Water cycle
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2008-06-22T08:50:17Z
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The [[Earth]]'s [[water]] is always in movement, and the '''water cycle''', also known as the '''hydrologic cycle''', describes the continuous movement of water on, above, and below the surface of the Earth. Since the water cycle is truly a "cycle," there is no beginning or end. Water can change states among [[liquid]], [[water vapor|vapor]], and [[ice]] at various places in the water cycle, with these processes happening in the blink of an eye and over millions of years. Although the balance of water on Earth remains fairly constant over time, individual water molecules can come and go in a hurry, but there is always the same amount of water on the surface of the earth.
==Description==
[[Image:Water cycle.png|thumb|250px|right|The water cycle.]]
The water cycle has no starting or ending point. The sun, which drives the water cycle, heats water in the oceans. Some of it [[evaporate]]s as vapor into the [[air]]. Ice and [[snow]] can [[sublimate]] directly into water vapor. Rising air currents take the vapor up into the atmosphere, along with water from evapotranspiration, which is water transpired from plants and evaporated from the soil. The vapor rises into the air where cooler temperatures cause it to condense into clouds. Air currents move clouds around the globe, cloud particles collide, grow, and fall out of the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, which can store frozen water for thousands of years. Snowpacks in warmer climates often thaw and melt when spring arrives, and the melted water flows overland as snowmelt. Most precipitation falls back into the oceans or onto land, where, due to gravity, the precipitation flows over the ground as surface runoff. A portion of runoff enters rivers in valleys in the landscape, with streamflow moving water towards the oceans. Runoff, and ground-water seepage, accumulate and are stored as freshwater in lakes. Not all runoff flows into rivers. Much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes [[aquifer]]s (saturated subsurface rock), which store huge amounts of freshwater for long periods of time. Some infiltration stays close to the land surface and can seep back into surface-water bodies (and the ocean) as ground-water discharge, and some ground water finds openings in the land surface and emerges as freshwater springs. Over time, the water continues flowing, some to reenter the ocean, where the water cycle renews itself.
The different processes are as follows:
:*'''[[Precipitation (meteorology)|Precipitation]]''' is condensed water vapor that falls to the Earth's surface. Most precipitation occurs as [[rain]], but also includes [[snow]], [[hail]], [[fog|fog drip]], [[graupel]], and [[sleet]].<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/precipitation.html Precipitation.] Retrieved on 2006-10-24.</ref> Approximately 505,000 km³ of water fall as precipitation each year, 398,000 km³ of it over the oceans.<ref>Dr. Art's Guide to Planet Earth. [http://www.planetguide.net/book/chapter_2/water_cycle.html The Water Cycle.] Retrieved on 2006-10-24.</ref>
:*'''[[Interception (water)|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 (hydrology)|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.
:*'''[[Infiltration (hydrology)|Infiltration]]''' is the flow of water from the ground surface into the ground. Once infiltrated, the water becomes [[soil moisture]] or [[groundwater]].<ref>National Weather Service Northwest River Forecast Center. [http://www.nwrfc.noaa.gov/info/water_cycle/hydrology.cgi Hydrologic Cycle.] Retrieved on 2006-10-24.</ref>
:*'''[[Subsurface Flow (hydrology)|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.<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/evaporation.html Evaporation.] Retrieved on 2006-10-24.</ref> The source of energy for evaporation is primarily [[solar radiation]]. Evaporation often implicitly includes '''[[transpiration]]''' from [[plant]]s, though together they are specifically referred to as '''[[evapotranspiration]]'''. Total annual evapotranspiration amounts to approximately 505,000 km³ of water, 434,000 km³ of which evaporates from the oceans.<ref>Dr. Art's Guide to Planet Earth. [http://www.planetguide.net/book/chapter_2/water_cycle.html The Water Cycle.] Retrieved on 2006-10-24.</ref>
:*'''[[Sublimation (chemistry)|Sublimation]]''' is the state change directly from solid water (snow or ice) to water vapor.<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/sublimation.html Sublimation.] Retrieved on 2006-10-24.</ref>
:*'''[[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.<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/advection.html Advection.] Retrieved on 2006-10-24.</ref>
:*'''[[Condensation]]''' is the transformation of water vapour to liquid water droplets in the air, producing [[cloud]]s and [[fog]].<ref>Arctic Climatology and Meteorology. [http://nsidc.org/arcticmet/glossary/condensation.html Condensation.] Retrieved on 2006-10-24.</ref>
