Response of thermal stratification in Longyangxia reservoir to climate change and adaptive management

The exacerbation of climate change has made the impacts of thermal stratification in reservoirs and changes in downstream river water temperatures on aquatic ecosystems a pivotal concern in water resources management. This study focuses on the Longyangxia Reservoir to explore the effects of climate change and reservoir operations on the stability of reservoir water temperature structure and the ecological implications of downstream water temperature releases. Utilizing the CE-QUAL-W2 hydrodynamic model, coupled with four distinct future socio-economic development pathways representing climate change and two reservoir operation strategies, simulations and analyses were conducted on the water temperature of Longyangxia Reservoir and downstream water temperatures. The findings reveal: (1) Under climate warming conditions and low-water level operations, the average thermocline thickness in Longyangxia Reservoir is 15.57 m, with an average thermocline strength of 0.55 °C/m and a stratified stability period ranging from 244 to 281 days. Under high-water level operations, the average thermocline thickness is 20.57 m, with an average thermocline strength of 0.37 °C/m and a stratified stability period ranging from 251 to 288 days. (2) Under the conventional development scenario (SSP-8.5), the temperature difference index between outflow temperatures and pre-reservoir construction downstream water temperatures is minimized, with values of 45.66 °C·d and 12.39 °C·d under different operation strategies. (3) Under the conventional development scenario (SSP-8.5), outflow temperatures are projected to reach the optimal spawning temperature for Carassius auratus approximately one month earlier. The results underscore the significant influence of climate change on water temperature characteristics in reservoir-river systems, highlighting the necessity for adopting appropriate operational strategies to address climate change impacts effectively.