This paper evaluates the combined impacts of climatic and demographic changes on a multipurpose reservoir system in the Harz mountains in northern Germany. For this purpose, the spatially distributed hydrological model PANTA RHEI was firstly calibrated and validated with gridded observational data. Secondly, the hydrological model was forced with data from two regional climate models. The possibilities and limitations of using current regional climate models as an input for hydrological modelling in very high resolution in space and time were evaluated. Their applicability in reservoir operation optimization on an hourly time step was demonstrated. For an increase in extreme events, indicated by one regional climate model, the impact of an adaptation of operation rules with respect to flood protection on the other uses of the system was assessed. Furthermore, a new method was presented, how economic information from national accounting in combination with demographic scenarios can be used in exposure analysis. The paper ends with a demonstration of the importance of combining the impacts of climatic and demographic change, while developing sound and cost-efficient water resources management strategies for the near and distant future.