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Atmosphere of Mars: 0.03%[24]

Atmosphere of Ceres[25]

Atmosphere of Jupiter: 0.0004%[26]

in icesonly; and that of itsmoon Europa[27]

Atmosphere of Saturnin icesonly;and that of itsmoons Titan(stratospheric), Enceladus

: 91%[28]and Dione(exosphere) Atmosphere of Uranusin trace

amounts below 50 bar[29]

Atmosphere of Neptunefound in thedeeper layers[30]

Extrasolar planetatmospheres:including those of HD 189733b[31]and HD 209458 b,[32]Tau Botisb,[33]HAT-P-11b,[34][35]XO-1b, WASP-12b, WASP-17b, and WASP-19b.[36]

Stellar atmospheres: not limited to

cooler stars and even detected in gianthot stars such as Betelgeuse, MuCephei,AntaresandArcturus.[35][37]

https://en.wikipedia.org/wiki/Atmosphere_of_Marshttps://en.wikipedia.org/wiki/Water#cite_note-Rapp2012-24https://en.wikipedia.org/wiki/Atmosphere_of_Cereshttps://en.wikipedia.org/wiki/Atmosphere_of_Cereshttps://en.wikipedia.org/wiki/Atmosphere_of_Jupiterhttps://en.wikipedia.org/wiki/Water#cite_note-26https://en.wikipedia.org/wiki/Volatileshttps://en.wikipedia.org/wiki/Europa_(moon)https://en.wikipedia.org/wiki/Europa_(moon)https://en.wikipedia.org/wiki/Atmosphere_of_Saturnhttps://en.wikipedia.org/wiki/Volatileshttps://en.wikipedia.org/wiki/Titan_(moon)https://en.wikipedia.org/wiki/Enceladus_(moon)https://en.wikipedia.org/wiki/Water#cite_note-Hansen-28https://en.wikipedia.org/wiki/Dione_(moon)https://en.wikipedia.org/wiki/Atmosphere_of_Uranushttps://en.wikipedia.org/wiki/Water#cite_note-FOOTNOTEEncrenaz200392-29https://en.wikipedia.org/wiki/Atmosphere_of_Neptunehttps://en.wikipedia.org/wiki/Water#cite_note-hubbard-30https://en.wikipedia.org/wiki/Extrasolar_planethttps://en.wikipedia.org/wiki/HD_189733_bhttps://en.wikipedia.org/wiki/HD_189733_bhttps://en.wikipedia.org/wiki/Water#cite_note-31https://en.wikipedia.org/wiki/HD_209458_bhttps://en.wikipedia.org/wiki/Water#cite_note-Space.com_water-32https://en.wikipedia.org/wiki/Tau_Bo%C3%B6tis_bhttps://en.wikipedia.org/wiki/Tau_Bo%C3%B6tis_bhttps://en.wikipedia.org/wiki/Water#cite_note-33https://en.wikipedia.org/wiki/HAT-P-11bhttps://en.wikipedia.org/wiki/Water#cite_note-NASA-20140924-34https://en.wikipedia.org/wiki/XO-1bhttps://en.wikipedia.org/wiki/WASP-12bhttps://en.wikipedia.org/wiki/WASP-12bhttps://en.wikipedia.org/wiki/WASP-17bhttps://en.wikipedia.org/wiki/WASP-19bhttps://en.wikipedia.org/wiki/Water#cite_note-NASA-20131203-36https://en.wikipedia.org/wiki/Stellar_atmospherehttps://en.wikipedia.org/wiki/Betelgeusehttps://en.wikipedia.org/wiki/Mu_Cepheihttps://en.wikipedia.org/wiki/Mu_Cepheihttps://en.wikipedia.org/wiki/Antareshttps://en.wikipedia.org/wiki/Arcturushttps://en.wikipedia.org/wiki/Water#cite_note-Hanslmeier2010-35https://en.wikipedia.org/wiki/Water#cite_note-Hanslmeier2010-35https://en.wikipedia.org/wiki/Water#cite_note-Hanslmeier2010-35https://en.wikipedia.org/wiki/Arcturushttps://en.wikipedia.org/wiki/Antareshttps://en.wikipedia.org/wiki/Mu_Cepheihttps://en.wikipedia.org/wiki/Mu_Cepheihttps://en.wikipedia.org/wiki/Betelgeusehttps://en.wikipedia.org/wiki/Stellar_atmospherehttps://en.wikipedia.org/wiki/Water#cite_note-NASA-20131203-36https://en.wikipedia.org/wiki/WASP-19bhttps://en.wikipedia.org/wiki/WASP-17bhttps://en.wikipedia.org/wiki/WASP-12bhttps://en.wikipedia.org/wiki/WASP-12bhttps://en.wikipedia.org/wiki/XO-1bhttps://en.wikipedia.org/wiki/Water#cite_note-NASA-20140924-34https://en.wikipedia.org/wiki/Water#cite_note-NASA-20140924-34https://en.wikipedia.org/wiki/HAT-P-11bhttps://en.wikipedia.org/wiki/Water#cite_note-33https://en.wikipedia.org/wiki/Tau_Bo%C3%B6tis_bhttps://en.wikipedia.org/wiki/Tau_Bo%C3%B6tis_bhttps://en.wikipedia.org/wiki/Water#cite_note-Space.com_water-32https://en.wikipedia.org/wiki/HD_209458_bhttps://en.wikipedia.org/wiki/Water#cite_note-31https://en.wikipedia.org/wiki/HD_189733_bhttps://en.wikipedia.org/wiki/HD_189733_bhttps://en.wikipedia.org/wiki/Extrasolar_planethttps://en.wikipedia.org/wiki/Water#cite_note-hubbard-30https://en.wikipedia.org/wiki/Atmosphere_of_Neptunehttps://en.wikipedia.org/wiki/Water#cite_note-FOOTNOTEEncrenaz200392-29https://en.wikipedia.org/wiki/Atmosphere_of_Uranushttps://en.wikipedia.org/wiki/Dione_(moon)https://en.wikipedia.org/wiki/Water#cite_note-Hansen-28https://en.wikipedia.org/wiki/Enceladus_(moon)https://en.wikipedia.org/wiki/Titan_(moon)https://en.wikipedia.org/wiki/Volatileshttps://en.wikipedia.org/wiki/Atmosphere_of_Saturnhttps://en.wikipedia.org/wiki/Europa_(moon)https://en.wikipedia.org/wiki/Europa_(moon)https://en.wikipedia.org/wiki/Volatileshttps://en.wikipedia.org/wiki/Water#cite_note-26https://en.wikipedia.org/wiki/Atmosphere_of_Jupiterhttps://en.wikipedia.org/wiki/Atmosphere_of_Cereshttps://en.wikipedia.org/wiki/Atmosphere_of_Cereshttps://en.wikipedia.org/wiki/Water#cite_note-Rapp2012-24https://en.wikipedia.org/wiki/Atmosphere_of_Mars

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Circumstellar disks: including those ofmore than half of T Tauri starssuch

asAA Tauri

[35]

as well as TWHydrae,[38][39]IRC +10216[40]andAPM08279+5255,[18][19]VY CanisMajorisand S Persei.[37]

Liquid water

Turquoise water with a bit of Sun.

