Reservoir water for drinking water production may undergo major short-term and long-term quality changes. These are results of natural processes in the water body and of the water's quality entering and leaving the reservoir. Long term quality changes are due to management of catchment areas, but also to a considerable extent by external impacts like climate change. Short term quality changes are impacted by extreme events like rain storms after drought periods, which might also be a result of climate change. Furthermore, short- and mid-term quality changes are impacted by reservoir management, which also influences the ecological state of rivers downstream the reservoir. The purpose of our work is to develop a decision support tool for reservoir management which takes into account short-, mid- and long-term factors for water quality change. With the tool it is intended to simulate not only water quality, but also management impact on flood risk prevention and drinking water quality (treatment efficiency and costs) and to assist decision making for reservoir management.