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Encyclopedia > Water cycle
The movement of water around, over, and through the Earth is called the water cycle.

The water cycle — technically known as the hydrologic cycle — is the continuous circulation of water within the Earth's hydrosphere, and is driven by solar radiation. This includes the atmosphere, land, surface water and groundwater. As water moves through the cycle, it changes state between liquid, solid, and gas phases. Water moves from compartment to compartment, such as from river to ocean, by the physical processes of evaporation, precipitation, infiltration, runoff, and subsurface flow.[1] Download high resolution version (860x589, 761 KB)Water cycle http://ga. ... Download high resolution version (860x589, 761 KB)Water cycle http://ga. ... Water is a tasteless, odourless substance that is essential to all known forms of life and is known as the universal solvent. ... Earth (IPA: , often referred to as the Earth, Terra, the World or Planet Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth largest. ... The movement of water around, over, and through the Earth is called the water cycle, a key process of the hydrosphere. ... Solar irradiance spectrum at top of atmosphere. ... Earths atmosphere is the layer of gases surrounding the planet Earth and retained by the Earths gravity. ... Surface water is water on the ground or in a stream, river, lake, sea or ocean; as opposed to groundwater. ... Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. ... In the physical sciences, a phase is a set of states of a macroscopic physical system that have relatively uniform chemical composition and physical properties (i. ... Water is a tasteless, odourless substance that is essential to all known forms of life and is known as the universal solvent. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Water vapor or water vapour, also aqueous vapour, is the gas phase of water. ... River upstream of an Australian trout farm A river is a large natural waterway. ... The worlds oceans as seen from the South Pacific Ocean, before the definition of the Southern Ocean in 2000 Oceans (from Okeanos in Greek, the ancient Greeks noticing the strong current that flowed off Gibraltar and assuming it was a great river) cover almost three quarters (71%) of the... This article or section is in need of attention from an expert on the subject. ... Infiltration is the process by which water on the ground surface enters the soil. ... Runoff is the flow of water across the earths surface, and is a major component in the hydrological cycle. ...



The water cycle is the continuous movement of water over, above, and beneath the Earth's surface.[2] It is powered by solar energy, and because it is a cycle, there is no beginning or end. As water moves around in the hydrosphere, it changes state among liquid, vapour, and ice. The time taken for water to move from one place to another varies from seconds to thousands of years, and the amount of water stored in different parts of the hydrosphere ranges up to 1.37 billion km³, which is contained in the oceans. Despite continual movement within the hydrosphere, the total amount of water at any one time remains essentially constant.

Movement of water takes place by a variety of physical and biophysical processes. The two processes responsible for moving the greatest quantities of water are precipitation and evaporation, transporting 505,000 km³ of water each year. The flow of water along rivers transports an intermediate amount of water, and sublimation of ice directly to vapour transports relatively very little. The different processes are as follows.

  • Precipitation is condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet.[3] Approximately 505,000 km³ of water fall as precipitation each year, 398,000 km³ of it over the oceans.[4]
  • Canopy interception is the precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground.
  • Snowmelt refers to the runoff produced by melting snow.
  • Runoff includes the variety of ways by which water moves across the land. This includes both surface runoff and channel runoff. As it flows, the water may infiltrate into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.
  • Subsurface Flow is the flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (eg. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures. Groundwater tends to move slowly, and is replenished slowly, so it can remain in aquifers for thousands of years.
  • Evaporation is the transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.[6] The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Approximately 90% of atmospheric water comes from evaporation, while the remaining 10% is from transpiration.[citation needed] Total annual evapotranspiration amounts to approximately 505,000 km³ of water, 434,000 km³ of which evaporates from the oceans.[7]
  • Sublimation is the state change directly from solid water (snow or ice) to water vapor.[8]
  • Advection is the movement of water—in solid, liquid, or vapour states—through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.[9]

