Freshwater occurs in minor amounts on the earth, but it is found virtually everywhere: in the atmosphere, streams, lakes, permafrost, glaciers, ice caps, icebergs, and below the ground. Although the salty oceans contain 97 percent (1,322,000,000 cubic kilometers) of the earth's water, glaciers contain about 2 percent (29,200,000 cubic kilometers); freshwater on land and in the atmosphere accounts for only 0.635 percent (8,630,000 cubic kilometers) of the total. It is estimated that ground-water (8,400,000 cubic kilometers) contains the significant portion of the total on the land; freshwater lakes (125,000 cubic kilometers), saline lakes and inland seas (104,000 cubic kilometers), soil moisture (67,000 cubic kilometers), atmosphere (13,000 cubic kilometers), and stream channels (1,250 cubic kilometers) make up the rest.
Many studies predicting the state of water resources of the United States in the future are pessimistic, because of not enough surface water, removal of too much groundwater, pollution of surface and groundwater, deterioration of drinking water quality, and flooding and destruction of wetlands. It has been estimated that the water table in the United States has been dropping 2 to 3 feet per year over large areas. Urban development, agriculture, deforestation, and desertification increase runoff and decrease infiltration to the water table.
Removing too much surface water for use in one place deprives other places of adequate water. Water taken for Los Angeles and irrigation removes so much from the Colorado River that the river is often dry where it flows into the Gulf of California. River diversions cause wetlands along rivers to dry up, causing waterfowl and other wildlife dependent on them to disappear. In the southern part of Florida, the Everglades has been drying up, because its original supply of water came from Lake Okeechobee. To aid farming, canals were cut from the lake, bypassing the Everglades and leaving the wetlands to subsist on local rainfall. In addition, a thousand-square-mile area of the Everglades was diked and ditched and equipped with large pumps, to provide land for sugar cane production. Recently Congress has passed legislation to remedy some of these problems.
Water on earth occurs in three forms: as a solid (ice), a liquid (water), and as a gas (water vapor). In liquid water, the molecules are not as tightly packed as they are in ice. Although water is densest at 4 degrees centigrade, ice is less dense, and it floats. As a result the water at the bottom of deeper lakes will not freeze.
The water table marks the division between the zone of aeration and the zone of saturation, with most of the water lying within 3,800 m of the surface and available with current technology. Rocks that contain water within the zone of saturation are called an aquifer, and the amount of water in an aquifer is determined by porosity—the amount of space and permeability, and how connected the pores are. Rocks with high porosity and low permeability will not transmit water easily. In rocks like granite, which contains a negligible number of pores, the water is located in cracks, while in some sandstone, it is in the spaces between the grains. Sandstone aquifers usually contain much more water than do granite aquifers. Some rocks, such as shale, are nearly impervious. As the water table drops, spring flow diminishes, wells become less productive or dry up, and surface flow in streams is also reduced. In addition, as the space the water occupied empties, the land above may subside, some
times by as much as 30 feet: building foundations may crack, as may roads, sewers, and water lines. Along the coast, saltwater often intrudes into the aquifer, and sinkholes develop where the supporting limestone is reduced as the groundwater drains away. Nearly half of the people in the United States use groundwater for drinking.
Under normal circumstance most plants get their water from soil moisture. However in dry environments, some plants, called phreato-phytes, can retrieve water from deep aquifers. They consume huge quantities of water and flourish along canals and reservoir where notable amounts of water are removed and returned to the air through leaves. Stream flow is very variable, depending on geology and climatic conditions. In the United States a network of gauging stations measures the amount of water passing by particular places along rivers, providing a good record of the amount of surface water. In general, a north-south line just east of the Oklahoma and Kansas boundary, for example, divides the country into two parts: a western part where evaporation exceeds precipitation and an eastern part, where precipitation exceeds evaporation. To ensure sufficient water supplies for homes, industries, and irrigation, dams are built to store water in times of excess flow.
It has been estimated that perhaps as many as 2 billion people worldwide do not have access to safe freshwater today. The amount of water we have on earth will remain essentially the same. Increases in personal per capita use, population increase, and the resulting increase in industrialization are going to stress both the quality and quantity of this resource.
See also: Dams; Interior Wetlands; Lakes; Rivers and Streams
Berner, Elizabeth K., and Robert A. Berner. 1996. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice Hall; Delsemme, Arman. H. 2001. "An Argument for the Cometary Origin of the Biosphere." American Scientist 89: 432-442; Hamblin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems, 9th ed. Upper Saddle River, NJ: Prentice Hall; Laing, David. 1991. The Earth System: An Introduction to Earth Science. Dubuque, IA: Wm. C. Brown.
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