:Water cycle

{{biogeochemical cycle sidebar|water}}

File:Earth's Water Cycle.ogv

{{Further|Water distribution on Earth}}

The water cycle is powered by the energy emitted from the sun. There are several ways in which this is accomplished, one of the first ways is through evaporation where the energy from the sun heats the water in oceans, lakes, streams, rivers, seas, ponds, etc. and that water goes through a phase change to become a gas (water vapor) that goes up into the atmosphere. Two other ways that water gets into the atmosphere is through snow and ice sublimating into water vapor and through evapotranspiration which is water transpired from plants and evaporated from the soil.

Clouds form because water molecules have a smaller molecular mass than the major gas components of the atmosphere (oxygen, O2; and nitrogen, N2); this smaller molecular mass leads to water having a lower density which drives the water molecules higher up in the atmosphere due to buoyancy. However, as altitude increases, air pressure decreases which causes a drop in temperature. The lower temperature forces the water vapor to go through another phase change, this time it forces it to condense into liquid water droplets which are supported by an updraft; if there is enough of these water droplets over a large area, it is considered a cloud. Condensation of the water vapour closer to the ground level is referred to as fog.

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 ice caps and glaciers, 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 surface runoff. A portion of this runoff enters rivers, with streamflow moving water towards the oceans. Runoff and water emerging from the ground (groundwater) may be 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 aquifers, 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 transpiration. Some groundwater finds openings in the land surface and emerges as freshwater springs. In river valleys and floodplains, there is often continuous water exchange between surface water and ground water in the 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".

File:HydrologicalCycle1.png

Important physical processes within the water cycle include (in alphabetical order):

• Advection: The movement of water through the atmosphere.{{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}} Without advection, water that evaporated over the oceans could not precipitate over land. Atmospheric rivers that move large volumes of water vapor over long distances are an example of advection.{{cite web |title=Atmospheric River Information Page |url=https://www.esrl.noaa.gov/psd/arportal/about/ |website=NOAA Earth System Research Laboratory}}
• Condensation: The transformation of water vapor to liquid water droplets in the air, creating clouds and fog.{{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}}
• Evaporation: The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.{{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}} 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. 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. 86% of global evaporation occurs over the ocean.
• Infiltration: The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater.{{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}} A recent global study using water stable isotopes, however, shows that not all soil moisture is equally available for groundwater recharge or for plant transpiration.{{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 }}
• Percolation: Water flows vertically through the soil and rocks under the influence of gravity.
• Precipitation: 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.{{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}} 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.{{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}} 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}}.{{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}} 78% of global precipitation occurs over the ocean.{{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}}
• 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.
• Subsurface flow: The flow of water underground, in the 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 gravity or gravity induced pressures. Groundwater tends to move slowly and is replenished slowly, so it can remain in aquifers for thousands of years.{{Cite web |date=2020-08-25 |title=Groundwater Movement |url=https://wellwater.oregonstate.edu/groundwater/understanding-groundwater/groundwater-movement |access-date=2025-05-01 |website=Well Water Program |language=en}}
• Transpiration: The release of water vapor from plants and soil into the air.

The residence time of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that reservoir (see 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.{{Cite journal |last1=Maxwell |first1=Reed M |last2=Condon |first2=Laura E |last3=Kollet |first3=Stefan J |last4=Maher |first4=Kate |last5=Haggerty |first5=Roy |last6=Forrester |first6=Mary Michael |date=2016-01-28 |title=The imprint of climate and geology on the residence times of groundwater |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2015GL066916 |journal=Geophysical Research Letters |language=en |volume=43 |issue=2 |pages=701–708 |doi=10.1002/2015GL066916 |bibcode=2016GeoRL..43..701M |issn=0094-8276}} 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, the residence time in the atmosphere is about 9 days before condensing and falling to the Earth as precipitation.

The major ice sheets – Antarctica and 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.{{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 }}

In hydrology, residence times can be estimated in two ways.{{Cite web |title=Hydrological Cycle |url=https://www.ldeo.columbia.edu/~martins/climate_water/lectures/hcycle.htm#:~:text=residence%20time:%20Tr%20=%20V/,the%20inflow%20or%20outflow%20rate. |access-date=2025-05-01 |website=www.ldeo.columbia.edu}}{{Cite web |title=Copernicus Meetings - 404 |url=https://meetings.copernicus.org/404.html |access-date=2025-05-01 |website=meetings.copernicus.org}} The more common method relies on the principle of conservation of mass (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 isotopic techniques. This is done in the subfield of isotope hydrology.