Rain falling Rain is a form of precipitation, other forms of which include snow, sleet, hail, and dew. ... Animation of snowcover changing with the seasons. ... This article or section does not cite its references or sources. ... Evening fog obscures Londons Tower Bridge from passers by. ... Graupel can be any of the following types of solid-ice precipitation: hail - large chunks of ice such as from a strong or severe thunderstorm sleet - small pellets of raindrops that have frozen in mid-air, in winter or a thunderstorm snow pellets - when freezing fog forms 2-5mm balls... Sleet can refer to at least two different forms of precipitation. ... Interception, or canopy interception, (geographical term) is when precipitation does not reach the soil, and is intercepted by some vegetation. ... Vegetation gives off heat, resulting in this circular snowmelt pattern. ... Runoff is the flow of water across the earths surface, and is a major component in the hydrological cycle. ... Runoff flowing into a stormwater drain Surface runoff is water, from rain, snowmelt, or other sources, that flows over the land surface, and is a major component of the water cycle[1][2]. Runoff that occurs on surfaces before reaching a channel is also called overland flow. ... Streamflow, or channel runoff, is the flow of water in streams, rivers, and other channels, and is a major element of the water cycle. ... Infiltration is the process by which water on the ground surface enters the soil. ... This article or section does not cite its references or sources. ... Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of geologic formations. ... Subsurface flow is the flow of water beneath ground surface in hydrology. ... Gravity is a force of attraction that acts between bodies that have mass. ... This article or section is in need of attention from an expert on the subject. ... Solar irradiance spectrum at top of atmosphere. ... Transpiration is the evaporation of water from aerial parts of plants, especially leaves but also stems, flowers and fruits. ... Divisions Green algae Chlorophyta Charophyta Land plants (embryophytes) Non-vascular plants (bryophytes) Marchantiophyta - liverworts Anthocerotophyta - hornworts Bryophyta - mosses Vascular plants (tracheophytes) †Rhyniophyta - rhyniophytes †Zosterophyllophyta - zosterophylls Lycopodiophyta - clubmosses †Trimerophytophyta - trimerophytes Equisetophyta - horsetails Pteridophyta - true ferns Psilotophyta - whisk ferns Ophioglossophyta - adderstongues Seed plants (spermatophytes) †Pteridospermatophyta - seed ferns Pinophyta - conifers Cycadophyta - cycads Ginkgophyta... Evapotranspiration (ET) is the sum of evaporation and plant transpiration. ... Sublimation of an element or substance is a conversion between the solid and the gaseous phases of matter, with no intermediate liquid stage. ... Advection is the transport of a conserved scalar quantity that is transported in a vector field. ... Water vapor condensing over a cup of hot tea Condensation is the change in matter of a substance to a denser phase, such as a gas (or vapor) to a liquid. ... Cumulonimbus capillatus incus floating over Swifts Creek, Victoria in Australia A cloud is a visible mass of condensed droplets or frozen crystals suspended in the atmosphere above the surface of the Earth or another planetary body. ... Evening fog obscures Londons Tower Bridge from passers by. ...


Volume of water stored in
the water cycle's reservoirs
Reservoir Volume of water
(106 km³)
of total
Oceans 1370 97.25
Ice caps & glaciers 29 2.05
Groundwater 9.5 0.68
Lakes 0.125 0.01
Soil moisture 0.065 0.005
Atmosphere 0.013 0.001
Streams & rivers 0.0017 0.0001
Biosphere 0.0006 0.00004

In the context of the water cycle, a reservoir represents the water contained in different steps within the cycle. The largest reservoir is the collection of oceans, accounting for 97% of the Earth's water. The next largest quantity (2%) is stored in solid form in the ice caps and glaciers. The water contained within all living organisms represents the smallest reservoir. To help compare the orders of magnitude of different volumes, here is a list of volumes between 1015 m3 and 1018 m3. ...

The volume of water in the fresh water reservoirs, particularly those that are available for human use, are important water resources.[12] Water resources are sources of water that are useful or potentially useful to humans. ...

Residence times

Average reservoir residence times[11]
Reservoir Average residence time
Oceans 3,200 years
Glaciers 20 to 100 years
Seasonal snow cover 2 to 6 months
Soil moisture 1 to 2 months
Groundwater: shallow 100 to 200 years
Groundwater: deep 10,000 years
Lakes 50 to 100 years
Rivers 2 to 6 months
Atmosphere 9 days

The residence time of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that reservoir (see the adjacent table). It is a measure of the average age of the water in that reservoir, though some water will spend much less time than average, and some much more. A residence time is the average time a substance spends within a specified region of space, such as a reservoir. ...

Groundwater can spend over 10,000 years beneath Earth's surface before leaving. Particularly old groundwater is called fossil water. Water stored in the soil remains there very briefly, because it is spread thinly across the Earth, and is readily lost by evaporation, transpiration, stream flow, or groundwater recharge. After evaporating, water remains in the atmosphere for about 9 days before condensing and falling to the Earth as precipitation. Fossil water is groundwater having remained in an aquifer for thousands or more years. ...

In hydrology, residence times can be estimated in two ways. The more common method relies on the principle of conservation of mass and assumes the amount of water in a given reservoir is roughly constant. With this method, residence times are estimated by dividing the volume of the reservoir by the rate by which water either enters or exits the reservoir. Conceptually, this is equivalent to timing how long it would take the reservoir to become filled from empty if no water were to leave (or how long it would take the reservoir to empty from full if no water were to enter). (The Lomonosov-Lavoisier law) states that the mass of a closed system of substances will remain constant, regardless of the processes acting inside the system. ...

