Study On Numerical Simulation Of Water Temperature And Effects Of Low Temperature Water Releases From Large Reservoir In The Upper Reaches Of The Yellow River

The influence of low-temperature water discharged from large reservoirs on the down-stream water ecological environment and its improvement measures are one of the important issues of concern for reservoir water environmental engineering.The study on the distribution of water temperature and the variation law of discharge water temperature in reservoirs can provide an important reference for formulating an effective selective withdrawal scheme.The Longyangxia Reservoir in the upper reaches of the Yellow River was taken as the research object.Based on the in-situ observation data of water temperature in the reservoir,the spatial and temporal distribution characteristics of water temperature in the reservoir were analyzed,and the three-dimensional numerical model of water temperature in the reservoir was established.Meanwhile,the main parameters of the model were calibrated and verified.Sensitivity analysis of influencing factors of the model was carried out by simulating the changes of water temperature over the years.Finally,the distribution of water temperature and the change of discharge water temperature under different stratified water intake conditions were simulated.The main achievements of this paper were as follows:（1）The water temperature distribution of the Longyangxia Reservoir presents a typical stable thermal stratification characteristic.The operating water level of the reservoir is closely related to the change of thermal stratification structure of the reservoir,and is also the main factor affecting the change of discharge temperature.The higher the operating water level of the reservoir,the more significant the temperature lag at the bottom of the reservoir.The higher the temperature of discharge water from December to March in next year,the lower the temperature of discharge water from May to October.（2）A three-dimensional water temperature model of the Longyangxia Reservoir was constructed by using MIKE3 software.The calibration of model heat exchange parameters and model validation were carried out.The errors of the simulation results of the model at different operating levels were analyzed.The root mean square error RMSE=0.44°C and coefficient of determination R²=0.99 at low water level.The root mean square error RMSE=0.60°C and coefficient of determination R²=0.98 at high water level.Sensitivity analysis shows that the sensitivity of water temperature to boundary conditions is different in different water levels and layers.The water temperature gradient of thermocline is most affected by the inflow rate at low operating water level,and the sensitivity is Ss=0.54305.The water temperature gradient of thermocline is most affected by the water temperature of the reservoir at high operating water level,and the sensitivity is Ss=0.67716.The Sun constants a₂ and ₂b in?ngstr?m’s law have the most significant influence on the model calculation results.（3）According to the parameters of submergence water depth,withdrawal height and withdrawal layer number,two different selective withdrawal schemes were designed for reservoir under low and high water level operating conditions.The effects of different selective withdrawal schemes on the improvement of low-temperature drainage water were simulated and studied.According to the target of water temperature for fish survival,an effective selective withdrawal scheme is proposed.