{{Further|Water resources|Water distribution on Earth}}

File:HumanIntegratedWaterCycle (2).jpg

The water cycle describes the processes that drive the movement of water throughout the 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 km³ of the world's water supply, about 1,338,000,000 km³ 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.{{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}} The Earth's ice caps, glaciers, and permanent snowpack store another 24,064,000 km³, 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.{{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 |date=8 September 2019 |publisher=US Department of the Interior |access-date=October 17, 2022}}

Image:Natural & impervious cover diagrams EPA.jpg and surface runoff]]

Human activities can alter the water cycle at the local or regional level. This happens due to changes in land use and land cover. Such changes affect "precipitation, evaporation, flooding, groundwater, and the availability of freshwater for a variety of uses".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.{{rp|1153}}

Examples of common land use changes include urbanization, agricultural expansion, and deforestation. These changes can increase soil compaction and impervious surface cover which decrease the infiltration capacity of soils and result in greater surface runoff rates.{{Cite journal |last1=Sun |first1=Di |last2=Yang |first2=Hong |last3=Guan |first3=Dexin |last4=Yang |first4=Ming |last5=Wu |first5=Jiabing |last6=Yuan |first6=Fenghui |last7=Jin |first7=Changjie |last8=Wang |first8=Anzhi |last9=Zhang |first9=Yushu |date=2018-06-01 |title=The effects of land use change on soil infiltration capacity in China: A meta-analysis |url=https://linkinghub.elsevier.com/retrieve/pii/S0048969718301244 |journal=Science of the Total Environment |volume=626 |pages=1394–1401 |doi=10.1016/j.scitotenv.2018.01.104 |pmid=29898546 |bibcode=2018ScTEn.626.1394S |issn=0048-9697|url-access=subscription }} Deforestation has local and regional effects; at the local level it reduces soil moisture, evaporation, rainfall, and snowfall; at the regional level it can cause temperature changes that affect that affect rainfall patterns.{{rp|1153}}

Water management structures such as dams, stormwater drains, and sewage pipes can also alter local hydrologic conditions. Dams can alter natural flow rates, decrease water quality, and lead to a loss of habitat for aquatic species.{{Cite web |last=Fisheries |first=NOAA |date=2024-10-29 |title=How Dams Affect Water and Habitat on the West Coast {{!}} NOAA Fisheries |url=https://www.fisheries.noaa.gov/west-coast/endangered-species-conservation/how-dams-affect-water-and-habitat-west-coast |access-date=2025-05-01 |website=NOAA |language=en}} Stormwater drains function to decrease runoff rates, regulate flow rates, and increase groundwater recharge.{{Cite web |title=Overview of stormwater infiltration - Minnesota Stormwater Manual |url=https://stormwater.pca.state.mn.us/index.php/Overview_of_stormwater_infiltration |access-date=2025-05-01 |website=stormwater.pca.state.mn.us}} Leakage from sewage pipes may artificially contribute to groundwater recharge, resulting in higher stream baseflow conditions and groundwater contamination.{{Cite journal |last1=Fletcher |first1=T. D. |last2=Andrieu |first2=H. |last3=Hamel |first3=P. |date=2013-01-01 |title=Understanding, management and modelling of urban hydrology and its consequences for receiving waters: A state of the art |url=https://linkinghub.elsevier.com/retrieve/pii/S0309170812002412 |journal=Advances in Water Resources |series=35th Year Anniversary Issue |volume=51 |pages=261–279 |doi=10.1016/j.advwatres.2012.09.001 |bibcode=2013AdWR...51..261F |issn=0309-1708|url-access=subscription }} Groundwater depletion, however, remains an ongoing concern as groundwater is being pumped at unsustainable rates to meet municipal, industrial, and agricultural water demands.{{Cite journal |last1=Jasechko |first1=Scott |last2=Seybold |first2=Hansjörg |last3=Perrone |first3=Debra |last4=Fan |first4=Ying |last5=Shamsudduha |first5=Mohammad |last6=Taylor |first6=Richard G. |last7=Fallatah |first7=Othman |last8=Kirchner |first8=James W. |date=January 2024 |title=Rapid groundwater decline and some cases of recovery in aquifers globally |journal=Nature |language=en |volume=625 |issue=7996 |pages=715–721 |doi=10.1038/s41586-023-06879-8 |pmid=38267682 |pmc=10808077 |bibcode=2024Natur.625..715J |issn=1476-4687}}

