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The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a
biogeochemical cycle In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Topics of interest include the bi ...
that describes the continuous movement of
water Water (chemical formula H2O) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known li ...

water
on, above and below the surface of the
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

Earth
. The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of
ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , eve ...

ice
,
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in ...

fresh water
, saline water (Salt Water) and
atmospheric water
atmospheric water
is variable depending on a wide range of climatic variables. The water moves from one reservoir to another, such as from river to
ocean The ocean (also the sea The sea, connected as the world ocean or simply the ocean The ocean (also the sea or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
, or from the ocean to the atmosphere, by the physical processes of
evaporation Evaporation is a type of vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phe ...

evaporation
,
condensation Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

condensation
,
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
,
infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (med ...
,
surface runoff Surface runoff (also known as overland flow) is the flow of water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it ac ...
, and subsurface flow. In doing so, the water goes through different forms: liquid, solid (
ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known forms of , eve ...

ice
) and
vapor In physics, a vapor (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United St ...
. The water cycle involves the exchange of energy, which leads to
temperature Temperature ( ) is a physical quantity that expresses hot and cold. It is the manifestation of thermal energy Thermal radiation in visible light can be seen on this hot metalwork. Thermal energy refers to several distinct physical concept ...

temperature
changes. When water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence
climate Climate is the long-term pattern of weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a la ...

climate
. The evaporative phase of the cycle purifies water which then replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the Earth, through processes including
erosion In earth science Earth science or geoscience includes all fields of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific ...

erosion
and
sedimentation Sedimentation is the deposition of sediments Sediment is a naturally occurring material that is broken down by processes of weathering Weathering is the deterioration of rocks In geology Geology (from the Ancient Greek γ ...
. The water cycle is also essential for the maintenance of most life and ecosystems on the planet.


Description

The sun, which drives the water cycle, heats water in the ocean and seas. Water evaporates as water vapor into the
air The atmosphere of Earth is the layer of gas Gas is one of the four fundamental states of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (ph ...

air
. Some ice and snow sublimates directly into water vapor.
Evapotranspiration Evapotranspiration (ET) is the sum of water evaporation and transpiration from a surface area to the Atmosphere of Earth, atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and ...

Evapotranspiration
is water
transpired
transpired
from plants and evaporated from the soil. The water molecule has smaller
molecular mass The molecular mass (''m'') is the mass of a given molecule: it is measured in dalton Dalton may refer to: Science * Dalton (crater), a lunar crater * Dalton (program), chemistry software * Dalton (unit) (Da), the atomic mass unit Entertainmen ...
than the major components of the atmosphere, nitrogen () and oxygen () and hence is less dense. Due to the significant difference in density,
buoyancy Buoyancy (), or upthrust, is an upward exerted by a that opposes the of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bo ...

buoyancy
drives humid air higher. As altitude increases, air pressure decreases and the temperature drops (see
Gas laws The gas laws were developed at the end of the 18th century, when scientists began to realize that relationships between pressure Pressure (symbol: ''p'' or ''P'') is the force In physics Physics (from grc, φυσική (ἐ ...
). The lower temperature causes water vapor to condense into tiny liquid water droplets which are heavier than the air, and which fall unless supported by an updraft. A huge concentration of these droplets over a large area in the atmosphere become visible as
cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planetary body or similar space. Wate ...

cloud
, while condensation near ground level is referred to as
fog Fog is a visible aerosol consisting of tiny water drop (liquid), droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus cloud, s ...

fog
.
Atmospheric circulation Atmospheric circulation is the large-scale movement of Atmosphere of Earth, air and together with ocean circulation is the means by which thermal energy is redistributed on the surface of the Earth. The Earth's atmospheric circulation varies from ...

Atmospheric circulation
moves water vapor around the globe; cloud particles collide, grow, and fall out of the upper atmospheric layers as
precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
. Some precipitation falls as snow, hail, or sleet, and can accumulate in
ice cap In glaciology Lateral moraine on a glacier joining the Gorner Glacier, Zermatt">Gorner_Glacier.html" ;"title="moraine on a glacier joining the Gorner Glacier">moraine on a glacier joining the Gorner Glacier, Zermatt, Swiss Alps. The moraine is ...
s and
glacier A glacier (; ) is a persistent body of dense ice Ice is into a state. Depending on the presence of such as particles of soil or bubbles of air, it can appear transparent or a more or less bluish-white color. In the , ice is abunda ...

