux are that the flux increases in flood season and decreases in non flood season.Taking Shuyuan Station as a representative,the results show that the contribution rate of non-point source pollution is more than point source pollution.(5)The results of pollutant flux estimation based on the simulation data show that in the same hydrological year,the pollutant flux is:inflow water quality is class V>inflow water quality is class Ⅳ,in the same inflow water quality case:wet year>normal year>dry year;in each simulation scenario,the daily flux and daily flow are significantly positively correlated.Based on this,the univariate linear equation of daily flux and daily flow is established..(6)The results show that the loading ratio of COD is the largest and that of TP is the smallest.