# Water cycle The **water cycle** is the continuous movement of [[Water|water]] among the ocean, the [[Atmosphere_of_Earth|atmosphere]], ice, the land surface and the ground. Solar energy and [[Gravity|gravity]] drive it: water evaporates, mostly from the ocean, is carried by the air, falls as rain and snow, and returns to the sea by runoff, rivers and groundwater, or to the air by evaporation and transpiration.[^theis-water] Each year about 449,500 km³ of water evaporate from the ocean and about 116,500 km³ fall on land.[^rodell2016] Read as stocks joined by flows, the cycle gives every reservoir a residence time: about nine days for the atmosphere, about three thousand years for the ocean. It is the central subject of [[Hydrology|hydrology]]. <div class="microsim-player"> <iframe src="https://wikitube-3d-microsims.netlify.app/Water_cycle.html" width="100%" height="620" frameborder="0" loading="lazy" sandbox="allow-scripts allow-same-origin" allow="accelerometer; gyroscope" title="Water cycle — p5.js microsim"></iframe> </div> *Microsim (THY-108): watch water move between reservoirs at the measured rates and time each one — residence time is stock ÷ outflow. ILLUSTRATIVE: boxes are not to scale.* Videos: [16:9](https://wikitube-3d-microsims.netlify.app/media/Water_cycle_16x9.mp4) · [9:16](https://wikitube-3d-microsims.netlify.app/media/Water_cycle_9x16.mp4) ## Description The oceans hold about 97% of all water; ice caps and glaciers are the largest stores of fresh water, and shallow groundwater is the largest store of usable fresh water. Rivers and lakes, though the most heavily used water resources, hold only a tiny share of the world's water.[^theis-water] ### Overall process Water moves from the ocean to the atmosphere by evaporation, forms clouds, and falls as precipitation on both water and land. On land it returns to the ocean by surface runoff, rivers, glaciers and subsurface groundwater flow, or to the atmosphere by evaporation and transpiration. Evaporation is a distillation: it leaves dissolved salts behind, which is why rain is fresh while the ocean, with about 35,000 mg of dissolved ions per litre, is salt.[^theis-water] Global fluxes, in thousands of km³ per year, are about 449.5 for ocean evaporation, 403.5 for precipitation on the ocean, 116.5 for precipitation on land, 70.6 for evapotranspiration from land, and 45.9 for runoff to the ocean, balanced by 45.8 carried from ocean to land as vapour.[^rodell2016] ### Residence times The residence time of water in a reservoir is roughly its stock divided by its outflow. The atmosphere holds about 12,900 km³[^usgs-where] and loses about 520,000 km³ a year to precipitation,[^rodell2016] so its water turns over in about nine days; rivers, holding about 2,120 km³ and discharging about 45,900 km³ a year, turn over in about seventeen days; the ocean, holding about 1,338,000,000 km³ and evaporating about 449,500 km³ a year, keeps its water for about three thousand years.[^usgs-where][^rodell2016] Deep groundwater moving through aquitards can take many thousands of years to travel short distances.[^theis-water] ### Water in storage Of Earth's water, about 96.5% is in the oceans; of the roughly 2.5% that is fresh, more than 68% is locked in ice and glaciers and about 30% is groundwater.[^usgs-where] If all the world's water were shrunk to a gallon, the fresh water would be about a third of a cup and the readily usable fresh water about two tablespoons.[^theis-water] ## Changes caused by humans By 2000, withdrawals from rivers and aquifers had risen to almost 4,000 km³ a year, mostly for irrigation, and people change the cycle further through land cover and infrastructure.[^theis-water] ### Land cover and land use changes Pavement and buildings reduce infiltration and speed runoff to streams, while forests and wetlands slow it; paving a catchment makes its floods larger and quicker.