An alternative method to estimate residence times, gaining in popularity particularly for dating groundwater, is the use of isotopic techniques. This is done in the subfield of isotope hydrology. Isotope hydrology is a fast, cheap, and reliable way to discover the age, origins, size, flow and fate of a water source for purposes of sound water-use policy, maping underground aquifers, conserving water supplies, and controling pollution. ...

Changes over time

Over the past century the water cycle has become more intense[13], with the rates of evaporation and precipitation both increasing. This is an expected outcome of global warming, as higher temperatures increase the rate of evaporation due to warmer air's higher capacity for holding moisture.[14] Global mean surface temperatures 1856 to 2005 Mean surface temperature anomalies during the period 1995 to 2004 with respect to the average temperatures from 1940 to 1980 Global warming is the observed increase in the average temperature of the Earths atmosphere and oceans in recent decades. ...

Glacial retreat is also an example of a changing water cycle, where the supply of water to glaciers from precipitation cannot keep up with the loss of water from melting and sublimation. Glacial retreat since 1850 has been extensive.[15] Grinnell Glacier in Glacier National Park (US) showing recession since 1850 of 1. ... Grinnell Glacier in Glacier National Park (US) showing recession since 1850 of 1. ...

Human activities that alter the water cycle include:

Scrivener Dam, in Canberra, Australia, was engineered to withstand a once-in-5000-years flood event A dam is a barrier across flowing water that obstructs, directs or retards the flow, often creating a reservoir, lake or impoundment. ... gfdjhgkhfDeforestation is the conversion of forested areas to non-forest land use such as arable land, urban use, logged area or wasteland. ... Afforestation is the process of converting open land into a forest by planting trees or their seeds. ... A water well is an artificial excavation or structure put down by any method such as digging, boring or drilling for the purposes of withdrawing water from underground aquifers. ... Water abstraction is the process of taking water from the environment for irrigation or treatment to produce drinking water. ...

Effects on climate

The water cycle is powered from solar energy. 86% of the global evaporation occurs from the oceans, reducing their temperature by evaporative cooling. Without the cooling effect of evaporation the greenhouse effect would lead to a much higher surface temperature of 67 °C, and a warmer planet.[16] Evaporative cooling is a system in which latent heat of evaporation is used to carry heat away from an object to cool it. ... Celsius is, or relates to, the Celsius temperature scale. ...

Most of the solar energy warms tropical seas. After evaporating, water vapour rises into the atmosphere and is carried by winds away from the tropics. Most of this vapour condenses as rain in the Intertropical convergence zone, also known as the ITCZ, releasing latent heat that warms the air. This in turn drives the atmospheric circulation.[citation needed] The ITCZ, or InterTropical Convergence Zone, is a belt of low pressure girdling the globe at the equator. ... Atmospheric circulation is the large-scale movement of air, and the means (together with the ocean circulation, which is smaller [1]) by which heat is distributed on the surface of the Earth. ...

Effects on biogeochemical cycling

While the water cycle is itself a biogeochemical cycle,[17] flow of water over and beneath the Earth is a key component of the cycling of other biogeochemicals. Runoff is responsible for almost all of the transport of eroded sediment and phosphorus[18] from land to waterbodies. The salinity of the oceans is derived from erosion and transport of dissolved salts from the land. Cultural eutrophication of lakes is primarily due to phosphorus, applied in excess to agricultural fields in fertilizers, and then transported overland and down rivers. Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to waterbodies.[19] The dead zone at the outlet of the Mississippi River is a consequence of nitrates from fertilizer being carried off agricultural fields and funnelled down the river system to the Gulf of Mexico. Runoff also plays a part in the carbon cycle, again through the transport of eroded rock and soil.[20] In ecology, a biogeochemical cycle is a circuit where a nutrient moves back and forth between both biotic and abiotic components of ecosystems. ... Severe soil erosion in a wheat field near Washington State University, USA. Erosion is the displacement of solids (soil, mud, rock and other particles) by the agents of wind, water or ice, by downward or down-slope movement in response to gravity or by living organisms (in the case of... Sediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. ... General Name, Symbol, Number phosphorus, P, 15 Chemical series nonmetals Group, Period, Block 15, 3, p Appearance waxy white/ red/ black/ colorless Atomic mass 30. ... A body of water is any significant natural pool of water such as an ocean, a lake, or a river, covering the Earth or another planet. ... Annual mean sea surface salinity for the World Ocean. ... Eutrophication is apparent as increased turbidity in the northern part of the Caspian Sea, imaged from orbit. ... A green field or paddock In agriculture, a field refers generally to an area of land enclosed or otherwise and used for agricultural purposes such as: Cultivating crops Usage as a paddock or generally an enclosure of livestock Land left to lie fallow or as arable land See also Pasture... spreading manure, an organic fertilizer Fertilizers are compounds given to plants to promote growth; they are usually applied either via the soil, for uptake by plant roots, or by foliar feeding, for uptake through leaves. ... Sediment from the Mississippi River carries fertilizer to the Gulf of Mexico Dead zones are hypoxic (low-oxygen) areas in the worlds oceans, the observed incidences of which have been increasing since oceanographers began noting them in the 1970s. ... The Mississippi River, derived from the old Ojibwe word misi-ziibi meaning great river (gichi-ziibi big river at its headwaters), is the second-longest river in the United States; the longest is the Missouri River, which flows into the Mississippi. ... An electrostatic potential map of the nitrate ion. ... For the Second World War frigate class, see River class frigate The Murray River in Australia A waterfall on the Ova da Fedoz, Switzerland A river is a large natural waterway. ... Gulf of Mexico in 3D perspective. ... The carbon cycle is the biogeochemical cycle by which carbon is exchanged between the biosphere, geosphere, hydrosphere and atmosphere of the Earth (other astronomical objects may have similar carbon cycles, but nothing is yet known about them). ...

