Evaluating the effects of climate-induced heatwaves on the thermal dynamics and water quality of a deep reservoir in Portugal

The study was conducted in the Torrão Reservoir, situated in the Tâmega River Basin in northern Portugal. This study evaluates the impact of future climate-induced heatwaves on the thermal structure of a deep reservoir using the 2D water quality model, CE-QUAL-W2. Climate projections for the end of the century were based on four EURO-CORDEX simulations under the RCP 8.5 scenario. A novel, in-depth approach is used to assess the spatial distribution of climate forcing effects across the reservoir's 2D space. Heatwaves are projected to increase mean water temperatures (1.7–3.6 °C) and enhance thermal stratification, affecting dissolved oxygen and chlorophyll-a concentrations. The findings highlight the dual impacts of climate change: while long-term trends suppress average phytoplankton biomass, episodic extremes exacerbate water quality risks. The study also underscores the importance of hydrodynamic forcing functions, such as inflow temperature and atmospheric variability, in shaping thermal stratification and circulation patterns. Importantly, heatwaves led to the formation of localized thermal barriers in the upstream region of the reservoir. Additionally, the analysis quantifies thermal inertia, providing insights into reservoirs’ resilience to climate stressors. Overall, the results indicate that as heatwave intensity (quantified by peak air temperature) increases, the response of reservoir water temperature exhibits a progressively larger temporal lag. Specifically, on average, the delay in water temperature response increases by 3.21 days for each unit rise in peak air temperature. The proposed framework can serve as a valuable tool for understanding and predicting the complex interactions between climate change, heatwaves, and reservoir dynamics, enabling the development of targeted management strategies. ▪ Heatwaves increased water temperatures (1.7–3.6°C) and enhance thermal stratification. ▪ Localized thermal barriers developed in the upstream reservoir region during heatwave events. ▪ Heatwaves influence DO and Chla levels, with phytoplankton blooms in certain areas. ▪ Study highlights the need for reservoir management strategies to mitigate heatwave impacts.