View source for Physics:Water cycle

{{Short description|Continuous movement of water on, above and below the surface of the Earth}}
[[File:USGS WaterCycle English ONLINE 20221013.png|thumb|upright=2|Diagram depicting the global water cycle.]]
The '''water cycle''', also known as the '''hydrologic cycle''' or the '''hydrological cycle''', is a [[Biology:Biogeochemical cycle|biogeochemical cycle]] that describes the continuous movement of [[Chemistry:Water|water]] on, above and below the surface of the [[Astronomy:Earth|Earth]]. The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of [[Earth:Ice|ice]], [[Earth:Fresh water|fresh water]], [[Chemistry:Saline water|saline water (salt water)]] and [[Astronomy:Atmosphere|atmospheric water]] is variable depending on a wide range of [[Earth:Climate change|climatic variables]]. The water moves from one reservoir to another, such as from river to [[Earth:Ocean|ocean]], or from the ocean to the atmosphere, by the physical processes of [[Physics:Evaporation|evaporation]], [[Earth:Transpiration|transpiration]], [[Physics:Condensation|condensation]], precipitation, [[Earth:Infiltration (hydrology)|infiltration]], [[Earth:Surface runoff|surface runoff]], and subsurface flow. In doing so, the water goes through different forms: liquid, solid ([[Earth:Ice|ice]]) and [[Physics:Water vapor|vapor]]. The ocean plays a key role in the water cycle as it is the source of 86% of global evaporation.<ref name=":4">{{Cite web |title=Water Cycle {{!}} Science Mission Directorate |url=https://science.nasa.gov/earth-science/oceanography/ocean-earth-system/ocean-water-cycle |url-status=live |archive-url=https://web.archive.org/web/20180115065559/https://science.nasa.gov/earth-science/oceanography/ocean-earth-system/ocean-water-cycle |archive-date=2018-01-15 |access-date=2018-01-15 |website=science.nasa.gov |language=en}}</ref>

The water cycle involves the exchange of energy, which leads to [[Physics:Temperature|temperature]] changes. When water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence [[Earth:Climate|climate]].

The evaporative phase of the cycle purifies water, causing salts and other solids picked up during the cycle to be left behind, and then the condensation phase in the atmosphere replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the Earth, through processes including [[Earth:Erosion|erosion]] and [[Sedimentation|sedimentation]]. The water cycle is also essential for the maintenance of most life and ecosystems on the planet.


== Description ==
[[File:Diagram of the Water Cycle.jpg|thumb|Diagram of the water cycle]]
[[File:Earth's Water Cycle.ogv|thumb|Video of the Earth's water cycle (NASA)<ref>{{Cite web |last=NASA |date=2012-01-12 |title=NASA Viz: The Water Cycle: Following The Water |url=https://svs.gsfc.nasa.gov/vis/a010000/a010800/a010885/ |access-date=2022-09-28 |website=svs.gsfc.nasa.gov |language=en}}</ref>]]

=== Overall process ===
{{Further|Earth:Water distribution on Earth}}
The water cycle is powered from the energy emitted by the sun. This energy heats water in the ocean and seas. Water evaporates as water vapor into the air. Some ice and snow sublimates directly into water vapor. [[Earth:Evapotranspiration|Evapotranspiration]] is water [[Earth:Transpiration|transpired]] from plants and evaporated from the soil. The water molecule {{chem|H|2|O}} has smaller [[Physics:Molecular mass|molecular mass]] than the major components of the atmosphere, nitrogen ({{chem|N|2}}) and oxygen ({{chem|O|2}}) and hence is less dense. Due to the significant difference in density, [[Physics:Buoyancy|buoyancy]] drives humid air higher. As altitude increases, air pressure decreases and the temperature drops (see [[Physics:Gas laws|Gas laws]]). The lower temperature causes water vapor to condense into tiny liquid water droplets which are heavier than the air, and which fall unless supported by an updraft. A huge concentration of these droplets over a large area in the atmosphere becomes visible as [[Earth:Cloud|cloud]], while condensation near ground level is referred to as fog.

