Cumulative Impact Of Cascade Hydropower Development In The Middle Reaches Of Lancang River On Reservoir Water Temperature

Water temperature as an important element of the hydrological environment,there is a very vital impact on the aquatic ecosystem.Studies have shown that the water temperature of the reservoir area and the river below the dam is different from the original natural river water temperature structure after the dam is impounded.The water temperature of the reservoir has vertical stratification and low-temperature underwater leakage.Under model of the cascade hydropower,the water temperature change is Accumulate in time and space,resulting in a cumulative effect.This research systematically collects the water temperature data of the Gongguoqiao-Xiaowan cascade development section.And based on topography,hydrology,meteorology,water temperature and other data,constructs the W2 model of the cascade reservoirs,and uses the measured water temperature data Carrying out parameter determination and result corroboration;combining the W2 model simulation results with the prototype observation water temperature data,revealing the rule of water temperature differentiation and its influencing factors in the Gongguoqiao-Xiaowan cascade development section,and quantitatively revealing the cumulative effect of water temperature in the GuogongqiaoXiaowan cascade development section.The main conclusions are as follows:（1）The W2 water temperature model can better simulate the annual variation of the water temperature and the vertical layering features of the cascade reservoirs.（2）The water temperature of Gongguoqiao Reservoir changes during the year in a mixed type.The water temperature in the reservoir area is basically not stratified in January,February,November and December,and the water temperature in the reservoir area is in a stratified state from March to October.In spring,the surface water temperature rises rapidly,while the bottom water temperature rises slowly,and the layering begins to increase;the overall water temperature in the reservoir area rises in summer,with the most obvious stratification;the reservoir area as a whole is in a cooling state in autumn,and the water temperature layering gradually weakens;in winter the reservoir area The water temperature layering is not obvious.（3）The Xiaowan Reservoir is a representative water temperature stratified reservoir,and the water temperature varies throughout the year.In spring,the surface water temperature rises faster,while the bottom layer rises slowly,and the layering phenomenon is gradually obvious.In summer,the water temperature in the reservoir area rises,the stratification phenomenon is the most obvious,and the stratification structure is stable.The surface temperature layer is at the water depth of 0-10 m,the thermocline layer is at the water depth of 10-100 m,and the stagnant layer is at the water depth of 100-200 m.Than in autumn,the surface water temperature decreases,the vertical temperature gradient gradually decreases,and the stratification begins to weaken.In winter,the overall water temperature is low,and the stratification phenomenon is weak.The surface temperature is at 0-10 m,the thermocline appears at the water depth of 10-60 m,and the stagnant layer appears at 60-200 m.（4）The operation of the cascade reservoirs has no significant cumulative effect on the river water temperature.Among them,the cumulative effect of water temperature time is not significant.The water storage of Gongguoqiao Reservoir has a small cumulative effect on the water temperature of the downstream Xiaowan Reservoir.However,after the Gongguoqiao and Xiaowan Reservoirs were put into operation,the water temperature change process is obviously lagging behind.The time change of the water temperature of Gongguoqiao Reservoir was about 16 days later,and that of Xiaowan Reservoir was as high as 39 days;the temperature of the discharged water tended to be “uniform” during the year,and the variation of water temperature is reduced.The spatial cumulative effect of water temperature is more significant.The water temperature of the GongguoqiaoXiaowan cascade development section shows discontinuous changes.The temperature increase rate along the river is significantly different from that of natural rivers,and its temporal and spatial differentiation is more prominent and changeable.