The Water Cycle across Scales

he transport and transformations of water sub-stance in the global water cycle are of fundamental and primary importance for understanding the ecosystems, the land surface hydrology that supports them, and the management of water resources for humans. Th e water cycle is also critical for predicting weather, and the long-term climate balance of the planet. Despite the importance of water and a tremendous body of work researching its processes, critical questions re-garding the global water cycle remain. Th ese questions range from fundamental details of the spectroscopy of water, to the forecasting of precipitation, to closing the budget of the hydrological cycle on many scales.Discussion of these and other topics relevant to contemporary water-cycle research led us to outline some themes, questions, and proposed opportunities. Many of these, and broader, issues have been brought to the forefront of scientific discourse through a number of strategic documents that have recently been published (see “For Further Reading” below). To minimize duplication with earlier eff orts, we focus on integrating themes and strategic opportunities identifi ed at the workshop.