The Evolution Of Temperature Stratification In A Deep Hydropower Reservoir Under Large Inflows

The thermal regimes of reservoir systems exert great influences on multiple hydrological and ecological processes.Reservoir impoundments have historically brought many benefits to human society,such as flood control and hydroelectric power.However,these benefits have come with documented costs on the environment and risks to human flourishing.This tension is particularly present in the Yangtze River of China,one of the most biologically diverse systems in Asia which is being heavily developed for hydropower.Downstream temperature alterations due to reservoir impoundments directly impact processes such as the spawning of fish and crop production.This indirectly impacts the human communities which depend on these resources to thrive.On the one hand,the unique climactic,morphological,and hydraulic conditions of this reservoir network have the potential to greatly deepen our understanding of inland waters.However,this unique environment also presents a challenge to traditional environmental remediation strategies.On the international stage,mitigating the impacts of reservoir thermal alterations is an ongoing operational debate.It therefore seems abundantly necessary to investigate this system,describe its governing processes,compare results with historic insights,and propose creative solutions.To that end,we chose to study a representative case for this reservoir network called Xiluodu Reservoir.Xiluodu Reservoir is a subtropical deep reservoir with high inflow rates and hydropeaking outflows.We inquired into the primary drivers for stratification and mixing within Xiluodu Reservoir using field measurements,numerical modeling,and statistical analysis.Thermocline response to meteorological forcing,inflows,and hydropeaking withdrawals were quantified from seasonal to sub-daily scales.Model scenarios considered the impacts of hydropeaking both on reservoir mixing and downstream thermal regimes.Results were able to establish the relative impacts of surface heating,wind forcing,and flows on stratification development as well as their respective spatio-temporal extents.The validity of a laterally-averaged two-dimensional model(CE-QUAL-W2)to simulate internal wave phenomena was established.Observed data and model scenarios suggest that hydropeaking operations at the dam outlet have minor impacts on overall mixing and outflow temperatures,in contrast to multiple studies in other reservoir environments.The internal wave field near the dam exhibits hydropeaking-induced V1H1 motions that are spatially ubiquitous and diurnal in periodicity but weak in overall energy content.During certain canyon funneled wind gusts,clear V2H1 responses were observed that disrupted the weaker diurnal oscillations.The primary driver of seasonal thermocline progression is both the rate of inflow and the time-varying temperature difference between the inflow and epilimnion.Despite epilimnetic withdrawals,the reservoir exhibits downstream thermal alterations normally associated with hypolimnetic outtakes.The causes for this phenomena are considered,along with the impacts of outflow scheduling on heat transport due to incoming density currents.