Many of those who provided input regarding a national assessment of water availability indicated a need for improved regional-scale estimates of recharge, evapotranspiration, interbasin transfers, and other components of the water cycle. Elements of the water cycle typically are organized and interpreted in terms of water budgets (fig. 8). State, local, and Federal water managers often rely on the USGS for quantitative information on water-cycle components to better understand water budgets and to support water-management decisions. Information on water-cycle components for regional water budgets, however, is not always available in a consistent form across the Nation. To meet these information needs, the assessment should provide regional estimates of selected water-cycle components across the Nation. These estimates would support analyses of water availability that are undertaken by many local and regional agencies, would benefit research quantifying variability and changes in the national and global water cycle, and would aid in the interpretation of trends identified in the national indicators of water storage, flow, and use. Some of the water-cycle components, such as evapotranspiration, have proven difficult to estimate accurately using existing measurement techniques. Therefore, the assessment should support development of improved methods for quantifying water-cycle components.
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| Figure 8. Water-cycle components and simplified water budget of a drainage basin. |
The effort for improved characterization of water-cycle components outlined here would complement, build upon, and extend the gains made through the existing USGS Cooperative Water Program. The Cooperative Water Program is a cost-sharing partnership between the USGS and water-resource agencies at the State, local, and Tribal levels. As part of the cooperative program, the USGS conducts analyses of water availability in support of water-resource management decisions from local to statewide levels. These analyses often are done to estimate specific components of the water cycle or to quantify interactions among multiple components of the water cycle.
Two examples from the USGS Cooperative Water Program illustrate the value of water-cycle information for water-resource management, as well as the level of effort that is required for water-cycle studies. In the Eastern United States, the USGS is studying interactions among water withdrawals, streamflow reductions, and streamflow requirements for aquatic-habitat protection (Box D). In the Southwest, the USGS developed improved methods for estimating evapotranspiration by native vegetation to better understand the water budget of Owens Valley (Box E). It has been difficult, however, to synthesize local analyses into regional and national pictures. Regional and national analyses of water-cycle components could be accomplished more effectively through a coordinated national initiative such as the assessment described in this report.
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