Editing Hydrosphere (section)

==Water cycle==
{{Main|Water cycle}}

[[File:Watercyclesummary.jpg|right|393x393px]]

The ''water cycle'' refers to the transfer of water from one state or reservoir to another. Reservoirs include [[Water vapor|atmospheric moisture]] (snow, rain and clouds), streams, oceans, rivers, lakes, [[groundwater]], [[wikt:subterranean#English|subterranean]] [[aquifer]]s, [[polar ice cap]]s and saturated soil. [[Solar energy]], in the form of heat and light ([[insolation]]), and [[gravity]] cause the transfer from one state to another over periods from hours to thousands of years. Most [[evaporation]] comes from the oceans and is returned to the earth as snow or rain.<ref name=deVilliersWater2003 />{{rp|27}}[[Sublimation (phase transition)|Sublimation]] refers to evaporation from snow and ice. Transpiration refers to the expiration of water through the minute pores or stomata of trees. [[Evapotranspiration]] is the term used by [[hydrology|hydrologists]] in reference to the three processes together, transpiration, sublimation and evaporation.<ref name=deVilliersWater2003 />

[[Marq de Villiers]] has described the hydrosphere as a [[closed system]] in which water exists. The hydrosphere is intricate, complex, interdependent, all-pervading, stable, and "seems purpose-built for regulating life."<ref name=deVilliersWater2003>{{cite book|author= Marq de Villiers|title= Water: The Fate of Our Most Precious Resource|year=2003|publisher= McClelland & Stewart|location=Toronto, Ontario|isbn=978-0-7710-2641-6|oclc = 43365804|edition=2|pages=453}}, revised 2003|[[Governor General's Award for English-language non-fiction|Governor General's Award]] (1999)</ref>{{rp|26}} De Villiers claimed that, "On earth, the total amount of water has almost certainly not changed since geological times: what we had then we still have. Water can be polluted, abused, and misused but it is neither created nor destroyed, it only migrates. There is no evidence that water vapor escapes into space."<ref name=deVilliersWater2003 />{{rp|26}}<blockquote>Every year the turnover of water on Earth involves 577,000 km<sup>3</sup> of water. This is water that evaporates from the oceanic surface (502,800 km<sup>3</sup>) and from land (74,200 km<sup>3</sup>). The same amount of water falls as atmospheric precipitation, 458,000 km<sup>3</sup> on the ocean and 119,000 km<sup>3</sup> on land. The difference between precipitation and evaporation from the land surface (119,000 − 74,200 = 44,800 km<sup>3</sup>/year) represents the total runoff of the Earth's rivers (42,700 km<sup>3</sup>/year) and direct groundwater runoff to the ocean (2100 km<sup>3</sup>/year). These are the principal sources of fresh water to support life necessities and man's economic activities.<ref name="Shiklomanov1998" /></blockquote>Water is a basic necessity of life. Since two thirds of the Earth is covered by water, the Earth is also called the blue planet and the watery planet.<ref group=" notes">According to the planetary geologist, [[Ronald Greeley]], "Water is very common in the outer solar system."{{citation needed|date=November 2014}} Europa holds more water than earth's oceans.</ref> The hydrosphere plays an important role in the existence of the atmosphere in its present form. Oceans are important in this regard. When the Earth was formed it had only a very thin atmosphere rich in hydrogen and helium similar to the present atmosphere of Mercury. Later the gases hydrogen and helium were expelled from the atmosphere. The gases and water vapor released as the Earth cooled became its present atmosphere. Other gases and water vapor released by volcanoes also entered the atmosphere. As the Earth cooled the water vapor in the atmosphere condensed and fell as rain. The atmosphere cooled further as atmospheric carbon dioxide dissolved into the rain water. In turn, this further caused water vapor to condense and fall as rain. This rain water filled the depressions on the Earth's surface and formed the oceans. It is estimated that this occurred about 4000 million years ago. The first life forms began in the oceans. These organisms did not breathe oxygen. Later, when [[cyanobacteria]] evolved, the process of conversion of carbon dioxide into food and oxygen began. As a result, Earth's atmosphere has a distinctly different composition from that of other planets and allowed for [[Evolutionary history of life|life to evolve on Earth]].

Human activity has had an impact on the water cycle. Infrastructure, like dams, have a clear, direct impact on the water cycle by blocking and redirecting water pathways. Human caused pollution has changed the biogeochemical cycles of some water systems, and climate change has significantly altered weather patterns.<ref>{{cite journal |last1=Western |first1=David |title=Human-Modified Ecosystems and Future Evolution |journal=Proceedings of the National Academy of Sciences of the United States of America |date=2001 |volume=98 |issue=10 |pages=5458–5465 |doi=10.1073/pnas.101093598 |jstor=3055648 |pmid=11344294 |pmc=33234 |bibcode=2001PNAS...98.5458W |doi-access=free }}</ref> Water withdrawals have exponentially increased because of agriculture, state and domestic use, and infrastructure.<ref>{{cite journal |last1=Haddeland |first1=Ingjerd |last2=Heinke |first2=Jens |last3=Biemans |first3=Hester |last4=Eisner |first4=Stephanie |last5=Flörke |first5=Martina |last6=Hanasaki |first6=Naota |last7=Konzmann |first7=Markus |last8=Ludwig |first8=Fulco |last9=Masaki |first9=Yoshimitsu |last10=Schewe |first10=Jacob |last11=Stacke |first11=Tobias |last12=Tessler |first12=Zachary D. |last13=Wada |first13=Yoshihide |last14=Wisser |first14=Dominik |title=Global water resources affected by human interventions and climate change |journal=Proceedings of the National Academy of Sciences of the United States of America |date=2014 |volume=111 |issue=9 |pages=3251–3256 |doi=10.1073/pnas.1222475110 |jstor=23770678 |pmid=24344275 |pmc=3948259 |bibcode=2014PNAS..111.3251H |doi-access=free }}</ref>