Water Cycle
The water, or hydrologic, cycle refers to the global scale continuous move­ment of water between the oceans, the atmosphere, and the continents. Wa­ter exists on Earth in three states: liquid water, solid (ice or snow), and gas (water vapor). Liquid water covers about 70 percent of Earth’s surface, pri­marily in oceans that store 96 percent of Earth’s total water volume (1460 x 10⁶ km³). The remaining water is mostly contained in the polar ice caps, glaciers, and groundwater, while only small amounts occur in rivers, lakes, soils, and the atmosphere. Freshwater lakes, rivers, and groundwaters com­prise only a tiny fraction of Earth’s water.
Evaporation and precipitation are the major exchange processes for wa­ter between the atmosphere and Earth’s surface. Water, evaporated from land, surface waters, and the oceans and transpired by vegetation, is returned to the atmosphere as water vapor. Water molecules cycle rapidly in the at­mosphere, with an average residence time of only eleven days. Falling as precipitation on land, water can enter the groundwater, later emerging in lakes and rivers; run off as surface flow into rivers and lakes and, eventually, the ocean; or evaporate back to the atmosphere. The residence time of wa­ter in rivers and lakes is extremely variable (from a few weeks for rivers to over one hundred years for the deepest lakes), but is fast compared to 20,000 years in deep groundwater layers and 39,000 years in the ocean.
Although the amount of water stored is small and the residence times short, atmospheric water, rivers, and lakes are extremely important for main­tenance of the world’s ecosystems. Global patterns of precipitation and evaporation determine the distribution and character of biological habitats from deserts to rain forests. Water transports minerals, sediments, nutri­ents, and pollutants across the landscape, and over long distances in the at­mosphere. Because of close links with global energy and carbon and nitrogen cycles, the water cycle is vitally important to Earth’s ecology.
References
Berner, Elizabeth Kay, and Robert A. Berner. Global Environment: Water, Air, and Geochemical Cycles. Upper Saddle River, NJ: Prentice Hall, 1996.
Maidment, David R., ed. Handbook of Hydrology. New York: McGraw-Hill, 1993.