Liquid water is known to be present onEarth, covering 71% of its surface.Scientists believe liquid water is presentin the Saturnian moons ofEnceladus, asa 10-kilometre thick ocean approximately3040 kilometres below Enceladus' southpolar surface,[41][42]and Titan, as a

https://en.wikipedia.org/wiki/Circumstellar_diskhttps://en.wikipedia.org/wiki/T_Tauri_starhttps://en.wikipedia.org/wiki/AA_Taurihttps://en.wikipedia.org/wiki/AA_Taurihttps://en.wikipedia.org/wiki/TW_Hydraehttps://en.wikipedia.org/wiki/TW_Hydraehttps://en.wikipedia.org/wiki/Water#cite_note-38https://en.wikipedia.org/wiki/IRC_%2B10216https://en.wikipedia.org/wiki/IRC_%2B10216https://en.wikipedia.org/wiki/APM_08279%2B5255https://en.wikipedia.org/wiki/APM_08279%2B5255https://en.wikipedia.org/wiki/Water#cite_note-Clavin-18https://en.wikipedia.org/wiki/VY_Canis_Majorishttps://en.wikipedia.org/wiki/VY_Canis_Majorishttps://en.wikipedia.org/wiki/S_Perseihttps://en.wikipedia.org/wiki/Water#cite_note-Lund_Observatory-37https://en.wikipedia.org/wiki/Enceladus_(moon)https://en.wikipedia.org/wiki/Water#cite_note-NASA-20140403-41https://en.wikipedia.org/wiki/Titan_(moon)https://en.wikipedia.org/wiki/File:Turquoise_Water.jpghttps://en.wikipedia.org/wiki/Titan_(moon)https://en.wikipedia.org/wiki/Water#cite_note-NASA-20140403-41https://en.wikipedia.org/wiki/Water#cite_note-NASA-20140403-41https://en.wikipedia.org/wiki/Enceladus_(moon)https://en.wikipedia.org/wiki/Water#cite_note-Lund_Observatory-37https://en.wikipedia.org/wiki/S_Perseihttps://en.wikipedia.org/wiki/VY_Canis_Majorishttps://en.wikipedia.org/wiki/VY_Canis_Majorishttps://en.wikipedia.org/wiki/Water#cite_note-Clavin-18https://en.wikipedia.org/wiki/Water#cite_note-Clavin-18https://en.wikipedia.org/wiki/APM_08279%2B5255https://en.wikipedia.org/wiki/APM_08279%2B5255https://en.wikipedia.org/wiki/IRC_%2B10216https://en.wikipedia.org/wiki/IRC_%2B10216https://en.wikipedia.org/wiki/Water#cite_note-38https://en.wikipedia.org/wiki/Water#cite_note-38https://en.wikipedia.org/wiki/TW_Hydraehttps://en.wikipedia.org/wiki/TW_Hydraehttps://en.wikipedia.org/wiki/AA_Taurihttps://en.wikipedia.org/wiki/AA_Taurihttps://en.wikipedia.org/wiki/T_Tauri_starhttps://en.wikipedia.org/wiki/Circumstellar_disk

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subsurface layer, possibly mixedwith ammonia.[43]Jupiter's

moon Europahas surface characteristicswhich suggest a subsurface liquid waterocean.[44]Liquid water may also exist onJupiter's moon Ganymedeas a layersandwiched between high pressure iceand rock.[45]

Currently, there are two planets known tohave flowing liquid water on theirsurfaces: Earth and Mars.[46]

Water ice

Water is present as ice on:

Mars: under the regolith and at thepoles

Earth-Moon system: mainly as icesheetson Earth and in Lunar cratersand volcanic rocks[47]NASA reported

the detection of water molecules byNASA's Moon Mineralogy Mapperaboard the Indian Space Research

https://en.wikipedia.org/wiki/Ammoniahttps://en.wikipedia.org/wiki/Water#cite_note-43https://en.wikipedia.org/wiki/Europa_(moon)https://en.wikipedia.org/wiki/Water#cite_note-44https://en.wikipedia.org/wiki/Ganymede_(moon)https://en.wikipedia.org/wiki/Water#cite_note-45https://en.wikipedia.org/wiki/Marshttps://en.wikipedia.org/wiki/Water#cite_note-46https://en.wikipedia.org/wiki/Water_on_Marshttps://en.wikipedia.org/wiki/Ice_sheethttps://en.wikipedia.org/wiki/Ice_sheethttps://en.wikipedia.org/wiki/Water#cite_note-47https://en.wikipedia.org/wiki/Water#cite_note-47https://en.wikipedia.org/wiki/Ice_sheethttps://en.wikipedia.org/wiki/Ice_sheethttps://en.wikipedia.org/wiki/Water_on_Marshttps://en.wikipedia.org/wiki/Water#cite_note-46https://en.wikipedia.org/wiki/Marshttps://en.wikipedia.org/wiki/Water#cite_note-45https://en.wikipedia.org/wiki/Ganymede_(moon)https://en.wikipedia.org/wiki/Water#cite_note-44https://en.wikipedia.org/wiki/Europa_(moon)https://en.wikipedia.org/wiki/Water#cite_note-43https://en.wikipedia.org/wiki/Ammonia

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Organization's Chandrayaan-1spacecraft in September 2009.[48]

Jupiter's moons: Europa's surface andalso that of Ganymede

Saturn: in the planet's ringsystem[49]and on the surface andmantle of Titanand Enceladus

Pluto-Charonsystem[49]

Cometsand related (Kuiperbeltand Oort cloudobjects).

And may also be present on:

Mercury's poles[50]

Ceres

Tethys

Exotic forms

Water and other volatilesprobablycomprise much of the internal structuresof Uranusand Neptuneand the water in

the deeper layers may be in the formof ionic waterin which the moleculesbreak down into a soup of hydrogen andoxygen ions, and deeper down

https://en.wikipedia.org/wiki/Water#cite_note-48https://en.wikipedia.org/wiki/Europa_(moon)https://en.wikipedia.org/wiki/Rings_of_Saturnhttps://en.wikipedia.org/wiki/Rings_of_Saturnhttps://en.wikipedia.org/wiki/Water#cite_note-Sparrow-49https://en.wikipedia.org/wiki/Titan_(moon)https://en.wikipedia.org/wiki/Enceladus_(moon)https://en.wikipedia.org/wiki/Plutohttps://en.wikipedia.org/wiki/Charon_(moon)https://en.wikipedia.org/wiki/Water#cite_note-Sparrow-49https://en.wikipedia.org/wiki/Cometshttps://en.wikipedia.org/wiki/Kuiper_belthttps://en.wikipedia.org/wiki/Kuiper_belthttps://en.wikipedia.org/wiki/Oort_cloudhttps://en.wikipedia.org/wiki/Water#cite_note-50https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)https://en.wikipedia.org/wiki/Tethys_(moon)https://en.wikipedia.org/wiki/Volatileshttps://en.wikipedia.org/wiki/Uranushttps://en.wikipedia.org/wiki/Neptunehttps://en.wikipedia.org/w/index.php?title=Ionic_water&action=edit&redlink=1https://en.wikipedia.org/w/index.php?title=Ionic_water&action=edit&redlink=1https://en.wikipedia.org/wiki/Neptunehttps://en.wikipedia.org/wiki/Uranushttps://en.wikipedia.org/wiki/Volatileshttps://en.wikipedia.org/wiki/Tethys_(moon)https://en.wikipedia.org/wiki/Ceres_(dwarf_planet)https://en.wikipedia.org/wiki/Water#cite_note-50https://en.wikipedia.org/wiki/Oort_cloudhttps://en.wikipedia.org/wiki/Kuiper_belthttps://en.wikipedia.org/wiki/Kuiper_belthttps://en.wikipedia.org/wiki/Cometshttps://en.wikipedia.org/wiki/Water#cite_note-Sparrow-49https://en.wikipedia.org/wiki/Charon_(moon)https://en.wikipedia.org/wiki/Plutohttps://en.wikipedia.org/wiki/Enceladus_(moon)https://en.wikipedia.org/wiki/Titan_(moon)https://en.wikipedia.org/wiki/Water#cite_note-Sparrow-49https://en.wikipedia.org/wiki/Rings_of_Saturnhttps://en.wikipedia.org/wiki/Rings_of_Saturnhttps://en.wikipedia.org/wiki/Europa_(moon)https://en.wikipedia.org/wiki/Water#cite_note-48

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as superionic waterin which the oxygencrystallises but the hydrogen ions float

around freely within the oxygen lattice.