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 Surface runoff (also known as overland flow) is the flow of water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it ac ...
. A portion of this runoff enters rivers, with streamflow moving water towards the oceans. Runoff and water emerging from the ground (
groundwater Groundwater is the water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). It is vital for all known form ...

groundwater
) may be stored as freshwater in lakes. Not all runoff flows into rivers; much of it soaks into the ground as
infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (med ...
. Some water infiltrates deep into the ground and replenishes
aquifer An aquifer is an underground layer of -bearing , rock fractures or unconsolidated materials (, , or ). can be extracted using a water . The study of water flow in aquifers and the characterization of aquifers is called . Related terms include a ...

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. Some groundwater finds openings in the land surface and emerges as freshwater springs. In river valleys and
floodplain A floodplain or flood plain or bottomlands is an area of land adjacent to a river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows int ...
s, there is often continuous water exchange between surface water and ground water in the
hyporheic zone The hyporheic zone is the region of sediment and porous space beneath and alongside a stream bed A stream is a body of water ( Lysefjord) in Norway Norway ( nb, ; nn, ; se, Norga; smj, Vuodna; sma, Nöörje), officially the ...
. Over time, the water returns to the ocean, to continue the water cycle. File:Earth's Water Cycle.ogv, Earth's water cycle File:The Water Cycle.ogv, As the Earth's surface water evaporates, wind moves water in the air from the sea to the land, increasing the amount of freshwater on land. File:The Water Cycle Watering the Land.ogv, Water vapor is converted to clouds that bring fresh water to land in the form of rain snow and sleet File:The Water Cycle - Following the Water.ogv, Precipitation falls on the ground, but what happens to that water depends greatly on the geography of the land at any particular place.


Deep water recycling

The deep water cycle (also called the geological water cycle) is the exchange of water with the Earth's mantle, via subduction zones and volcanic activity, and is distinguished from the cycling of water above and on the surface of the planet in the Hydrologic Cycle. The process of deep water recycling involves water entering the mantle by being carried down by subducting oceanic plates (a process known as regassing) being balanced by water being released at mid-ocean ridges (degassing). This is a central concept in the understanding of the long‐term exchange of water between the earth's interior and the
exosphere The exosphere ( grc, ἔξω "outside, external, beyond", grc, σφαῖρα "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density ...
and the transport of water bound in hydrous minerals. An imbalance in deep water recycling has been proposed as one mechanism that can affect global sea levels.


Processes

;
Precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw mod ...
: Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as
rain Rain is liquid water in the form of droplet Rain water flux from a canopy. Among the forces that govern drop formation: cohesion, Van der Waals force">Cohesion_(chemistry).html" ;"title="surface tension, Cohesion (chemistry)">cohesion, ...

rain
, but also includes
snow Snow comprises individual ice Ice is water Water (chemical formula H2O) is an , transparent, tasteless, odorless, and , which is the main constituent of 's and the s of all known living organisms (in which it acts as a ). ...

snow
,
hail Hail is a form of solid precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until ...

hail
,
fog drip Fog drip is water dripping to the ground during fog. It occurs when water droplets from the fog adhere to the needles or leaves of trees or other objects, coalesce into larger drops and then drop to the ground. Fog drip can be an important sourc ...
,
graupel Graupel (; ), also called soft hail, corn snow, hominy snow, or snow pellets, is precipitation In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmo ...

graupel
, and . Approximately of water falls as precipitation each year, of it over the oceans. The rain on land contains of water per year and a snowing only . 78% of global precipitation occurs over the ocean. ;
Subduction Subduction is a geological process in which the oceanic lithosphere A lithosphere ( grc, λίθος [] for "rocky", and [] for "sphere") is the rigid, outermost shell of a terrestrial planet, terrestrial-type planet or natural satellite. O ...

Subduction
&
Mineral hydration Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a '' hydrate''. In geological terms, the process of mineral hydration is known as ''ret ...
: Sea water seeps into the oceanic lithosphere through fractures and pores, and reacts with minerals in the crust and mantle to form hydrous minerals (such as serpentine) that store water in their crystal structures. Water is transported into the deep mantle ''via'' hydrous minerals in subducting slabs. During subduction, a series of minerals in these slabs such as
serpentine Serpentine may refer to: Music * Serpentine (album), ''Serpentine'' (album), a 2002 goth metal album by Flowing Tears * Serpentine (song), "Serpentine" (song), a 2011 country song by Tiffany * Serpentines (Ingrid Laubrock album), ''Serpentines'' ( ...

