Alpine regions are particularly sensitive to climate change due to the pronounced effect on snow and glacial melt. In this context, large perialpine lakes play a crucial role in modulating climate change impacts on water resources. Lake level management is the key challenge to bringing together diverse interests, such as fishery, shipping, energy production, nature conservation and mitigation of extremes. The question that remains open today is how to incorporate these regulatory effects into hydrologic models to project climate change impacts and to disentangle climatic and regulatory impacts. Despite the importance of lake level management, climate change studies on river systems only rarely include lakes or only in a simplified way. In this study, we focus on large perialpine lakes in Switzerland, which crucially influence the water cycle of all river basins. We combine a hydrologic model with the hydrodynamic model MIKE11 to simulate lake water level and outflow scenarios from 1981 to 2099, using the Swiss Climate Change Scenarios CH2018. We investigate one unregulated, one semi-regulated and two regulated lakes. The hydrological projections at the end of the century show a pronounced seasonal redistribution for both lake water levels and outflows, characterised by an increase in winter and a decrease in summer, intensifying with time and missing climate mitigation measures. In summer, the changes range from -0.39 m for the unregulated lake compared to -0.04 m to -0.22 m for the regulated lakes, which can lead to more frequent and severe drought events in late summer. Our climate change impact simulations demonstrate the importance of incorporating lake level management in hydrologic simulations and provide a data basis for disciplines such as limnology, water resources management and ecohydrology. Future work should focus on interannual variability to explore lake level management strategies under changing conditions.

<p>From snow-covered peaks to urban heat islands, this gradient, in its most concentrated form, is the essence of Alpine regions; it spans not only diverse ecosystems, but also diverse demands on water resources. Continuing climate change modifies the water supply and accentuates the pressure from competing water uses. Large Alpine lakes play hereby a key role, for water resource and natural hazard management, but surprisingly, are often only crudely modelled in available climate change impact studies on hydrology. Indeed, regulation of Alpine lake outlets, where daily specifications for lake level and outflow are defined, are the crux to bringing together diverse stakeholders. Ideally, a common regulation is agreed upon with an annual pattern that both corresponds to natural fluctuations and respects the different needs of the lake ecosystem, its immediate environment and upstream and downstream interests, such as fishery, shipping, energy production, nature conservation and the mitigation of high and low extremes. Surprisingly, a key question that remains open to date is how to incorporate these anthropogenic effects into a hydrological model?</p><p>To estimate climate change impacts, daily streamflow through this century was calculated with the hydrological model PREVAH, using 39 climate model chains in transient simulation from the new Swiss Climate Change Scenarios CH2018, corresponding to the three different CO<sub>2</sub> emission scenarios RCP2.6, RCP4.5 and RCP8.5. PREVAH is based on a 200×200 m grid resolution and consists of several model components covering the hydrological cycle: interception, evapotranspiration, snow, glacier, soil- and groundwater, runoff formation and transfer. In order to implement the anthropogenic effect of lake regulations, we created an interface for the hydrodynamic model MIKE11. In this work, we will present the two hydraulically connected Swiss lakes, Walensee (unregulated) and Zurichsee (regulated), that are located on the gradient between snow-covered peaks and urban environments. This catchment area was already affected by water scarcity in isolated years.</p><p>The hydrological projections at the end of the century show minor changes in mean annual lake levels and outflow for both lakes, but there is a pronounced seasonal redistribution of both level and outflow. The changes intensify over time, especially in the scenario without climate change mitigation measures (RCP8.5). In the winter, mean lake levels rise and outflow increases; in the summer, mean lake levels fall and outflow decreases. Walensee’s (unregulated) level change is significantly higher, with a difference of up to 50 cm under RCP8.5, than Zurichsee’s (regulated), which only changes around 5 cm; the changes in outflow are of the same order of magnitude in both lakes. The extremes show an increased frequency of reaching the drawdown limit, but no clear change in frequency of reaching the flood limit.</p><p>In order to estimate future hydrological developments on lakes and downstream rivers, it is important to use models that include the impact of such regulations. Hydrological models including anthropogenic effects allow a separation of climatic and regulatory impacts. Timely hydrological projections are crucial to allow the necessary time horizon for both lake and downstream interests to adapt.</p>