Identifying the nature of sustainable water resources in the United States is technically challenging. From an engineering standpoint, a balance for water--just like any other mass quantity--can be achieved by determining input, output, and accumulation within a defined area. While such a mass balance is conceptually simple, the practice of determining input, output, and accumulation quantities can be difficult and expensive. The list of major water inputs and outputs, at least, is straightforward and is shown in Table 2.1.
The difficulty for researchers is to translate the available data into water input, output, and accumulation rates for the various geographical regions of the United States. These rates are interdependent; for example, streamflow (output) is affected by the local aquifer level (accumulation), which is in turn affected by infiltration rates (input). Obtaining a water balance for even a single aquifer requires substantial data collection, mathematical modeling, and uncertainty analysis to compensate for the necessary assumptions and simplifications (Carter et al., 2000).
The interaction between groundwater and surface water is still an active area of geophysical research. Where data is available, competing models yield conflicting quantitative analyses of U.S. water resources. In other cases data are not available to make detailed analyses. At present, neither the United States Geological Survey nor the Environmental Protection Agency have definitive position papers on the quantitative analysis of future national water resource availability (USGS, 2001a).
However, there do exist some detailed local studies from which overall trends can be gleaned. In addition, while researchers disagree about the quantity of water available and the rate of water depletion nationally, there are important points of general agreement regarding U.S. water resources (Frederick, 1995):
In support of the last point of agreement, one author notes:
The largest hindrances to effective water management in virtually all countries are the outmoded economic and institutional policies (taxes, subsidies and regulations) that shape public and private decisions, development strategies and resource use patterns (Stakhiv, 1998).
This technology assessment discusses the above points of general agreement, along with the specific findings of regional studies, as a foundation for justifying potential changes in national policy. The technological and policy tools available to address these issues are explored and synthesized into a number of policy options. The remainder of this chapter will examine the foundational background and data on national water resource problems, thereby setting the stage for the analysis necessary to formulate effective policy.