Hydrologic Cycle

Heat from solar energy causes water to evaporate off the surface of the sea and the land. The vapor rises and condenses to form clouds from which rain and snow fall back to the surface. On the land, rivers return the water to the sea to be evaporated once again. The endless cycle is called the hydrologic cycle. When rain falls to the surface, some of it is immediately evaporated back to the atmosphere, a portion flows into streams, and the rest soaks into the ground. How much water goes to each depends on temperature, humidity, and the nature of the soil and bedrock. The portion that moves underground through soil and rock may enter aquifers (underground reservoirs), where water may take decades or more to be returned by seeping into streams and lakes and then to the atmosphere or the sea. However, some water that soaks into the soil may be utilized by plants and is returned to the atmosphere by transpiration, the loss of water through leaves. Studies have shown that transpiration contributes more water to the atmosphere than does direct evaporation from lakes, rivers, and soils. In many places snow usually melts and evaporates, returning the water quickly to the atmosphere; but in colder climates some snow remains and may be converted to glacial ice, where it is reserved for hundreds or thousands of years. If all of the glacial ice on earth were to melt, sea level would rise about 240 feet.

One important aspect of the hydrologic cycle is that the evaporative process purifies water. All of the materials in water—whether they are sea salts, clay particles, micro-organisms, or dissolved materials—are left behind. However, as water condenses and precipitates, it may pick up pollutants on the way down, and after reaching the surface may dissolve additional unwanted chemicals on or below the ground.

—Sidney Horenstein

See also: Atmosphere; Climatology; Freshwater; Lakes; Oceans; Rivers and Streams


Berner, Elizabeth K., and Robert A. Berner. 1996. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice Hall; Hamblin, W. Kenneth, and Eric H. Christiansen. 2000. The Earth's Dynamic Systems. Upper Saddle River, NJ: Prentice Hall; Laing, David. 1991. The Earth System: An Introduction to Earth Science. Dubuque, IA: Wm. C. Brown; Press, Frank, and Raymond Siever. 2000. Understanding Earth. New York: W. H. Freeman and Company.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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