{{Main|Effects of climate change on the water cycle|Effects of climate change on oceans}}

File:20211109_Frequency_of_extreme_weather_for_different_degrees_of_global_warming_-_bar_chart_IPCC_AR6_WG1_SPM.svg (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.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.{{rp|Figure SPM.6}}]]

File:Soil_moisture_and_climate_change.svg means that average soil moisture will approximately match the ninth driest year between 1850 and 1900 at that location.]]

Since the middle of the 20th century, human-caused climate change has resulted in observable changes in the global water cycle.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.{{rp|85}} The IPCC Sixth Assessment Report in 2021 predicted that these changes will continue to grow significantly at the global and regional level.{{rp|85}} These findings are a continuation of scientific consensus expressed in the IPCC Fifth Assessment Report from 2007 and other special reports by the Intergovernmental Panel on Climate Change which had already stated that the water cycle will continue to intensify throughout the 21st century.

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

While the water cycle is itself a biogeochemical cycle, flow of water over and beneath the Earth is a key component of the cycling of other biogeochemicals.{{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}} Runoff is responsible for almost all of the transport of eroded sediment and phosphorus from land to waterbodies.{{Cite web |url=https://enviroliteracy.org/air-climate-weather/biogeochemical-cycles/phosphorus-cycle/ |title=Phosphorus Cycle |work=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}} 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.{{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}} 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.{{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 }}

{{Main|Atmospheric escape}}

The hydrodynamic wind within the upper portion of a planet's atmosphere allows light chemical elements such as Hydrogen to move up to the exobase, the lower limit of the exosphere, where the gases can then reach escape velocity, entering outer space without impacting other particles of gas. This type of gas loss from a planet into space is known as planetary wind.{{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 }} Planets with hot lower atmospheres could result in humid upper atmospheres that accelerate the loss of hydrogen.{{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 }}

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 Homer ({{circa|800 BCE}}).

In Works and Days (ca. 700 BC), the Greek poet Hesiod outlines the idea of the water cycle: "[Vapour] is drawn from the ever-flowing rivers and is raised high above the earth by windstorm, and sometimes it turns to rain towards evening, and sometimes to wind when Thracian Boreas huddles the thick clouds."{{Cite web |title=HESIOD, WORKS AND DAYS - Theoi Classical Texts Library |url=https://www.theoi.com/Text/HesiodWorksDays.html |access-date=2025-05-01 |website=www.theoi.com}}

In the 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]).{{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}} 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 the Adityahridayam (a devotional hymn to the Sun God) of 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 Anaximander (570 BCE) (who also speculated about the evolution of land animals from fish{{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}}) and Xenophanes of Colophon (530 BCE).{{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}} Warring States period Chinese scholars such as Chi Ni Tzu (320 BCE) and Lu Shih Ch'un Ch'iu (239 BCE) had similar thoughts.{{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 }}

The idea that the water cycle is a closed cycle can be found in the works of Anaxagoras of Clazomenae (460 BCE) and Diogenes of Apollonia (460 BCE). Both Plato (390 BCE) and 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.{{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}}{{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}} Much like the earlier Aristotle, the Eastern Han Chinese scientist Wang Chong (27–100 AD) accurately described the water cycle of Earth in his Lunheng but was dismissed by his contemporaries.Needham, Joseph. (1986a). Science and Civilisation in China: Volume 3; Mathematics and the Sciences of the Heavens and the Earth. Taipei: Caves Books, Ltd, p. 468 {{ISBN|0-521-05801-5}}.

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 Athanasius Kircher (1644 CE).

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.{{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}}

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{{Reflist}}

{{Commons category|Water cycle}}

• [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] {{Webarchive|url=https://web.archive.org/web/20160323125433/https://svs.gsfc.nasa.gov/vis/a010000/a010800/a010885/ |date=2016-03-23 }} (NASA Visualization Explorer with videos)

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