Simulation For Hydrodynamics And Water Quality Of Poyang Lake During Dry Season Based On EFDC Model

Poyang Lake is the largest freshwater lake in China,which is affected by“five rivers”and the Yangtze River.Since the beginning of the 21st century,especially after 2003,the hydrological rhythm of Poyang Lake has changed significantly,which mainly manifests as follows:earlier drying,prolonged drying and lower dry water level.Furthermore,due to the continuous growth of pollution load into the lake and hydrological characteristics such as the flow rate and water level in the dry season,the water quality in the dry season is worse and has attracted widespread attention.Therefore,a coupled hydrodynamic and water quality model of Poyang Lake is constructed with the assistance of EFDC（Environmental Fluid Dynamics Code）,focusing on the new characteristics of hydrological regime in the dry season of Poyang Lake and the corresponding water environment problems.Three typical hydrological years（2010,2011 and2012）are selected for simulation,corresponding to the high,low and medium flow year.It aims at analyzing the hydrological and water quality characteristics between the dry and wet season.Finally,it is essential to set different scenarios and simulate pollutants of five main rivers into the lake,focusing on the changes of hydrological regimes and responses of water environment during the dry season.The impact range and degree from each river are analyzed to provide scientific basis for water environment management of Poyang Lake.The main research conclusions are as follows:（1）After 2003,the annual average,highest and lowest water level decrease significantly,and the annual mean water level decreases by 9%,9%and 7%,respectively.During the dry season,the water level basically shows a downward trend in October and November while an upward trend from December to March of the next year.Furthermore,the runoff of the five main rivers into Poyang Lake shows the same trend,decreasing first and then increasing.October witnesses the largest decline,with a drop of 536.41m³ and 28%reduction.The decrease of the water level of Poyang Lake,the maximum decrease of the discharge in October and the early arrival of the dry water level are related to the operation and storage time of the Three Gorges Reservoir.（2）During the year,most water quality indicators are shown to be lower in spring and summer,higher in autumn and winter,lowest in July and highest in January.Further analysis shows that trophic states of Poyang Lake are different,especially in winter and summer.Inter annually,the2019.Intra annually,the TLI of the whole lake first decreases and then increases（49.15 in spring,45.95 in summer,49.93 in autumn and 50.06 in winter）,smallest in summer.（3）In the verification of hydrodynamic module,the simulation accuracy of hydrodynamic module is high,with Nash efficiency coefficient（NSE）above 0.90.In terms of water quality module,the average absolute error（MAE）of COD is within the range of 0.5～1.8 mg/L while TP is less than 0.15 mg/L.Although the simulation effect of NH₄⁺-N is slightly worse,it is consistent with the hydrodynamic and water quality of Poyang Lake in a long time series.The water quality of the wet season in Poyang Lake is better than that of the dry season.The pollution distribution of Poyang Lake is similar in different water seasons,but the concentration of Poyang Lake varies greatly in different regions.COD is mainly distributed in the central lake area,while TN and TP pollution areas are mainly located in the eastern and southern areas of Poyang Lake.（4）During the dry season,the average pollution sharing rates of COD,TN and TP in the five main rivers into Poyang Lake basically presents an increasing trend.The average sharing rate of with higher impact is in the range of 55%to 85%.The rank of water quality indexes is:TP>TN>COD.Furthermore,the influence of the five main rivers into the Poyang Lake on the surrounding rivers is relatively high.At the same time,the influence range of different water quality indexes presents a rising trend.The rank of the influence range is:TP>TN>COD.In terms of spatial distribution,the influence range of pollution is also affected by discharge,largest in Ganjiang River and smallest in Xiuhe River.