Quantifying the contribution of external thermal energy and internal hydrodynamic processes to the water temperature of river-valley reservoirs

[Display omitted] •River-valley reservoir heat variation aligns with inflow and outflow heat transport variation annually.•Inflow heat contribution is greater than surface heat exchange during reservoir stratification period.•The evolution of reservoir thermal stratification is driven by shear instability.•Heat transport and internal dynamics contribute to interannual cycling of seasonally stratified reservoir structures. Hydropower plants utilize hydroelectric energy while also impacting the hydrodynamic and hydrothermal processes within river. To comprehend and address the environmental and ecological impacts of hydropower plants, it is crucial to quantify these processes. The study synthesizes methods for calculating reservoir heat transport and Richardson numbers, presenting a comprehensive framework for understanding the interaction of heat transport and hydrodynamic processes in reservoirs based on EFDC model simulations. It quantifies the external thermal and internal hydrodynamic effects on the water temperature evolution of a deep valley reservoir, using the Jinsha River Xiluodu Reservoir as a case study. Multi-interface heat transport calculations indicate that the Deep River Valley Reservoir is a hydrothermal body dominated by inflow and outflow. During the water temperature stratification period, the reservoir water temperature increases by 0.015 °C/day due to surface heat fluxes, incorporating hydraulic residence time, while the reservoir water temperature increases by 0.06 °C/day due to inflow heat fluxes. Furthermore, this study analyses the interrelationships between reservoir buoyancy and shear stress based on the gradient Richardson number, revealing the interaction between hydrodynamic and water temperature processes. This study not only provides a framework for understanding reservoir thermodynamics but also provides a theoretical basis for effective reservoir management and ecosystem assessment.