Future projections of thermal regimes and mixing characteristics in a monomictic reservoir under climate change

Stratified structures are basic physical characteristics of lakes and reservoirs, which play important roles in vertical convection and mixing processes. Global warming is expected to alter the thermal regimes and stratified characteristics of lakes and reservoirs, leading to potential environmental risks and ecological problems. In this study, we used a two-dimensional hydrodynamic model to explore the response characteristics of the thermal regime to climate change in the Xiluodu Reservoir. The study is conducted based on four representative concentration pathways (RCP 2.6, RCP 4.5, RCP 6.0, and RCP 8.5). The results show that the projected surface water temperature of the reservoir will significantly increase (0.13 °C/10a-0.34 °C/10a) from 2022 to 2099, and the warming rate of the water in the epilimnion and thermocline will be greater than that in the hypolimnion, leading to enhanced vertical stratification in the reservoir. The stratification stability index (SI) will increase by 14.4 % under RCP 8.5 compared to that under RCP 2.6. The main responses of the reservoir to climate change are earlier start date and later end date of stratification, which may increase the duration of hypoxia in the hypolimnion and promote the succession of planktonic algal communities. The withdrawal water temperature in the reservoir will significantly increase under the climate warming scenarios (0.03 °C/10a-0.22 °C/10a). Furthermore, our research results indicate that selective withdrawal is still needed to mitigate the adverse effects of cold water from the reservoir during the fish spawning period under the strongest warming scenario, and can also be an effective adaptive management strategy to regulate thermal stability in the reservoir. This study provides a scientific and theoretical reference for the sustainable management of similar deep monomictic reservoirs under climate change.

Ensemble warming projections in Germany's largest drinking water reservoir and potential adaptation strategies

The thermal structure in reservoirs affects the development of aquatic ecosystems, and can be substantially influenced by climate change and management strategies. We applied a two-dimensional hydrodynamic model to explore the response of the thermal structure in Germany's largest drinking water reservoir, Rappbode Reservoir, to future climate projections and different water withdrawal strategies. We used projections for representative concentration pathways (RCP) 2.6, 6.0 and 8.5 from an ensemble of 4 different global climate models. Simulation results showed that epilimnetic water temperatures in the reservoir strongly increased under all three climate scenarios. Hypolimnetic temperatures remained rather constant under RCP 2.6 and RCP 6.0 but increased markedly under RCP 8.5. Under the intense warming in RCP 8.5, hypolimnion temperatures were projected to rise from 5 °C to 8 °C by the end of the century. Stratification in the reservoir was projected to be more stable under RCP 6.0 and RCP 8.5, but did not show significant changes under RCP 2.6. Similar results were found with respect to the light intensity within the mixed-layer. Moreover, the results suggested that surface withdrawal can be an effective adaptation strategy under strong climate warming (RCP 8.5) to reduce surface warming and avoid hypolimnetic warming. This study documents how global scale climate projections can be translated into site-specific climate impacts to derive adaptation strategies for reservoir operation. Moreover, our results illustrate that the most intense warming scenario, i.e. RCP 8.5, demands far-reaching climate adaptation while the mitigation scenario (RCP 2.6) does not require adaptation of reservoir management before 2100.