==Reservoirs==
{| class="wikitable" style="margin-left:1em; text-align:center" align="right"
|+ '''Volume of water stored in <br> the water cycle's reservoirs'''<ref name="PHYS">PhysicalGeography.net. [http://www.physicalgeography.net/fundamentals/8b.html CHAPTER 8: Introduction to the Hydrosphere.] Retrieved on 2006-10-24.</ref>
! style="text-align:left" valign="top"| Reservoir || Volume of water<br>([[1 E+15 m³|10<sup>6</sup> km³]]) || valign="top"|Percent<br> of total
|-
| style="text-align:left"|Oceans
|1370
|97.25
|-
| style="text-align:left" |Ice caps & glaciers
|29
|2.05
|-
| style="text-align:left" |Groundwater
|9.5
|0.68
|-
| style="text-align:left" |Lakes
|0.125
|0.01
|-
| style="text-align:left" |Soil moisture
|0.065
|0.005
|-
| style="text-align:left" |Atmosphere
|0.013
|0.001
|-
| style="text-align:left" |Streams & rivers
|0.0017
|0.0001
|-
| style="text-align:left" |Biosphere
|0.0006
|0.00004
|}
In the context of the water cycle, a [[Reservoir (water)|reservoir]] represents the water contained in different steps within the cycle. The largest reservoir is the collection of [[ocean]]s, accounting for 97% of the Earth's water. The next largest quantity (2%) is stored in [[solid]] form in the [[ice cap]]s and [[glacier]]s. The water contained within all living organisms represents the smallest reservoir.
The volume of water in the [[fresh water]] reservoirs, particularly those that are available for human use, are important [[water resources]].<ref>Environmental Literacy Council. [http://www.enviroliteracy.org/article.php/704.php Water Cycle.] Retrieved on 2006-10-24.</ref>
{{clear}}
==Residence times==
{| class="wikitable" style="margin-left:1em; text-align:center" align="right"
|+ '''Average reservoir residence times'''<ref name="PHYS"/>
! style="text-align:left" | Reservoir || Average residence time
|-
| style="text-align:left" | Oceans
|3,200 years
|-
| style="text-align:left" | Glaciers
|20 to 100 years
|-
| style="text-align:left" | Seasonal snow cover
|2 to 6 months
|-
| style="text-align:left" | Soil moisture
|1 to 2 months
|-
| style="text-align:left" | Groundwater: shallow
|100 to 200 years
|-
| style="text-align:left" | Groundwater: deep
|10,000 years
|-
| style="text-align:left" | Lakes (see [[lake retention time]])
|50 to 100 years
|-
| style="text-align:left" | Rivers
|2 to 6 months
|-
| style="text-align:left" | 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.
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.
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).
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]].
{{clear}}
==Changes over time==
The water cycle describes the processes that drive the movement of water throughout the [[hydrosphere]]. However, much more water is "in storage" for long periods of time than is actually moving through the cycle. The storehouses for the vast majority of all water on Earth are the oceans. It is estimated that of the 332,500,000 cubic miles (mi<sup>3</sup>) (1,386,000,000 km<sup>3</sup>) of the world's water supply, about 321,000,000 mi<sup>3</sup> (1,338,000,000 km<sup>3</sup>) is stored in oceans,or about 95%. It is also estimated that the oceans supply about 90 percent of the evaporated water that goes into the water cycle.<ref name=USGS>http://ga.water.usgs.gov/edu/watercycleoceans.html USGS, ''The Water Cycle: Water Storage in Oceans'' - Retrieved on 2008-05-14</ref>
During colder climatic periods more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age glaciers covered almost one-third of Earth's land mass, with the result being that the oceans were about 400 feet (122 meters) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 18 feet (5.5. meters) higher than they are now. About three million years ago the oceans could have been up to 165 feet (50 meters) higher.<ref name=USGS/>
The scientific consensus expressed in the 2007 [[Intergovernmental Panel on Climate Change]] (IPCC) Summary for Policymakers<ref>Intergovernmental Panel on Climate Change. [http://www.ipcc.ch/SPM2feb07.pdf Climate Change 2007: The Physical Science Basis, WG1 Summary for Policymakers]</ref> is for the water cycle to continue to intensify throughout the 21st century, though this does not mean that precipitation will increase in all regions. In subtropical land areas — places that are already relatively dry — precipitation is projected to decrease during the 21st century, increasing the probability of [[drought]]. The drying is projected to be strongest near the poleward margins of the [[subtropics]] (for example, the [[Mediterranean Basin]], [[South Africa]], southern [[Australia]], and the [[Southwestern United States]]). Annual precipitation amounts are expected to increase in near-equatorial regions that tend to be wet in the present climate, and also at high latitudes. These large-scale patterns are present in nearly all of the [[climate model]] simulations conducted at several international research centers as part of the 4th Assessment of the IPCC.