[[Earth:Atmospheric circulation|Atmospheric circulation]] moves water vapor around the globe; cloud particles collide, grow, and fall out of the upper atmospheric layers as precipitation. Some precipitation falls as snow, hail, or sleet, and can accumulate in [[Earth:Ice cap|ice cap]]s and [[Earth:Glacier|glacier]]s, which can store frozen water for thousands of years. Most water falls as rain back into the ocean or onto land, where the water flows over the ground as [[Earth:Surface runoff|surface runoff]]. A portion of this runoff enters rivers, with streamflow moving water towards the oceans. Runoff and water emerging from the ground ([[Earth:Groundwater|groundwater]]) may be stored as freshwater in lakes. Not all runoff flows into rivers; much of it soaks into the ground as [[Earth:Infiltration (hydrology)|infiltration]]. Some water infiltrates deep into the ground and replenishes [[Earth:Aquifer|aquifer]]s, which can store 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 groundwater discharge or be taken up by plants and transferred back to the atmosphere as water vapor by [[Earth:Transpiration|transpiration]]. Some groundwater finds openings in the land surface and emerges as freshwater springs. In river valleys and [[Earth:Floodplain|floodplain]]s, there is often continuous water exchange between surface water and ground water in the [[Earth:Hyporheic zone|hyporheic zone]]. Over time, the water returns to the ocean, to continue the water cycle.

The ocean plays a key role in the water cycle. The ocean holds "97% of the total water on the planet; 78% of global precipitation occurs over the ocean, and it is the source of 86% of global evaporation".<ref name=":4" />

[[File:HydrologicalCycle1.png|thumb|upright=1.8| {{center|Processes leading to movements and phase changes in water}}]]
Important physical processes within the water cycle include the following (in alphabetical order):
* [[Advection]]: The movement of water through the atmosphere.<ref>{{Cite web|url=https://nsidc.org/cryosphere/glossary/term/advection|title=advection |website=National Snow and Ice Data Center |access-date=2018-01-15|url-status=live|archive-url=https://web.archive.org/web/20180116135136/https://nsidc.org/cryosphere/glossary/term/advection|archive-date=2018-01-16}}</ref> Without advection, water that evaporated over the oceans could not precipitate over land. [[Earth:Atmospheric river|Atmospheric river]]s that move large volumes of water vapor over long distances are an example of advection.<ref>{{cite web |title=Atmospheric River Information Page |url=https://www.esrl.noaa.gov/psd/arportal/about/ |website=NOAA Earth System Research Laboratory}}</ref>
* [[Physics:Condensation|Condensation]]: The transformation of water vapor to liquid water droplets in the air, creating [[Earth:Cloud|cloud]]s and fog.<ref>{{Cite web|url=https://nsidc.org/cryosphere/glossary/term/condensation|title=condensation |website=National Snow and Ice Data Center |access-date=2018-01-15|url-status=live|archive-url=https://web.archive.org/web/20180116135103/https://nsidc.org/cryosphere/glossary/term/condensation|archive-date=2018-01-16}}</ref>
* [[Physics:Evaporation|Evaporation]]: The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.<ref>{{Cite web |url=https://nsidc.org/cryosphere/glossary/term/evaporation |title=evaporation |website=National Snow and Ice Data Center |access-date=2018-01-15 |url-status=live |archive-url=https://web.archive.org/web/20180116135125/https://nsidc.org/cryosphere/glossary/term/evaporation |archive-date=2018-01-16}}</ref> The source of energy for evaporation is primarily [[Physics:Solar radiation|solar radiation]]. Evaporation often implicitly includes [[Earth:Transpiration|transpiration]] from [[Biology:Plant|plant]]s, though together they are specifically referred to as [[Earth:Evapotranspiration|evapotranspiration]]. Total annual evapotranspiration amounts to approximately {{convert|505000|km3|cumi|abbr=on}} of water, {{convert|434000|km3|cumi|abbr=on}} of which evaporates from the oceans.<ref name="The Water Cycle" /> 86% of global evaporation occurs over the ocean.<ref name=":2" />
