Numerical Simulation Of Hydrodynamic-Water Quality In Lake Erhai Based On EFDC Model

Lake Erhai is one of the ’new three lakes’ under key conservation in China,and the protection of it has been paid lots of attention by the government.Based on systematic measured data of hydrology,meteorology and water quality there,this study established a three-dimensional hydrodynamic-water quality model via EFDC,and the effects of different external load reduction measures on the water quality of Lake Erhai were discussed.The conclusions of this paper are as follows:(1)The water quality of Lake Erhai in 2017 showed the characteristics of ’good in winter and spring,poor in summer and autumn,heavy in the north and south and light in the middle’.The peak load of nitrogen and phosphorus mainly occurred from July to August,and was mainly distributed in the west(57%)and north(39%)of Lake Erhai.(2)Lake Erhai calculation districts were divided into 1056 grids,and the three-dimensional hydrodynamic model of Lake Erhai was established based on EFDC.The water temperature model was sensitive to the variations of Fraction of Fast Wave in Solar Shortwave Radiation and Fast Scale Solar Shortwave Radiation Attenuation coefficient.After adjustment,the average Nash coefficient of water level and water temperature exceeded 0.9 and 0.8 respectively,so the model could accurately simulate the hydrodynamic characteristics of Lake Erhai.(3)The hydrodynamic simulation results show that the spatial distribution of the water temperature in Lake Erhai varies significantly with the seasons.The water temperature increases from the north to the south in spring and summer,and it is low in the north and south,high in the middle during autumn and winter.The vertical water temperature difference of Lake Erhai is usually maintained within 1.5℃.The weak stratification of water temperature is of great significance for the vertical exchange of nutrients in Lake Erhai.Counterclockwise circulation is easy to form in the south under the influence of topography,while clockwise circulation is easy to form in the middle and north of Lake Erhai under the dominant southwest wind.(4)The three-dimensional water quality model of Lake Erhai was established,which could well reflect the temporal and spatial changes of the water quality of Lake Erhai.The sensitivity analysis results of the water quality parameters showed that the phosphorus half-saturation for cyanobacteria was the most sensitive parameter,which indicated that nutrient limitation rather than light limitation played an important role in algal and water quality variations in Lake Erhai.(5)Based on the constructed model of Lake Erhai,the influence of different external load reduction schemes on the water quality of Lake Erhai was studied by scenario analysis method.The simulation results of the project reduction scheme show that 20%reduction of nitrogen and phosphorus load in the western ditch is preferred,and its improvement effect of water quality in the whole lake is the best.Taking the effect of water quality improvement and economy into account,reducing nitrogen and phosphorus by 20%in the western ditch and 10%in the northern three rivers are the preferred combination of engineering solutions.The Boluo river load reduction project is the most effective to improve the water quality in the south of Lake Erhai.The high load reduction scheme shows that the western ditches and the northern three rivers are the important sources of nutrient load into Lake Erhai.The load reduction of the western gully plays an important role in improving the TN and TP water quality of Lake Erhai,while the effect of the load reduction of the northern three rivers on the water quality of Lake Erhai decreases from north to south.