The global water cycle encompasses all components of the Earth system that deal with water, and a subset of the possible water variables have been nominated as “essential” to focus attention on ensuring the necessary global supply of information required to initialize, calibrate and validate integrated water models, for operational decision support systems, and to monitor, predict and manage water cycle extremes. These observations are required at a range of space/time resolutions, accuracy and latency depending on specific research and applications needs, which compounds the difficulty of fully applying the EWV observations. Papers are invited that demonstrate the use of priority EWVs in critical water research, operational water resources management and prediction/mitigation of impacts of water cycle extremes, or that identify existing deficiencies and necessary improvements in observing, data exchange and modeling systems.