* [[Earth:Infiltration (hydrology)|Infiltration]]: The flow of water from the ground surface into the ground. Once infiltrated, the water becomes [[Chemistry:Soil moisture|soil moisture]] or groundwater.<ref>{{cite web |website=Northwest River Forecast Center |url=http://www.nwrfc.noaa.gov/info/water_cycle/hydrology.cgi |title=Hydrologic Cycle |publisher=NOAA |url-status=live |archive-url=https://web.archive.org/web/20060427170539/http://www.nwrfc.noaa.gov/info/water_cycle/hydrology.cgi |archive-date=2006-04-27 |access-date=2006-10-24}}</ref> A recent global study using water stable isotopes, however, shows that not all soil moisture is equally available for [[Earth:Groundwater recharge|groundwater recharge]] or for plant transpiration.<ref>{{cite journal |last1=Evaristo |first1=Jaivime |last2=Jasechko |first2=Scott |last3=McDonnell |first3=Jeffrey J. |title=Global separation of plant transpiration from groundwater and streamflow |journal=Nature |date=September 2015 |volume=525 |issue=7567 |pages=91–94 |doi=10.1038/nature14983 |pmid=26333467 |s2cid=4467297 |bibcode=2015Natur.525...91E }}</ref>
* [[Percolation]]: Water flows vertically through the soil and rocks under the influence of [[Physics:Gravity|gravity]].
* Precipitation: Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain, but also includes [[Physics:Snow|snow]], [[Earth:Hail|hail]], fog drip, graupel, and sleet.<ref>{{Cite web |url=https://nsidc.org/cryosphere/glossary/term/precipitation |title=precipitation |website=National Snow and Ice Data Center |access-date=2018-01-15 |url-status=live |archive-url=https://web.archive.org/web/20180116135147/https://nsidc.org/cryosphere/glossary/term/precipitation |archive-date=2018-01-16}}</ref> Approximately {{convert|505000|km3|cumi|abbr=on}} of water falls as precipitation each year, {{convert|398000|km3|cumi|abbr=on}} of it over the oceans.<ref name="The Water Cycle">{{cite web |website=Dr. Art's Guide to Planet Earth |url=http://www.planetguide.net/book/chapter_2/water_cycle.html |title=The Water Cycle |url-status=usurped |archive-url=https://web.archive.org/web/20111226143942/http://www.planetguide.net/book/chapter_2/water_cycle.html |archive-date=2011-12-26 |access-date=2006-10-24}}</ref><ref name="snow" /> The rain on land contains {{convert|107000|km3|cumi|abbr=on}} of water per year and a snowing only {{convert|1000|km3|cumi|abbr=on}}.<ref name="snow">{{Cite web |url=http://www3.geosc.psu.edu/~dmb53/DaveSTELLA/Water/global%20water/global_water.htm |title=Estimated Flows of Water in the Global Water Cycle |website=www3.geosc.psu.edu |access-date=2018-01-15 |url-status=live |archive-url=https://web.archive.org/web/20171107132734/http://www3.geosc.psu.edu/~dmb53/DaveSTELLA/Water/global%20water/global_water.htm |archive-date=2017-11-07}}</ref> 78% of global precipitation occurs over the ocean.<ref name=":2">{{Cite web|url=https://science.nasa.gov/earth-science/oceanography/physical-ocean/salinity/|title=Salinity {{!}} Science Mission Directorate|website=science.nasa.gov|language=en|access-date=2018-01-15 |url-status=live |archive-url=https://web.archive.org/web/20180115065323/https://science.nasa.gov/earth-science/oceanography/physical-ocean/salinity |archive-date=2018-01-15}}</ref>
* [[Earth:Runoff (hydrology)|Runoff]]: 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 seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.
* [[Earth:Subsurface flow|Subsurface flow]]: The flow of water underground, in the [[Earth:Vadose zone|vadose zone]] and aquifers. Subsurface water may return to the surface (e.g. 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 [[Physics:Gravity|gravity]] or gravity induced pressures. Groundwater tends to move slowly and is replenished slowly, so it can remain in aquifers for thousands of years.
* [[Earth:Transpiration|Transpiration]]: The release of water vapor from plants and soil into the air.

