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Hydrosphere -- Britannica Online Encyclopedia 25/3/2020 Hydrosphere -- Britannica Online Encyclopedia Hydrosphere Hydrosphere, discontinuous layer of water at or near Earth’s surface. It includes all liquid and TABLE OF CONTENTS frozen surface waters, groundwater held in soil Introduction and rock, and atmospheric water vapour. Distribution and quantity of Earth’s waters Water is the Biogeochemical properties of the most hydrosphere abundant The water cycle substance at Origin and evolution of the hydrosphere the surface of Impact of human activities on the Earth. About hydrosphere 1.4 billion cubic km (326 Earth's environmental spheres million cubic miles) of water in liquid and frozen form Earth's environment includes the make up the oceans, lakes, streams, glaciers, and atmosphere, the hydrosphere, the lithosphere, and the biosphere. groundwaters found there. It is this enormous volume of Encyclopædia Britannica, Inc. 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 water cycle (or 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 km (9 miles) in Earth’s atmosphere and downward to depths on the order of 5 km (3 miles) in its crust. This article examines the processes of the water 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 and pollution by human society. Details concerning the major water environments that make up the hydrosphere are hydrologic cycle provided in the articles ocean, lake, river, and ice. See also climate for specic information about the impact of https://www.britannica.com/print/article/279025 1/35 25/3/2020 Hydrosphere -- Britannica Online Encyclopedia In the hydrologic cycle, water is climatic factors on the water cycle. The principal transferred between the land surface, the ocean, and the atmosphere. concerns and methods of hydrology and its various allied Encyclopædia Britannica, Inc. disciplines are summarized in Earth sciences. Distribution and quantity of Earth’s waters Ocean waters and waters 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 of sedimentary rocks now in existence and about 5 percent of the mass of 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 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. Water masses at Earth's surface reservoir volume (in cubic kilometres) percent of total oceans 1,338,000,000 96.5 ice caps, glaciers, and permanent snow 24,064,000 1.74 ground ice and permafrost 300,000 0.22 groundwater (total) 23,400,000 1.69 groundwater (fresh) 10,530,000 0.76 groundwater (saline) 12,870,000 0.93 lakes (total) 176,400 0.013 lakes (fresh) 91,000 0.007 lakes (saline) 85,400 0.006 soil moisture 16,500 0.001 atmosphere* 12,900 0.001 swamp water 11,470 0.0008 *As liquid equivalent of water vapour. **Total surpasses 100 percent because of upward rounding of individual reservoir volumes. Source: Adapted from Igor Shiklomanov's chapter "World Fresh Water Resources" in Peter H. Gleick (ed.), Water in Crisis: A Guide to the World's Fresh Water Resources, copyright 1993, Oxford University Press, New York, N.Y. Table made available by the United States Geological Survey. https://www.britannica.com/print/article/279025 2/35 25/3/2020 Hydrosphere -- Britannica Online Encyclopedia reservoir volume (in cubic kilometres) percent of total rivers 2,120 0.0002 biota 1,120 0.0001 total** 1,409,560,910 101.67 *As liquid equivalent of water vapour. **Total surpasses 100 percent because of upward rounding of individual reservoir volumes. Source: Adapted from Igor Shiklomanov's chapter "World Fresh Water Resources" in Peter H. Gleick (ed.), Water in Crisis: A Guide to the World's Fresh Water Resources, copyright 1993, Oxford University Press, New York, N.Y. Table made available by the United States Geological Survey. 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 the glaciations of the Pleistocene 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 inuence on evaporation from soils. The biosphere, though primarily H O in composition, contains very little of the total water at 2 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, Earth’s waters are not pure H O but contain dissolved and 2 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 the transportation of these substances about the planet’s surface. Biogeochemical properties of the hydrosphere Rainwater About 107,000 cubic km (nearly 25,800 cubic miles) of rain fall on land each year. The total water in the atmosphere is 13,000 cubic km, and this water, owing to precipitation and evaporation, turns over every 9.6 days. Rainwater is not pure but rather contains dissolved gases and salts, ne-ground particulate material, organic substances, and even bacteria. The https://www.britannica.com/print/article/279025 3/35 25/3/2020 Hydrosphere -- Britannica Online Encyclopedia sources of the materials in rainwater are the oceans, soils, fertilizers, air pollution, and fossil fuel combustion. It has been observed that rains over oceanic islands and near coasts have ratios of major dissolved constituents very close to those found in seawater. The discovery of the high salt content of rain near coastlines was somewhat surprising because sea salts are not volatile, and it might be expected that the process of evaporation of water from the sea surface would “lter” out the salts. It has been demonstrated, however, that a large percentage of the salts in rain is derived from the bursting of small bubbles at the sea surface due to the impact of rain droplets or the breaking of waves, which results in the injection of sea aerosol into the atmosphere. This sea aerosol evaporates, with resultant precipitation of the salts as tiny particles that are subsequently carried high into the atmosphere by turbulent winds. These particles may then be transported over continents to fall in rain or as dry deposition. –3.5 Assuming equilibrium with the atmospheric carbon dioxide partial pressure (P ) of 10 CO2 (0.00035) atmosphere, the approximate mean composition of rainwater is in parts per million + + 2+ 2+ (ppm): sodium (Na ), 1.98; potassium (K ), 0.30; magnesium (Mg ), 0.27; calcium (Ca ), 0.09; − 2− − chloride (Cl ), 3.79; sulfate (SO ), 0.58; and bicarbonate (HCO ), 0.12. In addition to these 4 3 ions, rainwater contains small amounts of dissolved silica—about 0.30 ppm. The average pH value of rainwater is 5.6. (The term pH is dened as the negative logarithm of the hydrogen ion concentration in moles per litre. The pH scale ranges from 0 to 14, with lower numbers indicating increased acidity.) On a global basis, as much as 35 percent of the sodium, 55 percent of the chlorine, 15 percent of the potassium, and 37 percent of the sulfate in river water may be derived from the oceans through sea aerosol generation. A considerable amount of data has become available for marine aerosols. These aerosols are important because (1) they are vital to any description of the global biogeochemical cycle of an element, (2) they may have an impact on climate, (3) they are a sink, via heterogeneous chemical reactions, for trace atmospheric gases, and (4) they inuence precipitation of cloud and rain droplets. For many trace metals, the ratio of the atmospheric ux to the riverine ux for coastal and remote oceanic areas may be greater than one, indicating the importance of atmospheric transport. Figures have been prepared that illustrate the enrichment factors (EF) of North Atlantic marine aerosols and suspended matter in North Atlantic waters relative to the crust (that is, terrestrial sources), where https://www.britannica.com/print/article/279025 4/35 25/3/2020 Hydrosphere -- Britannica Online Encyclopedia and (X/Al) and (X/Al) refer, respectively, to the ratio of the concentration of the element air crust X to that of Al, aluminum (which is an easily observed terrestrial component of aerosols), in the atmosphere and in average crustal material.
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