What is clear is that to meet these challenges the management of water resources will need to become more effective and systematic (see, for example, an international example in Box 1-1). This will require improvements in our capacity to understand and quantify the hydrologic cycle and its spatial and temporal interactions with the natural and built environment.


Water on the Earth continuously moves between the atmosphere, oceans, and various water stores on and below the land surface (Figure 1-1). The primary land-based water stores, in order of capacity, are icecaps and glaciers, groundwater, lakes, terrestrial soils, wetlands, and rivers. Water use and management generally focus on groundwater, lakes, rivers, and soils. These stores provide water for domestic, commercial, industrial, and agricultural uses. They have also been used to dispose various kinds of wastewater, although such uses have become increasingly constrained.

Ecosystems rely on and participate in the cycling of water. Water supports most forms of life and water stores are often associated with characteristic ecosystems. Late in the 20th century, societies recognized the utility of protecting and restoring ecosystems to maintain a “healthy” hydrologic cycle. For example, wet-

BOX 1-1

An International Initiative for Integrated Observations

The international community is recognizing the importance of integrated, comprehensive Earth monitoring systems. Perhaps most notably, on February 16, 2005, at the Third Earth Observation Summit held in Brussels, Belgium, the Global Earth Observation System of Systems (GEOSS) 10-Year Implementation Plan was endorsed by nearly 60 governments and the European Commission. The vision for GEOSS is “to realize a future wherein decisions and actions for the benefit of humankind are informed via coordinated, comprehensive and sustained Earth observations and information.” The GEOSS implementation plan calls for developing these comprehensive Earth observations in order to improve monitoring the state of the Earth, to increase understanding of Earth processes, and to enhance prediction of the behavior of the Earth system. The ultimate goal is to deliver to Earth’s societies enhanced benefits from improved forecasting and management in areas such as weather and seasonal climate, water resources and ecosystem management, environmental factors affecting human health and understanding, climate variability and change. The intergovernmental Group on Earth Observations (GEO), which is leading the effort, is co-chaired by the United States, South Africa, China, and the European Commission.

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