serpentine
… can be stable at different pressures within the slab geotherms, and may transport significant amount of water into the Earth's interior. As plates sink and heat up, released fluids can trigger seismicity and induce melting within the subducted plate and in the overlying mantle wedge. This type of melting selectively concentrates volatiles and transports them into the overlying plate. If an eruption occurs, the cycle then returns the volatiles into the oceans and atmosphere ; Canopy interception: The precipitation that is intercepted by plant foliage eventually evaporates back to the atmosphere rather than falling to the ground. ; Snow melt: The runoff produced by melting snow. ;
Runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed (printing), bleed, printing that lies beyond the edges to wh ...
: 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. ;
Infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (med ...
: The flow of water from the ground surface into the ground. Once infiltrated, the water becomes
soil moisture Soil moisture is the water content 300px, Soil composition by Volume and Mass, by phase: air, water, void (pores filled with water or air), soil, and total. Water content or moisture content is the quantity of water Water is an Inorganic ...
or groundwater. A recent global study using water stable isotopes, however, shows that not all soil moisture is equally available for
groundwater recharge Groundwater recharge or deep drainage or deep percolation is a process, where moves downward from to . Recharge is the primary method through which water enters an . This process usually occurs in the below plant s and, is often expressed as ...
or for plant transpiration. ;
Subsurface flow Subsurface flow, in hydrology Hydrology (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its popul ...
: The flow of water underground, in the
vadose zone The vadose zone, also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone The phreatic zone, or zone of saturation, is the part of an aquifer, below the water table, in which relatively a ...

vadose zone
and
aquifers An aquifer is an underground layer of water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constitue ...

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 Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

gravity
or gravity induced pressures. Groundwater tends to move slowly and is replenished slowly, so it can remain in aquifers for thousands of years. ;
Evaporation Evaporation is a type of vaporization Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phe ...

Evaporation
: The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. The source of energy for evaporation is primarily
solar radiation Solar irradiance is the power Power typically refers to: * Power (physics) In physics, power is the amount of energy transferred or converted per unit time. In the International System of Units, the unit of power is the watt, equal to one j ...
. Evaporation often implicitly includes
transpiration in a tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as a tomato plant. The species originated in western South America South America is a continent entirely in the Western Hemisphere ...

transpiration
from
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

plant
s, though together they are specifically referred to as
evapotranspiration Evapotranspiration (ET) is the sum of water evaporation and transpiration from a surface area to the Atmosphere of Earth, atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and ...

evapotranspiration
. Total annual evapotranspiration amounts to approximately of water, of which evaporates from the oceans. 86% of global evaporation occurs over the ocean. ; Sublimation: The state change directly from solid water (snow or ice) to water vapor by passing the liquid state. ;
Deposition Deposition may refer to: * Deposition (law), taking testimony outside of court * List of deposed politicians, Deposition (politics), the removal of a person of authority from political power * Deposition (university), a widespread initiation ritual ...
: This refers to changing of water vapor directly to ice. ;
Advection In the field of physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy ...
: The movement of water through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land. ;
Condensation Condensation is the change of the state of matter In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space ...

Condensation
: The transformation of water vapor to liquid water droplets in the air, creating
cloud In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid drop (liquid), droplets, ice crystals, frozen crystals, or other particulates, particles suspended in the atmosphere of a planetary body or similar space. Wate ...

cloud
s and fog. ;
Transpiration in a tomato The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as a tomato plant. The species originated in western South America South America is a continent entirely in the Western Hemisphere ...

Transpiration
: The release of water vapor from plants and soil into the air. ;
Percolation Fig. 2: Percolation in a square lattice (Click to animate) In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ...

Percolation
: Water flows vertically through the soil and rocks under the influence of
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

gravity
. ;
Plate tectonics upright=1.35, Diagram of the internal layering of Earth showing the lithosphere above the asthenosphere (not to scale) Plate tectonics (from the la, label=Late Latin Late Latin ( la, Latinitas serior) is the scholarly name for the written L ...
: Water enters the mantle via subduction of oceanic crust. Water returns to the surface via volcanism. The water cycle involves many of these processes. File:Diagram of the Water Cycle.jpg, Diagram of the water cycle File:Watercyclesummary.jpg, Natural water cycle


Residence times

The ''
residence time The residence time of a fluid parcelIn fluid dynamics In physics and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids—liquids and gases. It has several subdisciplines, including aerodynamics ...
'' of a reservoir within the hydrologic cycle is the average time a water molecule will spend in that reservoir (''see adjacent 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. Particularly old groundwater is called
fossil water Fossil water or paleowater is an ancient body of water that has been contained in some undisturbed space, typically groundwater in an aquifer An aquifer is an underground layer of -bearing , rock fractures or unconsolidated materials (, , or ). ...
. 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 Antarctica ( or ) is Earth's southernmost continent. It contains the geographic South Pole and is situated in the Antarctic region of the Southern Hemisphere, almost entirely south of the Antarctic Circle, and is surrounded by the Southern Oc ...

