| The dam and reservoir construction will change the hydrological situation of the original rivers,form the phenomenon of water temperature stratification,and further affect the ecological environment of the reservoir area.As an important water quality parameter in aquatic ecosystems,water temperature has a profound impact on physical and chemical characteristic of water,aquatic organisms and aquatic ecosystem.The stratification structure of water temperature in a large reservoir is affected by a variety of different factors,such as climatic conditions,hydrological processes,and reservoir operation modes,and these influencing factors vary at different periods and time scales.In the future,with the continuous increase of greenhouse gas emissions,the trend of climate warming is more obvious,which will also profoundly affect the variation characteristics of reservoir thermal stratification.Therefore,based on different temporal and spatial scales,exploring the evolution mechanism of thermal stratification structure of large reservoir under the influences of climate conditions and reservoir operation is of great significance to response to the climate change and scientifically operate reservoirs to improve reservoir water environment.Taking Shanmei Reservoir as the research area,based on the watershed distributed hydrological model SWAT and the reservoir two-dimensional hydrodynamic water quality model CE-QUAL-W2,the Shanmei reservoir water temperature model was constructed to simulate the water temperature from 1991 to 2010.the paper analyzed the temporal and spatial evolution characteristics of the thermal stratification structure from the annual,seasonal and monthly scales,and explored the time scale differences of the water temperature stratification factors.On this basis,the influence of reservoir operation on the hierarchical structure of water temperature was evaluated by adjusting the operation factors such as inflow and outflow flow,reservoir water level,outlet location and flood discharge mode.Furthermore,the two emission scenarios(RCP4.5,RCP8.5)under the future climate model of Noer ESM1-M are combined with the current water temperature model to simulate the water temperature of the reservoir from2031 to 2090,and evaluate the impact of future climate change on the thermal stratification structure of the reservoir.The main conclusions of the study are as follows:(1)The water temperature stratification in the Shanmei Reservoir presents a typical subtropical single-cycle mixed pattern,with a double thermocline structure in spring and summer,a single thermocline in autumn,and a disappearance of the thermocline in winter.The strength of thermocline is strong in spring and autumn and weak in summer;the thickness of thermocline increases in summer and decreases in autumn;the change of thermocline depth is opposite to that of thickness.The influence of hydrological process is strong in wet years,and the stability and days of annual stratification are less than those in normal and dry years.(2)Different modes of reservoir operation schemes will have an impact on the water temperature structure of the reservoir.The decrease of inflow and increase of outflow will lead to the decrease of vertical water temperature,while the increase of inflow and decrease of outflow show the opposite trend.The change of initial water level mainly changes the characteristic quantity of thermocline by affecting the water depth,but has relatively little effect on the reservoir water temperature.The operation mode of complete discharge and no discharge will greatly change the vertical water temperature distribution of the reservoir.The change of the height of the discharge outlet will affect the flow movement at the height and reduce the temperature of local water body.Raising the gate height will enhance the water movement and increase the temperature of the lower thermocline.(3)Under the climate change from 2031 to 2090 in the future,the reservoir water temperature will increase significantly,and the surface temperature increase is greater than the bottom temperature.Compared with the baseline period from 1991 to 2010,the reservoir water temperature will increase in different seasons.The maximum temperature increase is concentrated in summer,and the minimum temperature increase is concentrated in winter.The depth of thermal stratification became shallow in spring and autumn,the thickness increased in spring and summer,and decreased in autumn,and the intensity increased in three seasons.Climate change enhances the thermal stratification stability of reservoir water and prolongs the thermal stratification time of reservoir. |
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