Hydropower is an important renewable energy source, but it can consume a lot of water due to evaporation from the reservoir surface, which may contribute to water scarcity. Previous studies mostly used a gross evaporation approach for water footprint assessment where all the evaporation is attributed to hydropower. They fail to consider both evapotranspiration before the dam construction, which should be deducted from the footprint, and the seasonal storage dynamics of water. These considerations are critical for assessing reservoir impacts on water scarcity using temporally explicit water stress indices. This study seeks to fill this gap: we calculate the water footprints of 1500 hydropower plants which cover 43% of the global annual hydroelectricity generation. Apart from reduced water availability, alterations of the flow regime can also adversely affect ecosystems. Therefore, environmental flow requirements are also analysed. This novel approach for the water footprint assessment of hydropower indicates that previous studies mostly overrated the impacts of hydropower on water scarcity, often because reservoirs store water in periods of low scarcity and release water during months of high water scarcity. By contrast, flow alterations generally affect the environment more than water consumption. Since impacts vary broadly among plants, plant-specific evaluations are necessary. (C) 2016 Elsevier Ltd. All rights reserved.