  The balancing provided by hydropower reservoirs is essential in the
transition towards a decarbonised European energy system, but the resource
might be impacted by future climate change. In this work, we first analyse the
hydropower operation needed to balance a wind and solar dominated European
energy system, to signify whether and to what extent hydropower is required to
operate differently due to the decarbonisation of the energy system. Second, we
apply runoff data achieved with 10 dynamically downscaled climate models with
0.11 x 0.11 deg horizontal and daily resolution to project the future reservoir
inflow at three CO2 emissions scenarios: low (RCP2.6), mid (RCP4.5), and high
emissions (RCP8.5). We show that the decarbonised energy system increases the
ramp rates and seasonality of the hydropower operation. Despite large
interannual and intermodel variability, we found a significant change in annual
inflow due to climate change in 20 out of 22 European countries at the mid and
high emissions scenarios. The seasonal profile, as well as the frequency and
duration of droughts and floods, is also projected to be impacted.