[51]

Water and habitable zoneFurther information: Water distribution onEarth

The existence of liquid water, and to alesser extent its gaseousand solid forms,on Earth are vital to the existence of lifeon Earthas we know it. The Earth islocated in the habitable zoneof the solarsystem; if it were slightly closer to orfarther from the Sun(about 5%, or about

8 million kilometers), the conditions whichallow the three forms to be presentsimultaneously would be far less likely toexist.[52][53]

Earth's gravityallows it to hold

an atmosphere. Water vapor and carbondioxide in the atmosphere provide atemperature buffer (greenhouse effect)which helps maintain a relatively steady

https://en.wikipedia.org/wiki/Superionic_waterhttps://en.wikipedia.org/wiki/Water#cite_note-newscientist.com-51https://en.wikipedia.org/wiki/Water_distribution_on_Earthhttps://en.wikipedia.org/wiki/Water_distribution_on_Earthhttps://en.wikipedia.org/wiki/Gashttps://en.wikipedia.org/wiki/Organismhttps://en.wikipedia.org/wiki/Organismhttps://en.wikipedia.org/wiki/Habitable_zonehttps://en.wikipedia.org/wiki/Solar_systemhttps://en.wikipedia.org/wiki/Solar_systemhttps://en.wikipedia.org/wiki/Sunhttps://en.wikipedia.org/wiki/Water#cite_note-52https://en.wikipedia.org/wiki/Gravityhttps://en.wikipedia.org/wiki/Celestial_body_atmospherehttps://en.wikipedia.org/wiki/Greenhouse_effecthttps://en.wikipedia.org/wiki/Greenhouse_effecthttps://en.wikipedia.org/wiki/Celestial_body_atmospherehttps://en.wikipedia.org/wiki/Gravityhttps://en.wikipedia.org/wiki/Water#cite_note-52https://en.wikipedia.org/wiki/Water#cite_note-52https://en.wikipedia.org/wiki/Sunhttps://en.wikipedia.org/wiki/Solar_systemhttps://en.wikipedia.org/wiki/Solar_systemhttps://en.wikipedia.org/wiki/Habitable_zonehttps://en.wikipedia.org/wiki/Organismhttps://en.wikipedia.org/wiki/Organismhttps://en.wikipedia.org/wiki/Gashttps://en.wikipedia.org/wiki/Water_distribution_on_Earthhttps://en.wikipedia.org/wiki/Water_distribution_on_Earthhttps://en.wikipedia.org/wiki/Water#cite_note-newscientist.com-51https://en.wikipedia.org/wiki/Superionic_water

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surface temperature. If Earth weresmaller, a thinner atmosphere would

allow temperature extremes, thuspreventing the accumulation of waterexcept in polar ice caps(as on Mars).

The surface temperature of Earth hasbeen relatively constant through geologictimedespite varying levels of incoming

solar radiation (insolation), indicating thata dynamic process governs Earth'stemperature via a combination ofgreenhouse gases and surface oratmospheric albedo. This proposal is

known as the Gaia hypothesis.The state of water on a planet dependson ambient pressure, which isdetermined by the planet's gravity. If aplanet is sufficiently massive, the wateron it may be solid even at hightemperatures, because of the highpressure caused by gravity, as it wasobserved on exoplanets Gliese 436b[54]and GJ 1214 b.[55]

https://en.wikipedia.org/wiki/Polar_ice_caphttps://en.wikipedia.org/wiki/Marshttps://en.wikipedia.org/wiki/Geologic_timehttps://en.wikipedia.org/wiki/Geologic_timehttps://en.wikipedia.org/wiki/Insolationhttps://en.wikipedia.org/wiki/Albedohttps://en.wikipedia.org/wiki/Gaia_hypothesishttps://en.wikipedia.org/wiki/Gliese_436_bhttps://en.wikipedia.org/wiki/Gliese_436_bhttps://en.wikipedia.org/wiki/Water#cite_note-54https://en.wikipedia.org/wiki/GJ_1214_bhttps://en.wikipedia.org/wiki/Water#cite_note-55https://en.wikipedia.org/wiki/Water#cite_note-55https://en.wikipedia.org/wiki/GJ_1214_bhttps://en.wikipedia.org/wiki/Water#cite_note-54https://en.wikipedia.org/wiki/Gliese_436_bhttps://en.wikipedia.org/wiki/Gliese_436_bhttps://en.wikipedia.org/wiki/Gaia_hypothesishttps://en.wikipedia.org/wiki/Albedohttps://en.wikipedia.org/wiki/Insolationhttps://en.wikipedia.org/wiki/Geologic_timehttps://en.wikipedia.org/wiki/Geologic_timehttps://en.wikipedia.org/wiki/Marshttps://en.wikipedia.org/wiki/Polar_ice_cap

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On Earth

Main articles: Hydrologyand Water

distribution on Earth

Water covers 71% of the Earth's surface;the oceans contain 96.5% of the Earth'swater. TheAntarctic ice sheet, whichcontains 61% of all fresh water on Earth,

is visible at the bottom. Condensedatmospheric water can be seenas clouds, contributing to theEarth's albedo.

Hydrology is the study of the movement,

distribution, and quality of waterthroughout the Earth. The study of thedistribution of water is hydrography. Thestudy of the distribution and movement of

https://en.wikipedia.org/wiki/Hydrologyhttps://en.wikipedia.org/wiki/Water_distribution_on_Earthhttps://en.wikipedia.org/wiki/Water_distribution_on_Earthhttps://en.wikipedia.org/wiki/Antarctic_ice_sheethttps://en.wikipedia.org/wiki/Cloudhttps://en.wikipedia.org/wiki/Albedohttps://en.wikipedia.org/wiki/Hydrographyhttps://en.wikipedia.org/wiki/File:The_Earth_seen_from_Apollo_17.jpghttps://en.wikipedia.org/wiki/Hydrographyhttps://en.wikipedia.org/wiki/Albedohttps://en.wikipedia.org/wiki/Cloudhttps://en.wikipedia.org/wiki/Antarctic_ice_sheethttps://en.wikipedia.org/wiki/Water_distribution_on_Earthhttps://en.wikipedia.org/wiki/Water_distribution_on_Earthhttps://en.wikipedia.org/wiki/Hydrology

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groundwater is hydrogeology, of glaciersis glaciology, of inland waters

is limnologyand distribution of oceansis oceanography. Ecological processeswith hydrology are in focusof ecohydrology.

The collective mass of water found on,under, and over the surface of a planet is

called the hydrosphere. Earth'sapproximate water volume (the totalwater supply of the world) is1,338,000,000 km3(321,000,000 mi3).[2]

Liquid water is found inbodies of water,

such as anocean, sea, lake,river,stream, canal, pond, or puddle. The majority of water onEarth is sea water. Water is also presentin the atmosphere in solid, liquid, andvapor states. It also exists asgroundwater in aquifers.

