hydrosphere, discontinuous layer ofwater at or near Earth’s surface. It includes all liquid and frozen surface waters,groundwater held insoil androck, and atmospheric water vapour.

Water is the most abundant substance at the surface ofEarth. About 1.4 billion cubic km (326 million cubic miles) of water in liquid and frozen form make up theoceans,lakes, streams,glaciers, and groundwaters found there. It is this enormous volume of water, in its variousmanifestations, that forms the discontinuous layer, enclosing much of the terrestrial surface, known as the hydrosphere.

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Central to any discussion of the hydrosphere is the concept of thewater cycle (or hydrologic cycle). This cycle consists of a group of reservoirs containing water, the processes by which water is transferred from onereservoir to another (or transformed from one state to another), and the rates of transfer associated with such processes. These transfer paths penetrate the entire hydrosphere, extending upward to about 15 km (9 miles) in Earth’satmosphere and downward to depths on the order of 5 km (3 miles) in itscrust.

This article examines the processes of thewater cycle and discusses the way in which the various reservoirs of the hydrosphere are related through the water cycle. It also describes the biogeochemical properties of Earth’s waters at some length and considers the distribution of global water resources and their use andpollution by human society. Details concerning the major waterenvironments that make up the hydrosphere are provided in the articlesocean,lake,river, andice.See alsoclimate for specific information about the impact of climatic factors on the water cycle. The principal concerns and methods ofhydrology and its various allieddisciplines are summarized inEarth sciences.

Distribution and quantity of Earth’s waters

Ocean waters andwaters trapped in the pore spaces of sediments make up most of the present-day hydrosphere. The total mass of water in the oceans equals about 50 percent of the mass ofsedimentary rocks now in existence and about 5 percent of the mass of Earth’s crust as a whole. Deep and shallow groundwatersconstitute a small percentage of the total water locked in the pores of sedimentary rocks—on the order of 3 to 15 percent. The amount of water in theatmosphere at any one time is trivial, equivalent to roughly 13,000 cubic km (about 3,100 cubic miles) of liquid water, or about 0.001 percent of the total at Earth’s surface. This water, however, plays an important role in the water cycle.

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At present, ice locks up a little more than 2 percent of Earth’s water and may have accounted for as much as 3 percent or more during the height of theglaciations of thePleistocene Epoch (2.6 million to 11,700 years ago). Although water storage in rivers, lakes, and the atmosphere is small, the rate of water circulation through the rain-river-ocean-atmosphere system is relatively rapid. The amount of water discharged each year into the oceans from the land is approximately equal to the total mass of water stored at any instant in rivers and lakes.

Soil moisture accounts for only 0.005 percent of the water at Earth’s surface. It is this small amount of water, however, that exerts the most direct influence onevaporation from soils. Thebiosphere, though primarily H₂O incomposition, contains very little of the total water at the terrestrial surface, only about 0.00004 percent, yet the biosphere plays a major role in the transport of water vapour back into the atmosphere by the process of transpiration.

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As will be seen in the next section, Earth’s waters are not pure H₂O but contain dissolved and particulate materials. Thus, the masses of water at Earth’s surface are major receptacles of inorganic and organic substances, and water movement plays a dominant role in thetransportation of these substances about theplanet’s surface.