discontinuous layer of water at or near the Earth’s surface. It includes all liquid and frozen surface waters, groundwater held in soil and rock, and atmospheric water vapour.
Water is the most abundant substance at the surface of the Earth. About 1.4 billion cubic kilometres (326 million cubic miles) of water in liquid and frozen form make up the oceans, lakes, streams, glaciers, and groundwaters found there. It is this enormous volume of water, in its various manifestations, that forms the discontinuous layer, enclosing much of the terrestrial surface, known as the hydrosphere.
Central to any discussion of the hydrosphere is the concept of the hydrologic cycle. This cycle consists of a group of reservoirs containing water, the processes by which water is transferred from one reservoir 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 kilometres (nine miles) in the Earth’s atmosphere and downward to depths on the order of five kilometres in its crust.
This article examines the processes of the hydrologic cycle and discusses the way in which the various reservoirs of the hydrosphere are related through the hydrologic cycle. It also describes the biogeochemical properties of the waters of the Earth at some length and considers the distribution of global water resources and their utilization and pollution by human society. Details concerning the major water environments that make up the hydrosphere are provided in the articles ocean, lake, river, and ice. See also climate for specific information about the impact of climatic factors on the hydrologic cycle. The principal concerns and methods of hydrology and its various allied disciplines are summarized in Earth sciences.
Ocean waters and waters trapped in the pore spaces of sediments make up most of the present-day hydrosphere (see Table 1). The total mass of water in the oceans equals about 50 percent of the mass of sedimentary rocks now in existence and about 5 percent of the mass of the Earth’s crust as a whole. Deep and shallow groundwaters constitute 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 the atmosphere at any one time is trivial, equivalent to 0.013 × 106 cubic kilometres of liquid water, or about 0.001 percent of the total at the Earth’s surface. This water, however, plays an important role in the water cycle.
| Water masses at the Earth’s surface | ||
| reservoir | volume (in millions of cubic kilometres) | percent of total |
| oceans | 1,370.0 | 97.25 |
| ice caps and glaciers | 29.0 | 2.05 |
| deep groundwater* (750–4,000 metres) | 5.3 | 0.38 |
| shallow groundwater (less than 750 metres) | 4.2 | 0.30 |
| lakes | 0.125 | 0.01 |
| soil moisture | 0.065 | 0.005 |
| atmosphere** | 0.013 | 0.001 |
| rivers | 0.0017 | 0.0001 |
| biosphere | 0.0006 | 0.00004 |
| total | 1,408. 7 | 100 |
| *The total interstitial water in the pores of sediments is on the order of 50 × 106 to 300 × 106 km3. **As liquid equivalent of water vapour. |
||
| Source: Adapted from Elizabeth Kay Berner and Robert A. Berner, The Global Water Cycle: Geochemistry and Environment, copyright 1987, Table 2.1, p. 13. Reproduced by permission of Prentice Hall, Inc., Englewood Cliffs, N.J. | ||
At present, ice locks up a little more than 1 percent of the Earth’s water and may have accounted for as much as 3 percent or more during the height of the glaciations of the Pleistocene epoch (from 1,600,000 to 10,000 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 the Earth’s surface. It is this small amount of water, however, that exerts the most direct influence on evaporation from soils. The biosphere, though primarily H2O in composition, 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.
As will be seen in the next section, the Earth’s waters are not pure H2O but contain dissolved and particulate materials. Thus, the masses of water at the Earth’s surface are major receptacles of inorganic and organic substances, and water movement plays a dominant role in the transportation of these substances about the planet’s surface.
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