=== Residence times ===
{| class="wikitable" style="margin-left:1em; text-align:center" align="right"
|+ '''Average reservoir residence times'''<ref name="PHYS">{{Cite book |website=PhysicalGeography.net |chapter-url=http://www.physicalgeography.net/fundamentals/8b.html |chapter=Chapter 8: Introduction to the Hydrosphere |url-status=live |archive-url=https://web.archive.org/web/20160126072955/http://www.physicalgeography.net/fundamentals/8b.html |archive-date=2016-01-26 |access-date=2006-10-24|title=8(b) the Hydrologic Cycle }}</ref>
! style="text-align:left" | Reservoir || Average residence time
|-
| style="text-align:left" | Antarctica
| 20,000 years
|-
| 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 [[Earth:Lake retention time|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 adjacent table''). It is a measure of the average age of the water in that reservoir.

Groundwater can spend over 10,000 years beneath Earth's surface before leaving. Particularly old groundwater is called [[Earth:Fossil water|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, the residence time in the atmosphere is about 9 days before condensing and falling to the Earth as precipitation.

The major ice sheets – [[Earth:Antarctica|Antarctica]] and [[Place:Greenland|Greenland]] – store ice for very long periods. Ice from Antarctica has been reliably dated to 800,000 years before present, though the average residence time is shorter.<ref>{{cite journal |last1=Jouzel |first1=J. |last2=Masson-Delmotte |first2=V. |last3=Cattani |first3=O. |last4=Dreyfus |first4=G. |last5=Falourd |first5=S. |last6=Hoffmann |first6=G. |last7=Minster |first7=B. |last8=Nouet |first8=J. |last9=Barnola |first9=J. M. |last10=Chappellaz |first10=J. |last11=Fischer |first11=H. |last12=Gallet |first12=J. C. |last13=Johnsen |first13=S. |last14=Leuenberger |first14=M. |last15=Loulergue |first15=L. |last16=Luethi |first16=D. |last17=Oerter |first17=H. |last18=Parrenin |first18=F. |last19=Raisbeck |first19=G. |last20=Raynaud |first20=D. |last21=Schilt |first21=A. |last22=Schwander |first22=J. |last23=Selmo |first23=E. |last24=Souchez |first24=R. |last25=Spahni |first25=R. |last26=Stauffer |first26=B. |last27=Steffensen |first27=J. P. |last28=Stenni |first28=B. |last29=Stocker |first29=T. F. |last30=Tison |first30=J. L. |last31=Werner |first31=M. |last32=Wolff |first32=E. W. |title=Orbital and Millennial Antarctic Climate Variability over the Past 800,000 Years |journal=Science |date=10 August 2007 |volume=317 |issue=5839 |pages=793–796 |doi=10.1126/science.1141038 |pmid=17615306 |bibcode=2007Sci...317..793J |s2cid=30125808 |url=http://epic.awi.de/16356/1/Fis2007b.pdf }}</ref>

In hydrology, residence times can be estimated in two ways.{{Citation needed|date=April 2022}} The more common method relies on the principle of [[Physics:Conservation of mass|conservation of mass]] ([[Earth:Water balance|water balance]]) 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, which is gaining in popularity for dating groundwater, is the use of [[Physics:Isotope|isotopic]] techniques. This is done in the subfield of [[Earth:Isotope hydrology|isotope hydrology]].

=== Water in storage ===
{{Further|Earth:Water resources|Earth:Water distribution on Earth}}
[[File:HumanIntegratedWaterCycle (2).jpg|thumb|upright=2|Water cycle showing human influences and major pools (storages) and fluxes.<ref>{{Cite journal |last1=Abbott |first1=Benjamin W. |last2=Bishop |first2=Kevin |last3=Zarnetske |first3=Jay P. |last4=Minaudo |first4=Camille |last5=Chapin |first5=F. S. |last6=Krause |first6=Stefan |last7=Hannah |first7=David M. |last8=Conner |first8=Lafe |last9=Ellison |first9=David |last10=Godsey |first10=Sarah E. |last11=Plont |first11=Stephen |last12=Marçais |first12=Jean |last13=Kolbe |first13=Tamara |last14=Huebner |first14=Amanda |last15=Frei |first15=Rebecca J. |date=2019 |title=Human domination of the global water cycle absent from depictions and perceptions |url=http://pure-oai.bham.ac.uk/ws/files/80318786/27364_2_merged_1554614894.pdf |journal=Nature Geoscience |language=en |volume=12 |issue=7 |pages=533–540 |bibcode=2019NatGe..12..533A |doi=10.1038/s41561-019-0374-y |issn=1752-0894 |s2cid=195214876}}</ref>]]
The water cycle describes the processes that drive the movement of water throughout the [[Earth:Hydrosphere|hydrosphere]]. However, much more water is "in storage" (or in "pools") 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 1,386,000,000&nbsp;km<sup>3</sup> of the world's water supply, about 1,338,000,000&nbsp;km<sup>3</sup> is stored in oceans, or about 97%. It is also estimated that the oceans supply about 90% of the evaporated water that goes into the water cycle.<ref name=":3">{{Cite web |title=The Water Cycle summary |url=https://water.usgs.gov/edu/watercyclesummary.html |url-status=live |archive-url=https://web.archive.org/web/20180116135448/https://water.usgs.gov/edu/watercyclesummary.html |archive-date=2018-01-16 |access-date=2018-01-15 |website=USGS Water Science School}}</ref> The Earth's ice caps, glaciers, and permanent snowpack stores another 24,064,000&nbsp;km<sup>3</sup> accounting for only 1.7% of the planet's total water volume. However, this quantity of water is 68.7% of all freshwater on the planet.<ref>{{cite web |last1=Water Science School |title=Ice, Snow, and Glaciers and the Water Cycle |url=https://www.usgs.gov/special-topics/water-science-school/science/ice-snow-and-glaciers-and-water-cycle#:~:text=Ice%20caps%20and%20global%20water%20distribution&text=As%20these%20charts%20and%20the,in%20ice%20caps%20and%20glaciers. |website=USGS |publisher=US Department of the Interior |access-date=October 17, 2022}}</ref>

