Water availability and use are a function of the total flow of water through a basin, its quality, and the structures, laws, regulations, and economic factors that control its use. Because water availability and water use are closely linked, “water availability” will be used for brevity in the following sections to include both water availability and water use.
It is evident that a national water-availability assessment means different things to different people. Many different types of assessments and assessment products were envisioned by those whose advice was solicited. Recommendations for products included forecasts of water availability, calculations of constructed flow regimes in the absence of human management, calculations of water demands for protection or restoration of wildlife, and an assessment that would include policy-related data linking the institutional and physical environments. The scope and magnitude of an effort to meet all of the recommendations that were received would be an immense undertaking.
A national assessment of water availability is proposed that would report on indicators of the status and trends in storage volumes, flow rates, and uses of water nationwide. Currently, this information is not available in an up-to-date, nationally comprehensive and integrated form. The assessment also would provide regional information on recharge, evapotranspiration, interbasin transfers, and other components of the water cycle across the country. This regional information would support analyses of water availability that are undertaken by many agencies nationwide and would benefit research quantifying variability and changes in the national and global water cycle.
An assessment would require basic hydrologic data collected by the USGS and by others to create the indicator variables. This process of computing indicators from the basic data would help to elucidate uncertainties in our knowledge of the Nation’s hydrologic conditions and would provide useful feedback to the design of data-collection networks. Improved networks for the collection of surface-water and ground-water data are defined by USGS plans for the National Streamflow Information Program (U.S. Geological Survey, 1998a and 1999) and the Ground-Water Resources Program (U.S. Geological Survey, 1998b), and as part of the Cooperative Water Program. Water-use information developed by the assessment would expand upon and strengthen existing water-use efforts along the lines suggested by the National Research Council (2002).
There are numerous examples from across the Nation where water quality is the limiting factor to water availability (Box A). Considerable information is currently synthesized about the Nation’s water quality at regional and national scales through a number of State and Federal water-quality programs. The USGS National Water-Quality Assessment (NAWQA) program, for example, collects and analyzes water-quality data in many of the Nation’s major river basins and aquifers covering nearly all 50 States. Rather than duplicate existing water-quality information, the indicators of the flows, storages, and uses of water developed as part of a national assessment should be used with water-quality information from existing programs to provide a more complete national picture of the quantity and quality aspects of water availability.
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