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Global Distribution and Circulation of Water
Ground water is a part of the hydrosphere (Nace, 1960), which includes all of the water of the oceans, rivers, lakes, lower atmosphere, and subterranean environments. Ground water accounts for less than 1% of all the water in the hydrosphere. As tiny as this amount might first appear, it is nearly seven times the amount of fresh surface water available at any one time.
There is a constant interchange of water throughout the hydrosphere from sea water to atmospheric moisture to surface water to ground water. This change of water forms is known as the hydrologic cycle. The basic components of this cycle are illustrated in Figure 1.
There is no beginning or end to the hydrologic cycle, which involves a layer of the atmosphere 10 miles thick and at least 1/2 mile of the lithosphere (soil and rock) (Chow, 1964). Precipitated water returns to the earth in a variety of forms and may be intercepted or transpired back to the atmosphere by plants. It may run over the ground into streams and lakes or evaporated back into the air. A small amount moves downward through the soil to a zone of saturation.
This ground water is later discharged directly to streams and lakes or to springs and seeps, from which it may run off to streams or be evaporated into the atmosphere. Ultimately, surface and ground water flow back to the oceans to complete the cycle, which may take hours, or thousands and even millions of years.
Last updated on March 25, 2009
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