“abrupt” or irreversible changes, and relies on basic understanding of interacting processes, that is, understanding how the world works.

Scientific and technological advances now offer exceptional opportunity to develop a comprehensive understanding of water’s pervasive activity throughout the Earth system over periods ranging from early epochs to the present and future. Although many disciplines can contribute to this opportunity, hydrologic science plays a central role, as highlighted in this report. Over the past several decades, the knowledge of Earth as a system has grown considerably through new observational techniques, analysis methods, and computing tools, all of which have helped hydrology mature as a discipline (see Chapter 1). This maturation has given hydrologic science a leading role in advancing understanding of the water cycle and the processes that affect and are affected by it over a range of scales and environments. As part of this leadership, hydrologists and engineers have forged links with closely related disciplines, especially the atmospheric, soil, plant, and cryospheric sciences (which deal with snow, ice, and frozen ground) to develop a more comprehensive and coherent view of water as a central component in Earth’s climate system.

Admittedly, how water acts varies significantly across time scales ranging from seconds to decades and longer, and spatial scales from millimeters to planetary, thus presenting a very complex dynamic picture and a monumental task in monitoring all its storage and transport aspects. However, advances in observing systems and computing allow use of computational techniques such as data assimilation that could support development of this comprehensive view by merging observation sources into a unified, global portrayal of water with unprecedented temporal and spatial detail. New observing systems such as space-based platforms coupled with global networks of existing observing tools could produce global, real-time views of where water is and where it is going, in all its phases (Gao et al., 2010; Wong et al., 2011). The sensor revolution is in its nascent stages, but for the first time the promise of closing the global and regional water budgets with direct measurement of flux and storage components may be just within reach. Opportunities also exist to extend this portrayal of water into the future. Scientific advances have contributed to progress in understanding the interactions between water and other Earth system components, leading to modeling of the water cycle as part of a comprehensive Earth system simulation system. Yet an opportunity exists for models providing plausible scenarios of the impact of climate change and land use change on the regional water cycle.

Stepping away from the contemporary and future perspective, examination of water flow and storage during periods ranging over the past decades, centuries, millennia, and into deep geologic time offers opportunity to understand how Earth’s water cycle evolved to its present state. Equally

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