Antarctica
and
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an autonomous territory An autonomous administrative division (also referred to as an autonomous area, entity, unit, region, subdivision, or territory) is a subnational administra ...

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. In hydrology, residence times can be estimated in two ways. The more common method relies on the principle of
conservation of mass In and , the law of conservation of mass or principle of mass conservation states that for any to all transfers of and , the of the system must remain constant over time, as the system's mass cannot change, so quantity can neither be added n ...
(
water balance The law of water balance states that the inflows to any water system or area is equal to its outflows plus change in storage during a time interval. In hydrology, a water balance equation can be used to describe the flow of water in and out of a ...

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 Isotope hydrology is a field of geochemistry and hydrology Hydrology (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a cou ...
.


Changes over time

The water cycle describes the processes that drive the movement of water throughout the
hydrosphere The hydrosphere (from Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is app ...
. However, much more water is "in storage" 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 332,500,000 mi3 (1,386,000,000 km3) of the world's water supply, about 321,000,000 mi3 (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. During colder climatic periods, more ice caps and glaciers form, and enough of the global water supply accumulates as ice to lessen the amounts in other parts of the water cycle. The reverse is true during warm periods. During the last ice age, glaciers covered almost one-third of Earth's land mass with the result being that the oceans were about 122 m (400 ft) lower than today. During the last global "warm spell," about 125,000 years ago, the seas were about 5.5 m (18 ft) higher than they are now. About three million years ago the oceans could have been up to 50 m (165 ft) higher. The scientific consensus expressed in the 2007
Intergovernmental Panel on Climate Change The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations responsible for advancing knowledge on Attribution of recent climate change, human-induced climate change. It was established in 1988 by the ...
(IPCC) Summary for Policymakers is for the water cycle to continue to intensify throughout the 21st century, though this does not mean that precipitation will increase in all regions. In subtropical land areasplaces that are already relatively dryprecipitation is projected to decrease during the 21st century, increasing the probability of
drought A drought is an event of prolonged shortages in the water supply, whether atmospheric (below-average precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorolog ...

drought
. The drying is projected to be strongest near the poleward margins of the
subtropics The subtropical zones or subtropics are geographical zone, geographical and Köppen climate classification, climate zones located to the north and south of the tropics, Torrid Zone. Geographically part of the Geographical zone#Temperate zones, ...

subtropics
(for example, the Mediterranean Basin, South Africa, southern Australia, and the
Southwestern United States The Southwestern United States, also known as the American Southwest or simply the Southwest, is a geographic and cultural list of regions of the United States, region of the United States that generally includes Arizona, New Mexico, and adjacen ...
). Annual precipitation amounts are expected to increase in near-equatorial regions that tend to be wet in the present climate, and also at high latitudes. These large-scale patterns are present in nearly all of the
climate model Numerical climate models use quantitative methods to simulate the interactions of the important drivers of climate, including atmosphere, oceans, land surface and ice. They are used for a variety of purposes from study of the dynamics of the ...
simulations conducted at several international research centers as part of the 4th Assessment of the IPCC. There is now ample evidence that increased hydrologic variability and change in climate has and will continue to have a profound impact on the water sector through the hydrologic cycle, water availability, water demand, and water allocation at the global, regional, basin, and local levels. Research published in 2012 in ''
Science Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is something that is truth, true. The usual test for a statement of ...
'' based on surface ocean salinity over the period 1950 to 2000 confirm this projection of an intensified global water cycle with salty areas becoming more saline and fresher areas becoming more fresh over the period:
Fundamental thermodynamics and climate models suggest that dry regions will become drier and wet regions will become wetter in response to warming. Efforts to detect this long-term response in sparse surface observations of rainfall and evaporation remain ambiguous. We show that ocean salinity patterns express an identifiable fingerprint of an intensifying water cycle. Our 50-year observed global surface salinity changes, combined with changes from global climate models, present robust evidence of an intensified global water cycle at a rate of 8 ± 5% per degree of surface warming. This rate is double the response projected by current-generation climate models and suggests that a substantial (16 to 24%) intensification of the global water cycle will occur in a future 2° to 3° warmer world.
An
instrument Instrument may refer to: Science and technology * Flight instruments two-seat light airplane. The flight instruments are visible on the left of the instrument panel Flight instruments are the instruments in the cockpit of an aircraft that pro ...
carried by the
SAC-D SAC-D ( es, Satélite de Aplicaciones Científicas-D, meaning ''Satellite for Scientific Applications-D''), also known as Aquarius after its primary instrument, is an Argentine Argentines (also known as Argentinians or Argentineans; es, ...
satellite Aquarius, launched in June, 2011, measured global sea surface
salinity Salinity () is the saltiness or amount of salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific ...

