<p>Hydropower production affects different stakeholders, levels of administration and ecosystems, which makes the question of its sustainability complex. Hydropower delivers energy, storage capacity, jobs, economic value, but also involves altered streamflow, water temperature and sediment transport conditions, fractioning of aquatic habitats and modification of the landscape. Thus, an increasing demand for renewable and climate friendly energy from hydropower also results in more pressure on aquatic habitats, thereby potentially calling into question its sustainability.</p><p>In this work, we compare climate change impacts on the future energy production of 21 hydropower plants in Switzerland, to impacts related to environmental flow requirements and to site-specific technical optimisation potential. The simulation-based study corresponds to three future periods (2020–2049, 2045–2074 and 2070–2099) under three emission scenarios (RCP2.6, RCP4.5, RCP8.5), assuming unchanged environmental flow requirements and installed machinery. The results show an increase of winter production and a decrease of summer production, which in conjunction leads to an annual decrease. The simulated impacts strongly depend on the elevation and the plant-specific characteristics. The climate induced changes in production are of a similar order of magnitude as the production loss due to environmental flow requirements and the increase potential due to technical optimisations. A key result is that the climate induced reduction is not linearly related to the underlying streamflow reduction, but is modulated by environmental flow requirements, the design discharge and streamflow projections. Taken a step further, a change in production does not necessarily mean a linear change in financial revenue. The Water-Energy Nexus in terms of hydropower concerns more than just a m<sup>3</sup>s<sup>-1</sup>–kWh relationship: it is part of a complex framework that is namely sensitive to legal adjustments and to long lasting technical decisions taken in the past.</p>

<p>The steadily growing demand for energy and the simultaneous pursuit of decarbonization are increasing interest in the expansion of renewable energies worldwide. Hydropower produces around 60% of Switzerland's electricity and plays a key role in this energy transition strategy. However, habitat and ecosystem protection and climate friendly renewable energy production are sometimes at odds. While the environmental impact is usually addressed at some level, there is a lack of standardization and tools for a global assessment are still scarce. The GIS-based tool HYDROpot_integral allows the consideration of the total hydropower potential as well as the ecological potential of a region. Based on ecological and socio-economic geodata, both the current state of each river reach and the hydropower potential is assigned a rank. To record the suitability, every river reach is ranked according to their ecological, cultural and economic ecosystem services. A low rank means that a river reach is more suitable for hydropower production at low cost in terms of ecological and cultural ecosystem services; a high rank indicates high ecological and cultural ecosystem services and low economic services and is therefore more suitable for protection. As the limit between hydropower use and protection is adjustable, different scenarios can be explored. Results from five mesoscale test catchments in Switzerland show the feasibility of the present method, to provide a comprehensive and meaningful basis that can support the decision-making process. Overall, the assessment method is to be understood as a flexible tool to address tradeoffs between hydropower potential and ecological potential.</p>