Water is important in many geologicalprocesses. Groundwater is present in

https://en.wikipedia.org/wiki/Hydrogeologyhttps://en.wikipedia.org/wiki/Glaciologyhttps://en.wikipedia.org/wiki/Limnologyhttps://en.wikipedia.org/wiki/Oceanographyhttps://en.wikipedia.org/wiki/Ecohydrologyhttps://en.wikipedia.org/wiki/Hydrospherehttps://en.wikipedia.org/wiki/Water#cite_note-b1-2https://en.wikipedia.org/wiki/Water#cite_note-b1-2https://en.wikipedia.org/wiki/Body_of_waterhttps://en.wikipedia.org/wiki/Seahttps://en.wikipedia.org/wiki/Lakehttps://en.wikipedia.org/wiki/Riverhttps://en.wikipedia.org/wiki/Streamhttps://en.wikipedia.org/wiki/Canalhttps://en.wikipedia.org/wiki/Pondhttps://en.wikipedia.org/wiki/Pondhttps://en.wikipedia.org/wiki/Puddlehttps://en.wikipedia.org/wiki/Sea_waterhttps://en.wikipedia.org/wiki/Aquiferhttps://en.wikipedia.org/wiki/Aquiferhttps://en.wikipedia.org/wiki/Sea_waterhttps://en.wikipedia.org/wiki/Puddlehttps://en.wikipedia.org/wiki/Pondhttps://en.wikipedia.org/wiki/Pondhttps://en.wikipedia.org/wiki/Canalhttps://en.wikipedia.org/wiki/Streamhttps://en.wikipedia.org/wiki/Riverhttps://en.wikipedia.org/wiki/Lakehttps://en.wikipedia.org/wiki/Seahttps://en.wikipedia.org/wiki/Body_of_waterhttps://en.wikipedia.org/wiki/Water#cite_note-b1-2https://en.wikipedia.org/wiki/Hydrospherehttps://en.wikipedia.org/wiki/Ecohydrologyhttps://en.wikipedia.org/wiki/Oceanographyhttps://en.wikipedia.org/wiki/Limnologyhttps://en.wikipedia.org/wiki/Glaciologyhttps://en.wikipedia.org/wiki/Hydrogeology

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most rocks, and the pressure of thisgroundwater affects patterns of faulting.

Water in the mantleis responsible for themelt thatproduces volcanoesat subduction zones.On the surface of the Earth, water isimportant in both chemical andphysical weatheringprocesses. Water,

and to a lesser but still significant extent,ice, are also responsible for a largeamount of sediment transportthat occurson the surface of the earth. Depositionoftransported sediment forms many typesof sedimentary rocks, which make upthe geologic recordof Earth history.

Water cycleMain article: Water cycle

Water cycle

https://en.wikipedia.org/wiki/Rock_(geology)https://en.wikipedia.org/wiki/Fault_(geology)https://en.wikipedia.org/wiki/Mantle_(geology)https://en.wikipedia.org/wiki/Volcanohttps://en.wikipedia.org/wiki/Subduction_zonehttps://en.wikipedia.org/wiki/Weatheringhttps://en.wikipedia.org/wiki/Sediment_transporthttps://en.wikipedia.org/wiki/Deposition_(geology)https://en.wikipedia.org/wiki/Sedimentary_rockhttps://en.wikipedia.org/wiki/Geologic_recordhttps://en.wikipedia.org/wiki/History_of_the_Earthhttps://en.wikipedia.org/wiki/Water_cyclehttps://en.wikipedia.org/wiki/Water_cyclehttps://en.wikipedia.org/wiki/File:Water_cycle.pnghttps://en.wikipedia.org/wiki/Water_cyclehttps://en.wikipedia.org/wiki/Water_cyclehttps://en.wikipedia.org/wiki/History_of_the_Earthhttps://en.wikipedia.org/wiki/Geologic_recordhttps://en.wikipedia.org/wiki/Sedimentary_rockhttps://en.wikipedia.org/wiki/Deposition_(geology)https://en.wikipedia.org/wiki/Sediment_transporthttps://en.wikipedia.org/wiki/Weatheringhttps://en.wikipedia.org/wiki/Subduction_zonehttps://en.wikipedia.org/wiki/Volcanohttps://en.wikipedia.org/wiki/Mantle_(geology)https://en.wikipedia.org/wiki/Fault_(geology)https://en.wikipedia.org/wiki/Rock_(geology)

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The water cycle(known scientifically asthe hydrologic cycle) refers to the

continuous exchange of water withinthe hydrosphere, betweenthe atmosphere, soilwater, surfacewater, groundwater, and plants.

Water moves perpetually through each ofthese regions in the water

cycleconsisting of following transferprocesses:

evaporationfrom oceans and otherwater bodies into the airand transpirationfrom land plants and

animals into air. precipitation, from water vapor

condensing from the air and falling toearth or ocean.

runofffrom the land usually reaching

the sea.Most water vapor over the oceansreturns to the oceans, but winds carrywater vapor over land at the same rate

https://en.wikipedia.org/wiki/Water_cyclehttps://en.wikipedia.org/wiki/Hydrospherehttps://en.wikipedia.org/wiki/Earth_atmospherehttps://en.wikipedia.org/wiki/Soilhttps://en.wikipedia.org/wiki/Surface_waterhttps://en.wikipedia.org/wiki/Surface_waterhttps://en.wikipedia.org/wiki/Groundwaterhttps://en.wikipedia.org/wiki/Planthttps://en.wikipedia.org/wiki/Evaporationhttps://en.wikipedia.org/wiki/Transpirationhttps://en.wikipedia.org/wiki/Precipitation_(meteorology)https://en.wikipedia.org/wiki/Runoff_(water)https://en.wikipedia.org/wiki/Seahttps://en.wikipedia.org/wiki/Seahttps://en.wikipedia.org/wiki/Runoff_(water)https://en.wikipedia.org/wiki/Precipitation_(meteorology)https://en.wikipedia.org/wiki/Transpirationhttps://en.wikipedia.org/wiki/Evaporationhttps://en.wikipedia.org/wiki/Planthttps://en.wikipedia.org/wiki/Groundwaterhttps://en.wikipedia.org/wiki/Surface_waterhttps://en.wikipedia.org/wiki/Surface_waterhttps://en.wikipedia.org/wiki/Soilhttps://en.wikipedia.org/wiki/Earth_atmospherehttps://en.wikipedia.org/wiki/Hydrospherehttps://en.wikipedia.org/wiki/Water_cycle

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as runoff into the sea, about 47 Ttperyear. Over land, evaporation and

transpiration contribute another 72 Tt peryear. Precipitation, at a rate of 119 Tt peryear over land, has several forms: mostcommonly rain, snow, and hail, withsome contributionfrom fogand dew.[56]Dew is small drops

of water that are condensed when a highdensity of water vapor meets a coolsurface. Dew usually forms in themorning when the temperature is thelowest, just before sunrise and when thetemperature of the earth's surface startsto increase.[57]Condensed water in theair may also refractsunlighttoproduce rainbows.

Water runoff often collectsover watershedsflowing into rivers. A

mathematical model used to simulateriver or stream flow and calculate waterquality parameters is a hydrologicaltransport model. Some water is diverted

https://en.wikipedia.org/wiki/Metric_tonne_unithttps://en.wikipedia.org/wiki/Rainhttps://en.wikipedia.org/wiki/Snowhttps://en.wikipedia.org/wiki/Hailhttps://en.wikipedia.org/wiki/Foghttps://en.wikipedia.org/wiki/Dewhttps://en.wikipedia.org/wiki/Water#cite_note-56https://en.wikipedia.org/wiki/Water#cite_note-57https://en.wikipedia.org/wiki/Refracthttps://en.wikipedia.org/wiki/Sunlighthttps://en.wikipedia.org/wiki/Rainbowhttps://en.wikipedia.org/wiki/Drainage_basinhttps://en.wikipedia.org/wiki/Hydrological_transport_modelhttps://en.wikipedia.org/wiki/Hydrological_transport_modelhttps://en.wikipedia.org/wiki/Hydrological_transport_modelhttps://en.wikipedia.org/wiki/Hydrological_transport_modelhttps://en.wikipedia.org/wiki/Drainage_basinhttps://en.wikipedia.org/wiki/Rainbowhttps://en.wikipedia.org/wiki/Sunlighthttps://en.wikipedia.org/wiki/Refracthttps://en.wikipedia.org/wiki/Water#cite_note-57https://en.wikipedia.org/wiki/Water#cite_note-56https://en.wikipedia.org/wiki/Dewhttps://en.wikipedia.org/wiki/Foghttps://en.wikipedia.org/wiki/Hailhttps://en.wikipedia.org/wiki/Snowhttps://en.wikipedia.org/wiki/Rainhttps://en.wikipedia.org/wiki/Metric_tonne_unit

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toirrigationfor agriculture. Rivers andseas offer opportunity

for traveland commerce.Through erosion, runoff shapes theenvironment creatingriver valleysand deltaswhich providerich soil and level ground for theestablishment of population centers.