==Changes caused by humans==
[[File:20211109_Frequency_of_extreme_weather_for_different_degrees_of_global_warming_-_bar_chart_IPCC_AR6_WG1_SPM.svg|thumb|[[Earth:Extreme weather|Extreme weather]] (heavy rains, droughts, heat waves) is one consequence of a changing water cycle due to global warming. These events will be progressively more common as the Earth warms more and more.<ref>IPCC, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_SPM.pdf Summary for Policymakers]. In: [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 3−32, doi:10.1017/9781009157896.001.</ref>{{rp|Figure SPM.6}}]]
[[File:Soil_moisture_and_climate_change.svg|thumb|Predicted changes in average soil moisture for a scenario of 2°C global warming. This can disrupt agriculture and ecosystems. A reduction in soil moisture by one [[Standard deviation|standard deviation]] means that average soil moisture will approximately match the ninth driest year between 1850 and 1900 at that location.]]

=== Water cycle intensification due to climate change ===

Since the middle of the 20th century, human-caused [[Earth:Climate change|climate change]] has resulted in observable changes in the global water cycle.<ref name=":0">Arias, P.A., N. Bellouin, E. Coppola, R.G. Jones, G. Krinner, J. Marotzke, V. Naik, M.D. Palmer, G.-K. Plattner, J. Rogelj, M. Rojas, J. Sillmann, T. Storelvmo, P.W. Thorne, B. Trewin, K. Achuta Rao, B. Adhikary, R.P. Allan, K. Armour, G. Bala, R. Barimalala, S. Berger, J.G. Canadell, C. Cassou, A. Cherchi, W. Collins, W.D. Collins, S.L. Connors, S. Corti, F. Cruz, F.J. Dentener, C. Dereczynski, A. Di Luca, A. Diongue Niang, F.J. Doblas-Reyes, A. Dosio, H. Douville, F. Engelbrecht, V.  Eyring, E. Fischer, P. Forster, B. Fox-Kemper, J.S. Fuglestvedt, J.C. Fyfe, et al., 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_TS.pdf Technical Summary]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 33−144. doi:10.1017/9781009157896.002.</ref>{{rp|85}} The [[Earth:IPCC Sixth Assessment Report|IPCC Sixth Assessment Report]] in 2021 predicted that these changes will continue to grow significantly at the global and regional level.<ref name=":0" />{{rp|85}} These findings are a continuation of scientific consensus expressed in the [[Earth:IPCC Fifth Assessment Report|IPCC Fifth Assessment Report]] from 2007 and other special reports by the [[Organization:Intergovernmental Panel on Climate Change|Intergovernmental Panel on Climate Change]] which had already stated that the water cycle will continue to intensify throughout the 21st century.<ref>{{Cite web |last=Alley |first=Richard |display-authors=0 |date=February 2007 |title=Climate Change 2007: The Physical Science Basis |url=http://www.ipcc.ch:80/SPM2feb07.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070203164304/http://www.ipcc.ch/SPM2feb07.pdf |archive-date=February 3, 2007 |publisher=International Panel on Climate Change}}</ref>

{{excerpt|Effects of climate change on the water cycle|paragraphs=1-3|file=no}}

=== Changes due to other human activities ===
[[Image:Natural & impervious cover diagrams EPA.jpg|thumb|400px|Relationship between impervious surfaces and surface runoff]]
Human activities, other than those that lead to global warming from [[Earth:Greenhouse gas emissions|greenhouse gas emissions]], can also alter the water cycle. The [[Earth:IPCC Sixth Assessment Report|IPCC Sixth Assessment Report]] stated that there is "abundant evidence that [[Engineering:Land development|changes in land use]] and [[Earth:Land cover|land cover]] alter the water cycle globally, regionally and locally, by changing precipitation, evaporation, flooding, groundwater, and the availability of freshwater for a variety of uses".<ref name=":22">Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D.  Jiang, A.  Khan, W.  Pokam Mba, D.  Rosenfeld, J. Tierney, and O.  Zolina, 2021: [https://www.ipcc.ch/report/ar6/wg1/downloads/report/IPCC_AR6_WGI_Chapter08.pdf Water Cycle Changes]. In [https://www.ipcc.ch/report/ar6/wg1/ Climate Change 2021: The Physical Science Basis. Contribution of Working Group I  to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, US, pp. 1055–1210, doi:10.1017/9781009157896.010.</ref>{{rp|1153}}

