America's Water Supply: Status and Prospects for the Future

Abstract

Dr. Kenneth D. Frederick is an economist and senior fellow at Resources for the Future in Washington, D.C., an independent organization that conducts research on the development, conservation, and use of natural resources and the quality of the environment. Prior to joining RFF he served on the faculty of economics at the California Institute of Technology and as an economic advisor in Brazil for the Agency for International Development. Water is critical for the health of both humans and ecological systems and an important element in many of our recreational and economic activities. Neither plants nor animals can survive long without it, and water is used in virtually everything we make and do. It is the most widely used resource by industry; it is used both directly and indirectly to produce energy; it provides the basis for much of our outdoor recreation; it is an important part of our transportation network; it serves as a vehicle for disposing of wastes; and it provides important cultural and amenity values. The quality of life, as well as life itself, depends on an adequate supply of freshwater. Water covers almost 70 percent of the surface of the globe and is the Earth's most abundant resource. About 97 percent of the water is in the oceans and is too salty for drinking, growing crops, or most other human uses. Almost all of the freshwater is held in the ice caps of Antarctica and Greenland or in deep underground aquifers where, for most practical purposes, it is inaccessible for human use. Only about 0.3 percent of the earth's freshwater, less than 100,000 cubic kilometers or 81 trillion acre-feet (the unit of water needed to cover one acre to a depth of one foot, which is about 326,000 gallons), is found in rivers and lakes. These surface waters together with accessible groundwater resources comprise the usable supply (Table 1). Water is also one of the Earth's most renewable resources. Globally, the total quantity of water is essentially constant and unaffected by human activities. Driven by energy from the Sun, water constantly circulates from the seas, lakes, and streams (through evaporation) or the plants (through transpiration) to the atmosphere and back to earth (through precipitation). The evaporative process removes salts and other impurities that may be picked up either naturally or as a result of human use, making it possible to use and reuse water virtually indefinitely. The United States is relatively well endowed with water. Annual precipitation averages nearly 30 inches or 4,200 billion gallons per day (bgd) throughout the conterminous