A floodoccurs when an area of land,usually low-lying, is covered with water. Itis when a river overflows its banks orflood comes from the sea. A droughtisan extended period of months or yearswhen a region notes a deficiency in itswater supply. This occurs when a regionreceives consistently below averageprecipitation.

Fresh water storage

https://en.wikipedia.org/wiki/Irrigationhttps://en.wikipedia.org/wiki/Travelhttps://en.wikipedia.org/wiki/Commercehttps://en.wikipedia.org/wiki/Erosionhttps://en.wikipedia.org/wiki/Valleyhttps://en.wikipedia.org/wiki/River_deltahttps://en.wikipedia.org/wiki/Floodhttps://en.wikipedia.org/wiki/Droughthttps://en.wikipedia.org/wiki/Droughthttps://en.wikipedia.org/wiki/Floodhttps://en.wikipedia.org/wiki/River_deltahttps://en.wikipedia.org/wiki/Valleyhttps://en.wikipedia.org/wiki/Erosionhttps://en.wikipedia.org/wiki/Commercehttps://en.wikipedia.org/wiki/Travelhttps://en.wikipedia.org/wiki/Irrigation

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The Bay of Fundyat high tide (left) andlow tide (right)

Main article: Water resources

Some runoff water is trapped for periodsof time, for example in lakes. At high

altitude, during winter, and in the farnorth and south, snow collects in icecaps, snow pack and glaciers. Water alsoinfiltrates the ground and goes intoaquifers. This groundwater later flows

back to the surface in springs, or morespectacularly in hot springsand geysers.Groundwater is also extracted artificiallyinwells. This water storage is important,

https://en.wikipedia.org/wiki/Bay_of_Fundyhttps://en.wikipedia.org/wiki/Water_resourceshttps://en.wikipedia.org/wiki/Spring_(hydrosphere)https://en.wikipedia.org/wiki/Hot_springhttps://en.wikipedia.org/wiki/Geyserhttps://en.wikipedia.org/wiki/Water_wellhttps://en.wikipedia.org/wiki/File:Bay_of_Fundy_Low_Tide.jpghttps://en.wikipedia.org/wiki/File:Bay_of_Fundy_High_Tide.jpghttps://en.wikipedia.org/wiki/Water_wellhttps://en.wikipedia.org/wiki/Geyserhttps://en.wikipedia.org/wiki/Hot_springhttps://en.wikipedia.org/wiki/Spring_(hydrosphere)https://en.wikipedia.org/wiki/Water_resourceshttps://en.wikipedia.org/wiki/Bay_of_Fundy

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since clean, fresh water is essential tohuman and other land-based life. In

many parts of the world, it is in shortsupply.

Sea water and tidesMain articles: Seawaterand Tides

Sea watercontains about 3.5% saltonaverage, plus smaller amounts of othersubstances. The physical properties ofsea water differ from fresh water in someimportant respects. It freezes at a lowertemperature (about 1.9C) and itsdensity increases with decreasing

temperature to the freezing point, insteadof reaching maximum density at atemperature above freezing. The salinityof water in major seas varies from about0.7% in the Baltic Seato 4.0% in the RedSea.

Tidesare the cyclic rising and falling oflocal sea levels caused by the tidalforcesof the Moon and the Sun acting on

https://en.wikipedia.org/wiki/Seawaterhttps://en.wikipedia.org/wiki/Tideshttps://en.wikipedia.org/wiki/Seawaterhttps://en.wikipedia.org/wiki/Sodium_chloridehttps://en.wikipedia.org/wiki/Baltic_Seahttps://en.wikipedia.org/wiki/Red_Seahttps://en.wikipedia.org/wiki/Red_Seahttps://en.wikipedia.org/wiki/Tidehttps://en.wikipedia.org/wiki/Tidal_forcehttps://en.wikipedia.org/wiki/Tidal_forcehttps://en.wikipedia.org/wiki/Tidal_forcehttps://en.wikipedia.org/wiki/Tidal_forcehttps://en.wikipedia.org/wiki/Tidehttps://en.wikipedia.org/wiki/Red_Seahttps://en.wikipedia.org/wiki/Red_Seahttps://en.wikipedia.org/wiki/Baltic_Seahttps://en.wikipedia.org/wiki/Sodium_chloridehttps://en.wikipedia.org/wiki/Seawaterhttps://en.wikipedia.org/wiki/Tideshttps://en.wikipedia.org/wiki/Seawater

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the oceans. Tides cause changes in thedepth of the marine and estuarinewater

bodies and produce oscillating currentsknown as tidal streams. The changingtide produced at a given location is theresult of the changing positions of theMoon and Sun relative to the Earthcoupled with the effects of Earth

rotationand the local bathymetry. Thestrip of seashore that is submerged athigh tide and exposed at low tide,the intertidal zone, is an importantecological product of ocean tides.

Effects on life

An oasisis an isolated watersourcewith vegetationin adesert.

https://en.wikipedia.org/wiki/Estuaryhttps://en.wikipedia.org/wiki/Coriolis_effecthttps://en.wikipedia.org/wiki/Coriolis_effecthttps://en.wikipedia.org/wiki/Bathymetryhttps://en.wikipedia.org/wiki/Intertidal_zonehttps://en.wikipedia.org/wiki/Oasishttps://en.wikipedia.org/wiki/Water_sourcehttps://en.wikipedia.org/wiki/Water_sourcehttps://en.wikipedia.org/wiki/Vegetationhttps://en.wikipedia.org/wiki/Deserthttps://en.wikipedia.org/wiki/File:Oasis_in_Libya.jpghttps://en.wikipedia.org/wiki/Deserthttps://en.wikipedia.org/wiki/Vegetationhttps://en.wikipedia.org/wiki/Water_sourcehttps://en.wikipedia.org/wiki/Water_sourcehttps://en.wikipedia.org/wiki/Oasishttps://en.wikipedia.org/wiki/Intertidal_zonehttps://en.wikipedia.org/wiki/Bathymetryhttps://en.wikipedia.org/wiki/Coriolis_effecthttps://en.wikipedia.org/wiki/Coriolis_effecthttps://en.wikipedia.org/wiki/Estuary

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Overview

of photosynthesisandrespiration. Water(at right), together with carbon dioxide(CO2), form oxygen and organiccompounds (at left), which can berespired to water and (CO2).