Examples for such land use changes are converting fields to urban areas or clearing forests. Such changes can affect the ability of soils to soak up surface water. Deforestation has local as well as regional effects. For example it reduces soil moisture, evaporation and rainfall at the local level. Furthermore, deforestation causes regional temperature changes that can affect rainfall patterns.<ref name=":22" />{{rp|1153}} 

Aquifer [[Earth:Drawdown (hydrology)|drawdown]] or [[Earth:Overdrafting|overdrafting]] and the pumping of fossil water increase the total amount of water in the hydrosphere. This is because the water that was originally in the ground has now become available for evaporation as it is now in contact with the atmosphere.<ref name=":22" />{{rp|1153}}

== Related processes ==

=== Biogeochemical cycling ===
While the water cycle is itself a [[Biology:Biogeochemical cycle|biogeochemical cycle]], flow of water over and beneath the Earth is a key component of the cycling of other biogeochemicals.<ref>{{cite web |publisher=The Environmental Literacy Council |url=http://www.enviroliteracy.org/subcategory.php/198.html |title=Biogeochemical Cycles |url-status=live |archive-url=https://web.archive.org/web/20150430133927/http://enviroliteracy.org/subcategory.php/198.html |archive-date=2015-04-30 |access-date=2006-10-24}}</ref> Runoff is responsible for almost all of the transport of eroded [[Earth:Sediment|sediment]] and [[Chemistry:Phosphorus|phosphorus]] from land to waterbodies.<ref>{{Cite web |url=https://enviroliteracy.org/air-climate-weather/biogeochemical-cycles/phosphorus-cycle/ |title=Phosphorus Cycle |publisher=The Environmental Literacy Council |access-date=2018-01-15 |url-status=live |archive-url=https://web.archive.org/web/20160820012731/http://enviroliteracy.org/air-climate-weather/biogeochemical-cycles/phosphorus-cycle/|archive-date=2016-08-20}}</ref> The [[Earth:Salinity|salinity]] of the oceans is derived from [[Earth:Erosion|erosion]] and transport of dissolved salts from the land. Cultural [[Earth:Eutrophication|eutrophication]] of lakes is primarily due to phosphorus, applied in excess to agricultural fields in [[Chemistry:Fertilizer|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>{{cite web |publisher=Ohio State University |website=Extension Fact Sheet |url=http://ohioline.osu.edu/aex-fact/0463.html |title=Nitrogen and the Hydrologic Cycle |url-status=dead |archive-url=https://web.archive.org/web/20060901071850/http://ohioline.osu.edu/aex-fact/0463.html |archive-date=2006-09-01 |access-date=2006-10-24}}</ref> The [[Earth:Dead zone (ecology)|dead zone]] at the outlet of the Mississippi River is a consequence of [[Chemistry:Nitrate|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 [[Earth:Carbon cycle|carbon cycle]], again through the transport of eroded rock and soil.<ref>{{cite web |publisher=NASA |website=Earth Observatory |title=The Carbon Cycle |url=http://earthobservatory.nasa.gov/Library/CarbonCycle/ |url-status=dead |archive-url=https://web.archive.org/web/20060928223836/http://earthobservatory.nasa.gov/Library/CarbonCycle/ |archive-date=2006-09-28 |access-date=2006-10-24|date=2011-06-16 }}</ref>

=== Slow loss over geologic time ===
{{Main|Earth:Atmospheric escape}}

The hydrodynamic wind within the upper portion of a planet's atmosphere allows light chemical elements such as [[Chemistry:Hydrogen|Hydrogen]] to move up to the [[Earth:Exobase|exobase]], the lower limit of the [[Earth:Exosphere|exosphere]], where the gases can then reach [[Astronomy:Escape velocity|escape velocity]], entering [[Astronomy:Outer space|outer space]] without impacting other particles of gas. This type of gas loss from a planet into space is known as planetary wind.<ref>{{cite web|url=http://www.astronomynotes.com/solarsys/s3.htm |author=Nick Strobel |date=June 12, 2010 |archive-date=September 17, 2010 |access-date=September 28, 2010 |archive-url=https://web.archive.org/web/20100917233236/http://astronomynotes.com/solarsys/s3.htm |title=Planetary Science |url-status=dead }}</ref> Planets with hot lower atmospheres could result in humid upper atmospheres that accelerate the loss of hydrogen.<ref name="Dvorak">{{cite book|author=Rudolf Dvořák|year=2007|url=https://books.google.com/books?id=lbIlI6gMNAYC&q=hydrodynamic+wind+planet+atmosphere&pg=PA140|title=Extrasolar Planets|publisher=Wiley-VCH|pages=139–40|isbn=978-3-527-40671-5|access-date=2009-05-05}}{{Dead link|date=August 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

==Historical interpretations==

===Floating land mass===
In ancient times, it was widely thought that the land mass floated on a body of water, and that most of the water in rivers has its origin under the earth. Examples of this belief can be found in the works of [[Biography:Homer|Homer]] ({{circa|800 BCE}}).

