Editing Water cycle (section)

=== Overall process ===
{{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 [[Sublimation (phase transition)|sublimating]] into water vapor and through [[evapotranspiration]] which is water [[Transpiration|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, [[Gas laws|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 [[Condensation|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 (meteorology)|precipitation]]. Some precipitation falls as snow, hail, or sleet, and can accumulate in [[ice cap]]s and [[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 [[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 (hydrology)|infiltration]]. Some water infiltrates deep into the ground and replenishes [[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 [[transpiration]]. Some groundwater finds openings in the land surface and emerges as freshwater springs. In river valleys and [[floodplain]]s, 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".<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 (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. [[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>
* [[Condensation]]: The transformation of water vapor to liquid water droplets in the air, creating [[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>
* [[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 [[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 {{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" />
* [[Infiltration (hydrology)|Infiltration]]: The flow of water from the ground surface into the ground. Once infiltrated, the water becomes [[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 [[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 [[gravity]].
* [[Precipitation (meteorology)|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 [[Ice pellets|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>
* [[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.
* [[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.<ref>{{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}}</ref>
* [[Transpiration]]: The release of water vapor from plants and soil into the air.