From a biologicalstandpoint, water hasmany distinct properties that are criticalfor the proliferation of life. It carries outthis role by allowing organiccompoundsto react in ways thatultimately allow replication. All known

forms of life depend on water. Water isvital both as a solventin which many ofthe body's solutes dissolve and as anessential part of

https://en.wikipedia.org/wiki/Photosynthesishttps://en.wikipedia.org/wiki/Cellular_respirationhttps://en.wikipedia.org/wiki/Biologyhttps://en.wikipedia.org/wiki/Lifehttps://en.wikipedia.org/wiki/Organic_compoundhttps://en.wikipedia.org/wiki/Organic_compoundhttps://en.wikipedia.org/wiki/Self-replicationhttps://en.wikipedia.org/wiki/Solventhttps://en.wikipedia.org/wiki/File:Auto-and_heterotrophs.svghttps://en.wikipedia.org/wiki/Solventhttps://en.wikipedia.org/wiki/Self-replicationhttps://en.wikipedia.org/wiki/Organic_compoundhttps://en.wikipedia.org/wiki/Organic_compoundhttps://en.wikipedia.org/wiki/Lifehttps://en.wikipedia.org/wiki/Biologyhttps://en.wikipedia.org/wiki/Cellular_respirationhttps://en.wikipedia.org/wiki/Photosynthesis

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manymetabolicprocesses within thebody. Metabolism is the sum total of

anabolism and catabolism. In anabolism,water is removed from molecules(through energy requiring enzymaticchemical reactions) in order to growlarger molecules (e.g. starches,triglycerides and proteins for storage of

fuels and information). In catabolism,water is used to break bonds in order togenerate smaller molecules (e.g.glucose, fatty acids and amino acids tobe used for fuels for energy use or otherpurposes). Without water, theseparticular metabolic processes could notexist.

Water is fundamental to photosynthesisand respiration. Photosynthetic cells usethe sun's energy to split off water's

hydrogen from oxygen. Hydrogen iscombined with CO2(absorbed from air orwater) to form glucose and releaseoxygen. All living cells use such fuels and

https://en.wikipedia.org/wiki/Metabolismhttps://en.wikipedia.org/wiki/Metabolism

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oxidize the hydrogen and carbon tocapture the sun's energy and reform

water and CO2in the process (cellularrespiration).

Water is also central to acid-baseneutrality and enzyme function. An acid,a hydrogen ion (H+, that is, a proton)donor, can be neutralized by a base, a

proton acceptor such as a hydroxide ion(OH) to form water. Water is consideredto be neutral, with a pH(the negative logof the hydrogen ion concentration) of7.Acidshave pH values less than 7

while baseshave values greater than 7.Aquatic life formsMain articles: HydrobiologyandAquaticplant

https://en.wikipedia.org/wiki/PHhttps://en.wikipedia.org/wiki/Acidshttps://en.wikipedia.org/wiki/Base_(chemistry)https://en.wikipedia.org/wiki/Hydrobiologyhttps://en.wikipedia.org/wiki/Aquatic_planthttps://en.wikipedia.org/wiki/Aquatic_planthttps://en.wikipedia.org/wiki/Aquatic_planthttps://en.wikipedia.org/wiki/Aquatic_planthttps://en.wikipedia.org/wiki/Hydrobiologyhttps://en.wikipedia.org/wiki/Base_(chemistry)https://en.wikipedia.org/wiki/Acidshttps://en.wikipedia.org/wiki/PH

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Some of the biodiversityof a coral reef

Some marine diatomsakeyphytoplanktongroup

Earth surface waters are filled with life.The earliest life forms appeared in water;nearly all fishlive exclusively in water,and there are many types of marinemammals, such as dolphinsand whales.

Some kinds of animals, suchas amphibians, spend portions of theirlives in water and portions on land.Plants such as kelpand algaegrow in

https://en.wikipedia.org/wiki/Biodiversityhttps://en.wikipedia.org/wiki/Coral_reefhttps://en.wikipedia.org/wiki/Diatomhttps://en.wikipedia.org/wiki/Phytoplanktonhttps://en.wikipedia.org/wiki/Fishhttps://en.wikipedia.org/wiki/Dolphinhttps://en.wikipedia.org/wiki/Whalehttps://en.wikipedia.org/wiki/Amphibianhttps://en.wikipedia.org/wiki/Kelphttps://en.wikipedia.org/wiki/Algaehttps://en.wikipedia.org/wiki/File:Diatoms_through_the_microscope.jpghttps://en.wikipedia.org/wiki/File:Blue_Linckia_Starfish.JPGhttps://en.wikipedia.org/wiki/Algaehttps://en.wikipedia.org/wiki/Kelphttps://en.wikipedia.org/wiki/Amphibianhttps://en.wikipedia.org/wiki/Whalehttps://en.wikipedia.org/wiki/Dolphinhttps://en.wikipedia.org/wiki/Fishhttps://en.wikipedia.org/wiki/Phytoplanktonhttps://en.wikipedia.org/wiki/Diatomhttps://en.wikipedia.org/wiki/Coral_reefhttps://en.wikipedia.org/wiki/Biodiversity

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the water and are the basis for someunderwater ecosystems. Planktonis

generally the foundation of the oceanfoodchain.

Aquatic vertebrates must obtain oxygento survive, and they do so in variousways. Fish have gillsinstead of lungs,although some species of fish, such as

the lungfish, have both. Marinemammals, such as dolphins,whales, otters, and sealsneed to surfaceperiodically to breathe air. Someamphibians are able to absorb oxygen

through their skin. Invertebrates exhibit awide range of modifications to survive inpoorly oxygenated waters includingbreathing tubes (see insectand molluscsiphons) and gills(Carcinus). Howeveras invertebrate life evolved in an aquatic

habitat most have little or nospecialisation for respiration in water.

Effects on human civilization

https://en.wikipedia.org/wiki/Planktonhttps://en.wikipedia.org/wiki/Food_chainhttps://en.wikipedia.org/wiki/Food_chainhttps://en.wikipedia.org/wiki/Gillshttps://en.wikipedia.org/wiki/Lungshttps://en.wikipedia.org/wiki/Lungfishhttps://en.wikipedia.org/wiki/Marine_mammalhttps://en.wikipedia.org/wiki/Marine_mammalhttps://en.wikipedia.org/wiki/Otterhttps://en.wikipedia.org/wiki/Pinnipedhttps://en.wikipedia.org/wiki/Siphon_(insect)https://en.wikipedia.org/wiki/Siphon_(mollusc)https://en.wikipedia.org/wiki/Siphon_(mollusc)https://en.wikipedia.org/wiki/Gillshttps://en.wikipedia.org/wiki/Carcinushttps://en.wikipedia.org/wiki/Carcinushttps://en.wikipedia.org/wiki/Gillshttps://en.wikipedia.org/wiki/Siphon_(mollusc)https://en.wikipedia.org/wiki/Siphon_(mollusc)https://en.wikipedia.org/wiki/Siphon_(insect)https://en.wikipedia.org/wiki/Pinnipedhttps://en.wikipedia.org/wiki/Otterhttps://en.wikipedia.org/wiki/Marine_mammalhttps://en.wikipedia.org/wiki/Marine_mammalhttps://en.wikipedia.org/wiki/Lungfishhttps://en.wikipedia.org/wiki/Lungshttps://en.wikipedia.org/wiki/Gillshttps://en.wikipedia.org/wiki/Food_chainhttps://en.wikipedia.org/wiki/Food_chainhttps://en.wikipedia.org/wiki/Plankton

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Water fountain

Civilization has historically flourishedaround rivers and major

waterways; Mesopotamia, the so-calledcradle of civilization, was situatedbetween the majorrivers Tigrisand Euphrates; the ancientsociety of the Egyptiansdependedentirely upon the Nile.