=== Hebrew Bible ===
In the [[History:Ancient Near East|ancient Near East]], Hebrew scholars observed that even though the rivers ran into the sea, the sea never became full. Some scholars conclude that the water cycle was described completely during this time in this passage: "The wind goeth toward the south, and turneth about unto the north; it whirleth about continually, and the wind returneth again according to its circuits. All the rivers run into the sea, yet the sea is not full; unto the place from whence the rivers come, thither they return again" ([https://en.wikisource.org/wiki/Bible_(King_James)/Ecclesiastes Ecclesiastes 1:6-7]).<ref name="Morris1988p.15">{{Cite book |last=Morris |first=Henry M. |year=1988|title=Science and the Bible |edition=Trinity Broadcasting Network |location=Chicago, IL |publisher=Moody Press |page=15}}</ref> Scholars are not in agreement as to the date of Ecclesiastes, though most scholars point to a date during the time of King Solomon, son of David and Bathsheba, "three thousand years ago,<ref name="Morris1988p.15"/> there is some agreement that the time period is 962–922 BCE.<ref name=":1"/> Furthermore, it was also observed that when the clouds were full, they emptied rain on the earth ([https://en.wikisource.org/wiki/Bible_(King_James)/Ecclesiastes#Chapter_11 Ecclesiastes 11:3]). In addition, during 793–740 BCE a Hebrew prophet, Amos, stated that water comes from the sea and is poured out on the earth ([https://en.wikisource.org/wiki/Bible_(King_James)/Amos#Chapter_5 Amos 5:8]).<ref>Merrill, Eugene H.; Rooker, Mark F.; Grisanti, Michael A. (2011). The World and the Word. Nashville, TN: B&H Academic. p. 430. {{ISBN|9780805440317}}.</ref>

In the Biblical Book of Job, dated between 7th and 2nd centuries BCE,<ref name=":1">{{Cite book |title=The Oxford Companion to the Bible |last1=Metzger |first1=Bruce M. |last2=Coogan |first2=Michael D. |publisher=Oxford University Press |year=1993 |isbn=978-0195046458 |location=New York, NY |pages=[https://archive.org/details/isbn_9780195046458/page/369 369] |url-access=registration |url=https://archive.org/details/isbn_9780195046458/page/369 }}</ref> there is a description of precipitation in the hydrologic cycle,<ref name="Morris1988p.15"/> "For he maketh small the drops of water: they pour down rain according to the vapour thereof; which the clouds do drop and distil upon man abundantly" ([https://en.wikisource.org/wiki/Bible_(King_James)/Job#Chapter_36 Job 36:27-28]).

===Understanding of precipitation and percolation===
In the Adityahridayam (a devotional hymn to the Sun God) of [[Biology:Ramayana|Ramayana]], a Hindu epic dated to the 4th century BCE, it is mentioned in the 22nd verse that the Sun heats up water and sends it down as rain. By roughly 500 BCE, Greek scholars were speculating that much of the water in rivers can be attributed to rain. The origin of rain was also known by then. These scholars maintained the belief, however, that water rising up through the earth contributed a great deal to rivers. Examples of this thinking included [[Biography:Anaximander|Anaximander]] (570 BCE) (who also speculated about the evolution of land animals from fish<ref>{{cite web |url=http://palaeos.com/science/timeline/pre19C.html |title=Palaeos: History of Evolution and Paleontology in science, philosophy, religion, and popular culture : Pre 19th Century |first=M.Alan |last=Kazlev |url-status=live |archive-url=https://web.archive.org/web/20140302123918/http://palaeos.com/science/timeline/pre19C.html |archive-date=2014-03-02}}</ref>) and [[Biography:Xenophanes|Xenophanes of Colophon]] (530 BCE).<ref>{{cite web |author=James H. Lesher |url=http://philosophy.unc.edu/people/faculty/james-lesher/xenophanes%20scepticism.pdf |title=Xenophanes' Scepticism |access-date=2014-02-26 |url-status=dead |archive-url=https://web.archive.org/web/20130728071132/http://philosophy.unc.edu/people/faculty/james-lesher/xenophanes%20scepticism.pdf |archive-date=2013-07-28 |pages=9–10}}</ref> Chinese scholars such as Chi Ni Tzu (320 BCE) and Lu Shih Ch'un Ch'iu (239 BCE) had similar thoughts.<ref>{{cite book |url=https://books.google.com/books?id=JI65-MygMm0C&pg=PA4|title=The Basis of Civilization – water Science? |date=2004 |publisher=International Association of Hydrological Science |via=Google Books|isbn=9781901502572 }}</ref>

