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Quantifying physical water security at the global scale remains hampered by a lack of systematically produced observational data. Here we combine the observed trends in global freshwater availability from the recently completed Gravity Recovery and Climate Experiment satellite mission1 with more than a dozen other global datasets and provide the missing observational basis to numerous existing perceptions of global water security. We find the disparity between the water ‘haves’ and ‘have nots’ of the world continues to widen2. Nearly one in two people who live in areas of extreme water shortage experienced drying over the 14-year observation period while a fifth of crop calories produced for human food are grown in regions that dried yet already suffer from water shortage. The global water availability trends reveal a clear human imprint1 and reflect a world-wide inability to manage water resources for long term water security. We identify 21 regions that stand to face especially high social-ecological system pressures from the water availability trends and assess flooding and water scarcity vulnerability at the global scale. This application of remotely sensed water availability trends contributes to the quantitative diagnosis of the world’s contemporary water security challenges that will be useful in global policy directive setting.

PrefaceWater is a key component of the earth and human systems due to its strong interactions with the energy cycle and its vital roles in the energy-water-land system. Uncertainties in predicting the integrated water cycle can limit our abilities to address the energy and environmental challenges today and in the future. Modeling the integrated water cycle contributes to the Department of Energy's (DOE) core competencies in integrative modeling, drawing from unique and highly relevant research on cloud, aerosol, terrestrial ecosystem, carbon cycle, and subsurface processes, as well as climate and earth system modeling and integrated assessment modeling. Synthesizing new process knowledge and innovative computational methods in integrated models of the human-earth system can advance predictive capabilities relevant to DOE missions. This report describes the DOE workshop onCommunity Modeling and Long-Term Predictions of the Integrated Water Cycle, held September 2012 in Washington DC. The workshop serves as a launching point and major organizing event to identify challenges and plan the development of nextgeneration human-earth system models for improving long-term predictions of the regional-scale integrated water cycle. More specifically, the workshop aims to: Community Modeling and Long-Term Predictions of the Integrated Water Cycle