Water is essential for a wide range of life-sustaining human activities and is a major component underlying a suite of important climate processes and feedbacks that affect regional and global climates. As the hydrological cycle is projected to intensify in a warmer climate, the impacts on human and natural systems will be profound, in particular those on energy production and use, land use, and ultimately, feedbacks to the climate system. Today’s scientific uncertainties in predicting long-term changes in global and regional hydrologic cycles and the implications for water supplies and energy production fundamentally limit the Nation’s ability to develop sustainable energy solutions.