Large metropoliseslike Rotterdam, London, Montreal, Paris, New YorkCity, BuenosAires, Shanghai, Tokyo, Chicago,and Hong Kongowe their success in part

to their easy accessibility via water andthe resultant expansion of trade. Islandswith safe water ports, like Singapore,have flourished for the same reason. In

https://en.wikipedia.org/wiki/Fountainhttps://en.wikipedia.org/wiki/Mesopotamiahttps://en.wikipedia.org/wiki/Tigrishttps://en.wikipedia.org/wiki/Euphrateshttps://en.wikipedia.org/wiki/Egyptianshttps://en.wikipedia.org/wiki/Nilehttps://en.wikipedia.org/wiki/Metropolishttps://en.wikipedia.org/wiki/Rotterdamhttps://en.wikipedia.org/wiki/Londonhttps://en.wikipedia.org/wiki/Londonhttps://en.wikipedia.org/wiki/Montrealhttps://en.wikipedia.org/wiki/Parishttps://en.wikipedia.org/wiki/New_York_Cityhttps://en.wikipedia.org/wiki/New_York_Cityhttps://en.wikipedia.org/wiki/Buenos_Aireshttps://en.wikipedia.org/wiki/Buenos_Aireshttps://en.wikipedia.org/wiki/Shanghaihttps://en.wikipedia.org/wiki/Tokyohttps://en.wikipedia.org/wiki/Chicagohttps://en.wikipedia.org/wiki/Hong_Konghttps://en.wikipedia.org/wiki/Singaporehttps://en.wikipedia.org/wiki/File:Longwood_Gardens-Italian_Garden.jpghttps://en.wikipedia.org/wiki/Singaporehttps://en.wikipedia.org/wiki/Hong_Konghttps://en.wikipedia.org/wiki/Chicagohttps://en.wikipedia.org/wiki/Tokyohttps://en.wikipedia.org/wiki/Shanghaihttps://en.wikipedia.org/wiki/Buenos_Aireshttps://en.wikipedia.org/wiki/Buenos_Aireshttps://en.wikipedia.org/wiki/New_York_Cityhttps://en.wikipedia.org/wiki/New_York_Cityhttps://en.wikipedia.org/wiki/Parishttps://en.wikipedia.org/wiki/Montrealhttps://en.wikipedia.org/wiki/Londonhttps://en.wikipedia.org/wiki/Londonhttps://en.wikipedia.org/wiki/Rotterdamhttps://en.wikipedia.org/wiki/Metropolishttps://en.wikipedia.org/wiki/Nilehttps://en.wikipedia.org/wiki/Egyptianshttps://en.wikipedia.org/wiki/Euphrateshttps://en.wikipedia.org/wiki/Tigrishttps://en.wikipedia.org/wiki/Mesopotamiahttps://en.wikipedia.org/wiki/Fountain

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places such as North Africaandthe Middle East, where water is more

scarce, access to clean drinking waterwas and is a major factor in humandevelopment.

Health and pollution

An environmental science programastudent from Iowa StateUniversitysampling water

Water fit for human consumption is calleddrinking water or potable water. Waterthat is not potable may be made potableby filtration or distillation, or by a rangeof other methods.

Water that is not fit for drinking but is notharmful for humans when used forswimming or bathing is called by variousnames other than potable or drinking

https://en.wikipedia.org/wiki/North_Africahttps://en.wikipedia.org/wiki/Middle_Easthttps://en.wikipedia.org/wiki/Iowa_State_Universityhttps://en.wikipedia.org/wiki/Iowa_State_Universityhttps://en.wikipedia.org/wiki/Potable_waterhttps://en.wikipedia.org/wiki/Distillationhttps://en.wikipedia.org/wiki/Water_treatmenthttps://en.wikipedia.org/wiki/File:Field_Trip-_water_sampling.jpghttps://en.wikipedia.org/wiki/Water_treatmenthttps://en.wikipedia.org/wiki/Distillationhttps://en.wikipedia.org/wiki/Potable_waterhttps://en.wikipedia.org/wiki/Iowa_State_Universityhttps://en.wikipedia.org/wiki/Iowa_State_Universityhttps://en.wikipedia.org/wiki/Middle_Easthttps://en.wikipedia.org/wiki/North_Africa

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water, and is sometimes called safewater, or "safe for bathing". Chlorine is a

skin and mucous membrane irritant thatis used to make water safe for bathing ordrinking. Its use is highly technical and isusually monitored by governmentregulations (typically 1 part per million(ppm) for drinking water, and 12 ppm of

chlorine not yet reacted with impuritiesfor bathing water). Water for bathing maybe maintained in satisfactorymicrobiological condition using chemicaldisinfectants such as chlorineor ozoneorby the use of ultravioletlight.

In the USA, non-potable formsof wastewatergenerated by humans maybe referred to as greywater, which istreatable and thus easily able to be madepotable again, and blackwater, which

generally contains sewageand otherforms of waste which require furthertreatmentin order to be made reusable.Greywater composes 5080% of

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residential wastewater generated by ahousehold's sanitation equipment

(sinks, showersand kitchenrunoff, butnot toilets, which generate blackwater.)These terms may have differentmeanings in other countries and cultures.

This natural resource is becomingscarcer in certain places, and its

availability is a major social andeconomic concern. Currently, about abillion people around the world routinelydrink unhealthy water. Most countriesaccepted the goal of halving by 2015 the

number of people worldwide who do nothave access to safe waterand sanitationduring the 2003 G8 Eviansummit.[58]Even if this difficult goal ismet, it will still leave more than anestimated half a billion people without

access to safe drinking water and over abillion without access to adequatesanitation. Poor water qualityand badsanitation are deadly; some five million

https://en.wikipedia.org/wiki/Sinkhttps://en.wikipedia.org/wiki/Showerhttps://en.wikipedia.org/wiki/Kitchenhttps://en.wikipedia.org/wiki/Toilethttps://en.wikipedia.org/wiki/Sanitationhttps://en.wikipedia.org/wiki/29th_G8_summithttps://en.wikipedia.org/wiki/29th_G8_summithttps://en.wikipedia.org/wiki/Water#cite_note-58https://en.wikipedia.org/wiki/Water_qualityhttps://en.wikipedia.org/wiki/Water_qualityhttps://en.wikipedia.org/wiki/Water#cite_note-58https://en.wikipedia.org/wiki/29th_G8_summithttps://en.wikipedia.org/wiki/29th_G8_summithttps://en.wikipedia.org/wiki/Sanitationhttps://en.wikipedia.org/wiki/Toilethttps://en.wikipedia.org/wiki/Kitchenhttps://en.wikipedia.org/wiki/Showerhttps://en.wikipedia.org/wiki/Sink

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deaths a year are caused by polluteddrinking water. The World Health

Organizationestimates that safewatercould prevent 1.4 million childdeaths from diarrheaeach year.[59]

Water, however, is not a finite resource,but rather re-circulated as potable waterin precipitation in quantities many

degrees of magnitude higher than humanconsumption. Therefore, it is therelatively small quantity of water inreserve in the earth (about 1% of ourdrinking water supply, which is

replenished in aquifers around every 1 to10 years), that is a non-renewableresource, and it is, rather, the distributionof potable and irrigation water which isscarce, rather than the actual amount ofit that exists on the earth. Water-poor

countries use importation of goods as theprimary method of importing water (toleave enough for local humanconsumption), since the manufacturing

https://en.wikipedia.org/wiki/World_Health_Organizationhttps://en.wikipedia.org/wiki/World_Health_Organizationhttps://en.wikipedia.org/wiki/Safe_waterhttps://en.wikipedia.org/wiki/Safe_waterhttps://en.wikipedia.org/wiki/Diarrheahttps://en.wikipedia.org/wiki/Water#cite_note-59https://en.wikipedia.org/wiki/Water#cite_note-59https://en.wikipedia.org/wiki/Water_supplyhttps://en.wikipedia.org/wiki/Non-renewable_resourcehttps://en.wikipedia.org/wiki/Non-renewable_resourcehttps://en.wikipedia.org/wiki/Non-renewable_resourcehttps://en.wikipedia.org/wiki/Non-renewable_resourcehttps://en.wikipedia.org/wiki/Water_supplyhttps://en.wikipedia.org/wiki/Water#cite_note-59https://en.wikipedia.org/wiki/Diarrheahttps://en.wikipedia.org/wiki/Safe_waterhttps://en.wikipedia.org/wiki/Safe_waterhttps://en.wikipedia.org/wiki/World_Health_Organizationhttps://en.wikipedia.org/wiki/World_Health_Organization

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process uses around 10 to 100 timesproducts' masses in water.

