Water Quality Simulation Study Of Xiaowan Reservoir In Lancang River Based On ECOlab Model

The construction of the reservoir dam has a great potential impact on the water environment of the river basin,especially the multi-year regulation of the reservoir,which completely changes the hydraulic characteristics and material migration laws of natural rivers.As the key river for the development of China's water conservancy and hydropower projects,the Lancang River has developed and planned 14 cascade hydropower stations in Yunnan Province.The impact of the reservoir dam project on the water environment in the Lancang River Basin has attracted extensive attention.This study selects the Xiaowan Reservoir,a typical alpine gorge-type high dam in the Lancang River Basin.Through the combination of on-site observation sampling,construction of numerical models and theoretical analysis,the distribution characteristics and time-space variation of important water quality indicators are studied.It provides necessary basic support for the in-depth study of the water ecological environment effect of the cascade development of the Lancang River,and provides reference for the scientific dispatch operation and water ecological protection of the Xiaowan Reservoir.The main findings are as follows:?1?Based on MIKE ECOlab,the one-dimensional hydrodynamic-water quality model of Xiaowan Reservoir was constructed,and the model was verified by the measured data.Seven sampling points are set in the main reservoir area of Minjiang River,and two points are set in the Heihuijiang branch area.The water quality parameters of the key biochemical processes in the ECOlab model were verified by using the measured water sample data at each point.The model verification results were good and can be used for water quality simulation in the reservoir area.At the same time,the sensitivity of the important water quality parameters of the model was analyzed.The results showed that the primary degradation rate of COD,ammonia nitrogen yield and nitrification rate were the most sensitive parameters,and the denitrification rate only had a greater impact on NO₃^--N content and has little effect on other indicators.?2?The one-dimensional water quality model was used to simulate the temporal and spatial variation characteristics of COD,DO and nitrogen nutrients?NH₄⁺-N,NO₃^--N?under typical operating conditions of Xiaowan Reservoir.The results show that the COD content is closely related to the water inflow,showing a decreasing trend along the way,and showing a periodic change law of increasing in the wet season and decreasing in the dry season.With the degradation and consumption of COD in the water,DO presents a downward trend along the same process,showing a cyclical law of decline in the wet season,rise and fall in the dry season.The distribution of NH₄⁺-N has a certain relationship with COD along the way,rising slowly at first and then falling,and the law of change with time is negatively correlated with COD.The content of NO₃^--N is affected by nitrification reaction,and the distribution rule along the way is similar to that of NH₄⁺-N.The change rule over time is positively correlated with NH₄⁺-N.The nitrification reaction and denitrification reaction are in the dynamic balance of nitrogen cycle.?3?The three-dimensional water quality model was used to simulate the vertical distribution characteristics of COD,DO and nitrogen nutrients.The cross-section results corresponding to the one-dimensional water quality model were extracted as boundary conditions,and the three-dimensional model before the dam was constructed.The three-dimensional model was verified by using the measured water sample data in front of the dam.The simulation results show that Xiaowan reservoir is a typical stratified reservoir.In summer,the water temperature appears stratification of variable temperature layer,thermocline layer and constant temperature layer,and causes stratification of other water quality factors.DO and COD show the same stratification rule as water temperature.In summer,they show a stratification structure with high surface value and low bottom value.On the contrary,NH₄⁺-N shows the stratification structure of the bottom layer higher than the surface layer in summer.NO₃^--N has no obvious stratification in summer.When it gradually enters winter,the external environment temperature and water temperature gradually decrease,and the water gradually mixes.In winter,water temperature,DO,COD and nitrogen nutrient salts all have no obvious stratification structure,and are basically evenly mixed.