FIGURE 11.1 Water in many parts of the United States, especially in the Southwest, is a critically scarce resource for most of the year. This view of the Snake Range was taken along the Great Basin National Park access road. Great Basin National Parkin eastern Nevada is known for its ecological diversity ranging from low, desert basin to high, alpine tundra, with many ecozones and habitats in between. SOURCE: Courtesy of the U.S. Geological Survey.

improve the ability to manage water and to provide the water-related infrastructure that is needed to provide for human needs and to protect and enhance the natural environment and associated biological systems.

The scientific challenge posed by the need to observe the global water cycle is to integrate in situ and space-borne observations to quantify the key water-cycle state variables and fluxes. The vision to address that challenge is a series of Earth observation missions that will measure the states, stocks, flows, and residence times of water on regional to global scales followed by a series of coordinated missions that will address the processes, on a global scale, that underlie variability and changes in water in all its three phases.

The accompanying societal challenge is to foster the improved use of water data and information as a basis for enlightened management of water resources, to protect life and property from effects of extremes in the water cycle—especially droughts and floods. The recent western U.S. drought (see Box 11.1) has renewed a focus on more effective management of water resources in the perennially water-stressed West. More generally, a major change in thinking about water science that goes beyond its physics to include its



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