[[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. [[Retreat of glaciers since 1850|Glacial retreat since 1850]] has been extensive.<ref>U.S. Geologic Survey. [http://nrmsc.usgs.gov/research/glacier_retreat.htm GLACIER RETREAT IN GLACIER NATIONAL PARK, MONTANA.] Retrieved on 2006-10-24.</ref>
Human activities that alter the water cycle include:
* [[agriculture]]
* alteration of the chemical composition of the atmosphere
* construction of [[dam]]s
* [[deforestation]] and [[afforestation]]
* removal of groundwater from [[water well|wells]]
* [[water abstraction]] from rivers
* [[urbanization]]
==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 [[Celsius|°C]], and a warmer planet.<ref>Science at NASA. [http://science.hq.nasa.gov/oceans/system/water.html NASA Oceanography: The Water Cycle.] Retrieved on 2006-10-24.</ref>
==Effects on biogeochemical cycling==
While the water cycle is itself a [[biogeochemical cycle]],<ref>The Environmental Literacy Council. [http://www.enviroliteracy.org/subcategory.php/198.html Biogeochemical Cycles.] Retrieved on 2006-10-24.</ref> 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 [[erosion|eroded]] [[sediment]] and [[phosphorus]]<ref>The Environmental Literacy Council. [http://www.enviroliteracy.org/article.php/480.html Phosphorus Cycle.] Retrieved on 2006-10-24.</ref> from land to [[waterbody|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 [[field (agriculture)|agricultural fields]] in [[fertilizer]]s, and then transported overland and down rivers. Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to waterbodies.<ref>Ohio State University Extension Fact Sheet. [http://ohioline.osu.edu/aex-fact/0463.html Nitrogen and the Hydrologic Cycle.] Retrieved on 2006-10-24.</ref> The [[dead zone (ecology)|dead zone]] at the outlet of the [[Mississippi River]] is a consequence of [[nitrate]]s 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.<ref>NASA's Earth Observatory. [http://earthobservatory.nasa.gov/Library/CarbonCycle/ The Carbon Cycle.] Retrieved on 2006-10-24.</ref>
==See also==
{{portalpar|Water|Drinking water.jpg}}
* [[Flood]]
* [[Drought]]
==References==
<div class="references-small">
<references/>
</div>
==External links==
* [http://ga.water.usgs.gov/edu/watercycle.html The Water Cycle], United States Geological Survey
* [http://www.planetguide.net/book/chapter_2/water_cycle.html The water cycle], from ''Dr. Art's Guide to the Planet''.
* [http://managingwholes.com/flash/wcSlideshow.htm Water cycle slideshow], 1 Mb Flash multilingual animation highlighting the often-overlooked evaporation from bare soil, from managingwholes.com.
* [http://www.gfdl.noaa.gov/research/climate/highlights/index.html#precip Will the wet get wetter and the dry drier?] - Climate research summary from [[NOAA]] [[Geophysical Fluid Dynamics Laboratory]] including text, graphics and animations
{{Biogeochemical cycle}}
[[Category:Hydrology]]
[[Category:Water]]
[[Category:Forms of water]]
[[Category:Soil physics]]
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