The idea that the water cycle is a closed cycle can be found in the works of [[Biography:Anaxagoras|Anaxagoras of Clazomenae]] (460 BCE) and [[Biography:Diogenes of Apollonia|Diogenes of Apollonia]] (460 BCE). Both [[Biography:Plato|Plato]] (390 BCE) and [[Biography:Aristotle|Aristotle]] (350 BCE) speculated about percolation as part of the water cycle. Aristotle correctly hypothesized that the sun played a role in the Earth's hydraulic cycle in his book ''Meteorology,'' writing "By it [the sun's] agency the finest and sweetest water is everyday carried up and is dissolved into vapor and rises to the upper regions, where it is condensed again by the cold and so returns to the earth.", and believed that clouds were composed of cooled and condensed water vapor.<ref>{{Cite book|last=Roscoe|first=Kelly|url=https://books.google.com/books?id=i3ZhDwAAQBAJ&pg=PT71|title=Aristotle: The Father of Logic|publisher=Rosen Publishing Group|year=2015|isbn=9781499461275|pages=70}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=UDiJbEIO-RsC&pg=PA7|title=Precipitation: Theory, Measurement and Distributio|publisher=Cambridge University Press|year=2006|isbn=9781139460019|pages=7}}</ref>

Up to the time of the Renaissance, it was wrongly assumed that precipitation alone was insufficient to feed rivers, for a complete water cycle, and that underground water pushing upwards from the oceans were the main contributors to river water. Bartholomew of England held this view (1240 CE), as did Leonardo da Vinci (1500 CE) and [[Biography:Athanasius Kircher|Athanasius Kircher]] (1644 CE).

=== Discovery of the correct theory ===
The first published thinker to assert that rainfall alone was sufficient for the maintenance of rivers was Bernard Palissy (1580 CE), who is often credited as the discoverer of the modern theory of the water cycle. Palissy's theories were not tested scientifically until 1674, in a study commonly attributed to Pierre Perrault. Even then, these beliefs were not accepted in mainstream science until the early nineteenth century.<ref>{{cite conference |url=http://hydrologie.org/ACT/OH2/actes/03_dooge.pdf |author=James C.I. Dodge |conference=International Symposium {{chem|O|H|2}} 'Origins and History of Hydrology', Dijon, May 9–11, 2001 |title=Concepts of the hydrological Cycle. Ancient and modern |access-date=2014-02-26 |url-status=live |archive-url=https://web.archive.org/web/20141011185410/http://hydrologie.org/ACT/OH2/actes/03_dooge.pdf |archive-date=2014-10-11}}</ref>

==See also==
* {{annotated link|Biology:Bioprecipitation}}
* {{annotated link|Physics:Cryosphere}}
* {{annotated link|Earth:Deep water cycle}}
* {{annotated link|Earth:Ecohydrology}}
* {{annotated link|Physics:Global meteoric water line}}
* {{annotated link|Earth:Moisture advection}}
* {{annotated link|Earth:Moisture recycling}}
* {{annotated link|Earth:Planetary boundaries}}
* {{annotated link|Earth:Water resources}}
* [[Earth:Biotic pump|Biotic pump]]

==References==
{{Reflist}}

==External links==
* [http://water.usgs.gov/edu/watercycle.html The Water Cycle], United States Geological Survey
* [http://water.usgs.gov/edu/watercycle-kids.html The Water Cycle for Kids], United States Geological Survey
* [https://svs.gsfc.nasa.gov/vis/a010000/a010800/a010885/ The Water Cycle: Following The Water] (NASA Visualization Explorer with videos)

{{Water}}
{{Biogeochemical cycle}}

{{DEFAULTSORT:Water Cycle}}
[[Category:Biogeochemical cycle]]
[[Category:Forms of water]]
[[Category:Hydrology]]
[[Category:Soil physics]]
[[Category:Water]]
[[Category:Oceanography]]

{{Sourceattribution|Water cycle}}