DO WE HAVE ENOUGH WATER FOR THE FUTURE? Page 2

In this country concerns about the availability of freshwater to meet the demands of a growing and increasingly affluent population while sustaining a healthy natural environment are based on several factors: (1) uncertainties as to the availability of supplies stemming from the vicissitudes of the hydrologic cycle and the threat that a greenhouse warming might alter the cycle; (2) the high costs of developing additional surface-water supplies; (3) the vulnerability of the resource and the problems of restoring and protecting valued surface and groundwater resources; (4) the importance of reliable supplies of high-quality water for human and environmental health and economic development; and (5) the shortcomings of our institutions for allocating scarce supplies in response to changing supply and demand conditions.

Uncertainty of Supply.

Timing, location, and reliability are important dimensions of the potential value of supplies. Because of the spatial and temporal variations in the distribution of water, national and long-term annual averages of precipitation and runoff are poor indicators, for practical purposes, of available supplies and potential problems. Precipitation generally declines as one moves from east to west in the United States. Average annual precipitation ranges from less than 1 inch in some desert areas in the Southwest to more than 60 inches in parts of the Southeast. Underlying these regional averages are large seasonal and annual variations that can result in droughts and floods. In the absence of flow regulation and storage, the ratio of the maximum to minimum stream flow within a year may exceed 500 to 1. Natural climatic variability results in interannual fluctuations.

The ratio of very high annual flows (amounts exceeded in five percent of the years) to very low (exceeded in 95 percent of the years) is 2.9 for the conterminous United States; the ratio for the nation's arid and semiarid regions is significantly higher. But almost any region lacking adequate storage is likely to encounter both periods when supplies are relatively plentiful or even excessive as well as periods of shortages. Water resource issues tend to be local or regional in nature: abundant supplies in one area are of no help to water-deficit areas unless there are facilities to transport supplies among regions. Water flows naturally within hydrologic basins and can be moved between basins where transfer facilities have been constructed. But water is too expensive relative to its marginal value to transport long distances out of these existing channels in response to climate- induced changes in supply or demand.

Thus, large seasonal, annual, interannual, and regional variations in precipitation and runoff pose major challenges for planners and down-to-earth risks for water users and occupants of the flood plains. Human efforts to alter the hydrologic cycle date back to ancient times. Primitive societies tried to bring rain through prayer, rain dances, human and animal sacrifices, and other rituals. Cloud seeding (dropping silver iodide crystals or dry ice into selected clouds to stimulate ice crystal formation and induce precipitation) represents today a more recent and more scientific, but still uncertain, attempt to influence rainfall. Although it is questionable whether any of these intentional efforts have succeeded in significantly modifying the rainfall, human activities are inadvertently altering the climate.

Changes in land use and land cover can affect atmospheric circulation and the movement of moisture locally. Evaporation from neighboring states, which depends on land use, can be the source of as much as one-third of the precipitation of inland areas. The anthropogenic increase in the atmospheric concentration of carbon dioxide and other greenhouse gases is expected to increase the average global surface temperature. Such a change would also affect precipitation patterns, evapo-transpiration rates, the timing and magnitude of runoff, and the frequency and intensity of storms as well as the demand for water. But the magnitude and even the nature of these impacts on the supply and demand for water in specific regions are largely unknown.

It remains to be seen if the U.S. can take full advantage of its abundant water resources and funding to gain the knowledge necessary to help the rest of the world secure and protect its water resources as the global environment begins to change, and thus creating new and quickly emerging climate issues.