See also

Look up flood in Wiktionary, the free dictionary. ... A drought is a period of time when there is not enough water to support agricultural, urban or environmental water needs. ...


  1. ^ U.S. Geologic Survey. Water Cycle. Retrieved on 2006-10-24.
  2. ^ U.S. Geologic Survey. Water Cycle. Retrieved on 2006-10-24.
  3. ^ Arctic Climatology and Meteorology. Precipitation. Retrieved on 2006-10-24.
  4. ^ Dr. Art's Guide to Planet Earth. The Water Cycle. Retrieved on 2006-10-24.
  5. ^ National Weather Service Northwest River Forecast Center. Hydrologic Cycle. Retrieved on 2006-10-24.
  6. ^ Arctic Climatology and Meteorology. Evaporation. Retrieved on 2006-10-24.
  7. ^ Dr. Art's Guide to Planet Earth. The Water Cycle. Retrieved on 2006-10-24.
  8. ^ Arctic Climatology and Meteorology. Sublimation. Retrieved on 2006-10-24.
  9. ^ Arctic Climatology and Meteorology. Advection. Retrieved on 2006-10-24.
  10. ^ Arctic Climatology and Meteorology. Condensation. Retrieved on 2006-10-24.
  11. ^ a b PhysicalGeography.net. CHAPTER 8: Introduction to the Hydrosphere. Retrieved on 2006-10-24.
  12. ^ Environmental Literacy Council. Water Cycle. Retrived on 2006-10-24.
  13. ^ U.S. Geologic Survey. Century of data shows intensification of water cycle but no increase in storms or floods. Retrieved on 2006-10-24.
  14. ^ University of Massachusetts. Reducing Humidity in the Greenhouse. Retrieved on 2006-10-24.
  15. ^ U.S. Geologic Survey. GLACIER RETREAT IN GLACIER NATIONAL PARK, MONTANA. Retrieved on 2006-10-24.
  16. ^ Science at NASA. NASA Oceanography: The Water Cycle. Retrieved on 2006-10-24.
  17. ^ The Environmental Literacy Council. Biogeochemical Cycles. Retrieved on 2006-10-24.
  18. ^ The Environmental Literacy Council. Phosphorus Cycle. Retrieved on 2006-10-24.
  19. ^ Ohio State University Extension Fact Sheet. Nitrogen and the Hydrologic Cycle. Retrieved on 2006-10-24.
  20. ^ NASA's Earth Observatory. The Carbon Cycle. Retrieved on 2006-10-24.

External links

Biogeochemical cycles
Carbon cycle - Hydrogen cycle - Nitrogen cycle
Oxygen cycle - Phosphorus cycle - Sulfur cycle - Water cycle

  Results from FactBites:
Water Cycle - MSN Encarta (979 words)
The water cycle consists of four distinct stages: storage, evaporation, precipitation, and runoff.
Water may be stored temporarily in the ground; in oceans, lakes, and rivers; and in ice caps and glaciers.
However, because the water that evaporates from the ocean is almost free of salt, the rain and snow that fall on the earth are relatively fresh.
EO Library: The Water Cycle (442 words)
The notion that water is continually circulating from the ocean to the atmosphere to the land and back again to the ocean has interested scholars through most of recorded history.
Water is everywhere on Earth and is the only known substance that can naturally exist as a gas, liquid, and solid within the relatively small range of air temperatures and pressures found at the Earth's surface.
In all, the Earth's water content is about 1.39 billion cubic kilometers (331 million cubic miles) and the vast bulk of it, about 96.5%, is in the global oceans.
  More results at FactBites »



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