The potential of epilimnetic withdrawal to counteract global warming: Insights from the Lichtenberg drinking water reservoir, Germany

Lichtenberg drinking water reservoir, Germany. Coupled hydrological-hydrodynamic modeling is used to evaluate an adaptation strategy for the dimictic Lichtenberg reservoir under climate warming in a realistic operational setup. An ensemble of three one-dimensional lake models, coupled with a rainfall-runoff model, simulated reservoir thermal dynamics through the end of the century under RCP2.6, RCP4.5, and RCP8.5, comparing current and adapted management. The current management strategy releases cold water from near the reservoir bottom to the downstream river, facilitating downward heat transfer within the reservoir. Under this strategy, the ensemble predicted consistent increases in surface and deep water temperatures, highest under RCP8.5 at 0.4 and 0.1 K/decade, respectively. To mitigate this impact, the water release depth to the downstream river is shifted closer to the surface. Surface water temperature, which is primarily driven by meteorology, was insensitive to this strategy. Conversely, the adapted strategy kept deep water isolated through thermal stratification for a longer period and reduced its temperature by about 1.5 K over time and across climate scenarios. This prevented early-summer hypolimnetic depletion and increased the availability of cold deep water for drinking water production. Epilimnetic withdrawal thus emerges as an effective, operationally feasible measure to help preserve water quality and supply in dimictic reservoirs under climate change. • Epilimnetic withdrawal counteracts hypolimnetic warming in a dimictic reservoir. • Coupled hydrological-hydrodynamic ensemble simulation reduces uncertainty. • Reproducing site-specific operation improves simulation realism and applicability.