[^usgs-infil][^epa-urban] ### Water cycle intensification due to climate change Global warming over the past century has accelerated the melting of glaciers, depleting one of the cycle's long-term stores.[^theis-water] ## Related processes The water cycle carries heat, sediment and dissolved chemicals with it. ### Biogeochemical cycling Water moving through soil and rock dissolves minerals and carries nutrients such as nitrogen and phosphorus to rivers and the sea, linking the water cycle to the carbon and nutrient cycles.[^theis-water] ## Historical interpretations For centuries the prevailing view held that springs and rivers were fed from underground sources rather than by rain. ### Discovery of the correct theory Measurements in the seventeenth century settled the question: Pierre Perrault showed that the rain and snow falling on the upper Seine basin were about six times the river's flow, more than enough to feed it — the result that founded quantitative [[Hydrology|hydrology]].[^perrault] ## Minnesota *This section is specific to Wikitube.* Minnesota counts 11,842 lakes of ten acres or more and 6,564 natural rivers and streams totaling 69,200 miles, and its water leaves by three routes: north to Hudson Bay, east to the Atlantic through the Great Lakes, and south to the Gulf of Mexico.[^dnr-water][^hmdb-3waters] ## See also - [[Hydrology]] - [[Groundwater]] - [[Infiltration_(hydrology)]] - [[Drainage_basin]] - [[PORTAL_Stock_and_flow|Stock and flow portal]] ## References [^theis-water]: Theis, Tom; Tomkin, Jonathan, eds. (2015). *Sustainability: A Comprehensive Foundation*, ch. 7, module "Water Cycle and Fresh Water Supply." https://open.umn.edu/opentextbooks/textbooks/sustainability-a-comprehensive-foundation [^rodell2016]: Rodell, Matthew; Beaudoing, Hiroko Kato; Koster, Randal; Peters-Lidard, Christa D.; Famiglietti, James S.; Lakshmi, Venkat (2016). "Large-Scale and Global Hydrology" (fig. 92.1, global water fluxes). NASA Technical Reports Server. https://ntrs.nasa.gov/api/citations/20170002756/downloads/20170002756.pdf [^usgs-where]: U.S. Geological Survey, Water Science School. "Where is Earth's Water?" (table after Shiklomanov, in Gleick, ed., *Water in Crisis*, 1993). https://www.usgs.gov/water-science-school/science/where-earths-water [^usgs-infil]: U.S. Geological Survey, Water Science School. "Infiltration and the Water Cycle." https://www.usgs.gov/water-science-school/science/infiltration-and-water-cycle [^epa-urban]: U.S. Environmental Protection Agency, CADDIS. "Urbanization – Hydrology." https://www.epa.gov/caddis/urbanization-hydrology [^perrault]: *Encyclopaedia Britannica*. "Pierre Perrault." https://www.britannica.com/biography/Pierre-Perrault [^dnr-water]: Minnesota Department of Natural Resources. "Lakes, rivers, and wetlands facts." https://www.dnr.state.mn.us/faq/mnfacts/water.html [^hmdb-3waters]: Historical Marker Database. "Hill of Three Waters or the Triple Divide." https://www.hmdb.org/m.asp?m=27715 ### Portal Books - Theis, Tom; Tomkin, Jonathan, eds. (2015). *Sustainability: A Comprehensive Foundation* — [OTL record](https://open.umn.edu/opentextbooks/textbooks/sustainability-a-comprehensive-foundation); on the [[PORTAL_Energy]] shelf. - Murphy, Tom W., Jr. (2021). *Energy and Human Ambitions on a Finite Planet* — [eScholarship](https://escholarship.org/uc/item/9js5291m); on the [[PORTAL_Energy]] shelf. ## External links - [The Water Cycle](https://www.usgs.gov/water-science-school/science/where-earths-water), USGS Water Science School ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Water_cycle) : [Wikitube](https://en.wikitube.io/wiki/Water_cycle) · pinned revision [1368240581](https://en.wikipedia.org/w/index.php?oldid=1368240581) · 2026-09-10 ## Previous hub tags Hubs: `Life_Physics`, `Systems`. Portals: [[PORTAL_Thury_Hydrodynamics_Apex_Spine]], [[PORTAL_Stock_and_flow]]. --- *Thury station wave · 2026-09-10 · drafted · microsim THY-108 (p5.js) · parent [[Hydrology]].*