Water Resources

Use

Off-stream use occurs when water is diverted away from its natural source (e.g., irrigation, thermoelectric power, industry, and public drinking water), while in-stream is the use of water in its place (e.g., boating and transportation, hydroelectric power, and recreation). In 2000, the generation of thermoelectric power accounted for 48 percent of off-stream water usage, followed by irrigation (34 percent); public water supply (11 percent); industry (5 percent); and domestic, livestock, aquaculture, and mining (3 percent). For most off-stream uses, water is consumed or not returned to the natural source. U.S. water consumption amounts to approximately 100 billion gallons daily.

Supply

Water exists in two forms: seawater (97.5 percent) and freshwater (2.5 percent of the world's water supply). Of the 2.5 percent of freshwater, 1.5 percent is in the polar ice sheets, 0.65 percent is in groundwater, and 0.35 percent is surface water (streams and lakes). U.S. water supplies include seawater, surface water (streams and lakes), and groundwater. Because seawater requires desalination before most of its off-stream uses, only Texas, Florida, and California consider seawater a viable water resource. The major sources of U.S. surface water are five major watersheds: the Columbia River Watershed (Northwest), the Mississippi River Watershed (Great Plains and Midwest), the Colorado River Watershed (Southwest), the Rio Grande Watershed (Southwest and Texas), and [Page 1018]the St. Lawrence River Watershed (Great Lakes and Northeast). U.S. lakes and reservoirs hold approximately 26,550 cubic kilometers of water, with 22,573 cubic kilometers just in the Great Lakes. U.S. rivers hold approximately 250 cubic kilometers of water. U.S. groundwater comes from the many aquifers that underlie the landscape. The most heavily used aquifers are the High Plains or Ogallala Aquifer (Great Plains), the Central Valley aquifer system (California), the Mississippi River Valley alluvial aquifer (Mississippi River Valley), the Basin and Range basin-fill aquifers (West and Southwest), and the Floridan aquifer system (Florida).

Demand

The demand for water continues to grow along with the population: In 1960 in the United States, total withdrawals were an estimated 175 billion gallons daily when the population was 179 million; in 2000, total withdrawals were an estimated 408 billion gallons daily for a population of 281 million. In the past 30 years, the U.S. population has grown 52 percent while total water usage has tripled. On average, Americans use 100 gallons per day for everything from drinking, bathing, laundry, and watering lawns. Water demand is greatest in the western United States where per capita use is roughly 50 percent greater than in eastern states. This disparity is due mostly to the large volume of water used for crop irrigation in California. Crop irrigation in California, Idaho, Colorado, and Nebraska accounted for more than half the total withdrawals for irrigation in 2000.

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