In the developing world, 90% ofall wastewaterstill goes untreated intolocal rivers and streams.[60]Some 50countries, with roughly a third of theworld's population, also suffer frommedium or high water stress, and 17 of

these extract more water annually than isrecharged through their natural watercycles.[61]The strain not only affectssurface freshwater bodies like rivers andlakes, but it also degrades groundwater

resources.Human usesFurther information: Water supply

Agriculture

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Water distribution in subsurface dripirrigation

Irrigationof field crops

The most important use of waterin agricultureis for irrigation, which is akey component to produce enough food.Irrigation takes up to 90% of waterwithdrawn in some developingcountries[62]and significant proportions in

more economically developed countries(in the United States, 30% of freshwaterusage is for irrigation).[63]

Fifty years ago, the common perceptionwas that water was an infinite resource.

At this time, there were fewer than halfthe current number of people on theplanet. People were not as wealthy astoday, consumed fewer calories and ate

https://en.wikipedia.org/wiki/Drip_irrigationhttps://en.wikipedia.org/wiki/Drip_irrigationhttps://en.wikipedia.org/wiki/Irrigationhttps://en.wikipedia.org/wiki/Agriculturehttps://en.wikipedia.org/wiki/Irrigationhttps://en.wikipedia.org/wiki/Water#cite_note-62https://en.wikipedia.org/wiki/Water#cite_note-Water_Use_in_the_United_States-63https://en.wikipedia.org/wiki/File:SiphonTubes.JPGhttps://en.wikipedia.org/wiki/Water#cite_note-Water_Use_in_the_United_States-63https://en.wikipedia.org/wiki/Water#cite_note-62https://en.wikipedia.org/wiki/Irrigationhttps://en.wikipedia.org/wiki/Agriculturehttps://en.wikipedia.org/wiki/Irrigationhttps://en.wikipedia.org/wiki/Drip_irrigationhttps://en.wikipedia.org/wiki/Drip_irrigation

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less meat, so less water was needed toproduce their food. They required a third

of the volume of water we presently takefrom rivers. Today, the competition forthe fixed amount of water resources ismuch more intense, giving rise to theconcept of peak water.[64]This is becausethere are now nearly seven billion people

on the planet, their consumption of water-thirsty meat and vegetables is rising, andthere is increasing competition for waterfrom industry, urbanisationand biofuelcrops. In future, even more water will beneeded to produce food because theEarth's population is forecast to rise to 9billion by 2050.[65]

An assessment of water management inagriculture was conducted in 2007 bythe International Water Management

Institutein Sri Lankato see if the worldhad sufficient water to provide food for itsgrowing population.[66]It assessed thecurrent availability of water for agriculture

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on a global scale and mapped outlocations suffering from water scarcity. It

found that a fifth of the world's people,more than 1.2 billion, live in areasof physical water scarcity, where there isnot enough water to meet all demands. Afurther 1.6 billion people live in areasexperiencing economic water scarcity,

where the lack of investment in water orinsufficient human capacity make itimpossible for authorities to satisfy thedemand for water. The report found thatit would be possible to produce the foodrequired in future, but that continuation oftoday's food production andenvironmental trends would lead to crisesin many parts of the world. To avoid aglobal water crisis, farmers will have tostrive to increase productivity to meet

growing demands for food, while industryand cities find ways to use water moreefficiently.[67]

As a scientific standard

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On 7 April 1795, the gramwas definedin Franceto be equal to "the absolute

weight of a volume of pure water equal toa cube of one hundredth of a meter, andat the temperature of melting ice."[68]Forpractical purposes though, a metallicreference standard was required, onethousand times more massive,

the kilogram. Work was thereforecommissioned to determine precisely themass of one literof water. In spite of thefact that the decreed definition of thegram specified water at 0 C a highlyreproducible temperaturethescientists chose to redefine the standardand to perform their measurements at thetemperature of highest water density,which was measured at the time as 4 C(39 F).[69]

The Kelvin temperature scaleof the SIsystem is based on the triple pointofwater, defined as exactly 273.16 K or0.01 C. The scale is an absolute

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temperaturescale with the sameincrement as the Celsius temperature

scale, which was originally definedaccording the boiling point(set to 100 C)and melting point(set to 0 C) of water.

Natural water consists mainly of theisotopes hydrogen-1 and oxygen-16, butthere is also a small quantity of heavier

isotopes such as hydrogen-2(deuterium). The amount of deuteriumoxides or heavy wateris very small, but itstill affects the properties of water. Waterfrom rivers and lakes tends to contain

less deuterium than seawater. Therefore,standard water is defined in the ViennaStandard Mean OceanWaterspecification.

For drinkingMain article: Drinking water

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A young girl drinking bottled water

Water availability: fraction of populationusing improved water sources by country

The human bodycontains from 55% to78% water, depending on bodysize.[70]To function properly, the bodyrequires between one and seven liters ofwater per dayto avoid dehydration; theprecise amount depends on the level ofactivity, temperature, humidity, and otherfactors. Most of this is ingested through

foods or beverages other than drinkingstraight water. It is not clear how muchwater intake is needed by healthy people,though most specialists agree that

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approximately 2 liters (6 to 7 glasses) ofwater daily is the minimum to maintain

proper hydration.

[71]

Medical literaturefavors a lower consumption, typically 1liter of water for an average male,excluding extra requirements due to fluidloss from exercise or warm weather.[72]

For those who have healthy kidneys, it is

rather difficult to drink too much water,but (especially in warm humid weatherand while exercising) it is dangerous todrink too little. People can drink far morewater than necessary while exercising,

however, putting them at risk ofwaterintoxication(hyperhydration), which canbe fatal.[73][74]The popular claim that "aperson should consume eight glasses ofwater per day" seems to have no realbasis in science.[75]Studies have shown

that extra water intake, especially up to500 ml at mealtime was conducive toweight loss.[76][77][78][79][80][81]Adequate fluid

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intake is helpful in preventingconstipation.[82]

Hazard symbolfor non-potable water

An original recommendation for waterintake in 1945 by the Food and NutritionBoard of the United States NationalResearch Councilread: "An ordinary

standard for diverse persons is 1 milliliterfor each calorie of food. Most of thisquantity is contained in preparedfoods."[83]The latest dietary referenceintake report by theUnited StatesNational Research Councilin generalrecommended (including food sources):3.7 liters for men and 2.7 liters of watertotal for women.[84]

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Specifically, pregnantand breastfeedingwomen need additional fluids to stay

hydrated. The Institute of Medicine(U.S.)recommends that, on average, menconsume 3.0 liters and women 2.2 liters;pregnant women should increase intaketo 2.4 liters (10 cups) and breastfeedingwomen should get 3 liters (12 cups),

since an especially large amount of fluidis lost during nursing.[85]Also noted isthat normally, about 20% of water intakecomes from food, while the rest comesfrom drinking water and beverages(caffeinatedincluded). Water is excretedfrom the body in multiple forms;through urineand feces,through sweating, and by exhalation ofwater vapor in the breath. With physicalexertion and heat exposure, water loss

will increase an

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