salinity
.
Glacial retreat Image:Glacial lakes, Bhutan.jpg, 300px, Termini of the glaciers in the Bhutan-Himalaya. Glacial lakes have been rapidly forming on the surface of the debris-covered glaciers in this region during the last few decades. USGS researchers have found ...
is also an example of a changing water cycle, where the supply of water to glaciers from precipitation cannot keep up with the loss of water from melting and sublimation. Glacial retreat since 1850 has been extensive. Human activities that alter the water cycle include: *
agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise trends, Increases in sedentary behaviors su ...

agriculture
*
industry Industry may refer to: Economics * Industry (economics) In macroeconomics, an industry is a branch of an economy that produces a closely related set of raw materials, goods, or services. For example, one might refer to the wood industry ...
* alteration of the chemical composition of the atmosphere * construction of
dam A dam is a barrier that stops or restricts the flow of surface water An example of surface water is Lake Kinney. Surface water is water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tas ...

dam
s *
deforestation deforestation in 1750-2004 (net loss) showing anthropogenic modification of remaining forest. File:MODIS (2020-08-01).jpg, 300px, Dry seasons, exacerbated by climate change, and the use of slash-and-burn methods for clearing tropical forest ...

deforestation
and
afforestation Afforestation is the establishment of a forest or stand of trees (forestation Forestation is either growing existing forests (proforestation) or establishing forest growth on areas that either had forest or lacked it naturally. In the first ...
* removal of groundwater from
wells Wells most commonly refers to: * Wells, Somerset, a cathedral city in Somerset, England * Well, an excavation or structure created in the ground * Wells (name) Wells may also refer to: Places ;Canada *Wells, British Columbia ;England * Wells ( ...

wells
* water abstraction from rivers *
urbanization Urbanization (or urbanisation) refers to the population shift from rural File:Rural landscape in Finland.jpg, A rural landscape in Lappeenranta, South Karelia, Finland. 15 July 2000. In general, a rural area or a countryside is a geographi ...
- to counteract its impact,
water-sensitive urban design Water-sensitive urban design (WSUD) is a land planning and engineering design approach which integrates the urban water cycle, including stormwater, groundwater and wastewater management and water supply, into urban design to minimise environmen ...
can be practiced


Effects on climate

The water cycle is powered from solar energy. 86% of the global evaporation occurs from the oceans, reducing their temperature by
evaporative cooling An evaporative cooler (also evaporative air conditioner, swamp cooler, swamp box, desert cooler and wet air cooler) is a device that cools air through the evaporation Evaporation is a type of that occurs on the of a as it changes int ...
. Without the cooling, the effect of evaporation on the
greenhouse effect The greenhouse effect is the process by which radiation from a planet's atmosphere warms the planet's surface to a temperature above what it would be without this atmosphere. Radiatively active gases (i.e., greenhouse gas A greenhou ...

greenhouse effect
would lead to a much higher surface temperature of , and a warmer planet. Aquifer drawdown or
overdrafting Overdrafting is the process of extracting groundwater beyond the equilibrium yield of the aquifer. Groundwater is the fresh water Fresh water (or freshwater) is any naturally occurring water Water is an Inorganic compound, inorgan ...
and the pumping of fossil water increases the total amount of water in the hydrosphere, and has been postulated to be a contributor to sea-level rise.


Effects on biogeochemical cycling

While the water cycle is itself a
biogeochemical cycle In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Topics of interest include the bi ...
, flow of water over and beneath the Earth is a key component of the cycling of other biogeochemicals. Runoff is responsible for almost all of the transport of
eroded In earth science Earth science or geoscience includes all fields of natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific ...

eroded
sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently sediment transport, transported by the action of wind, water, or ice or by the force of gravity acting on the particles. ...

sediment
and
phosphorus Phosphorus is a chemical element In chemistry, an element is a pure Chemical substance, substance consisting only of atoms that all have the same numbers of protons in their atomic nucleus, nuclei. Unlike chemical compounds, chemical el ...

phosphorus
from land to
waterbodies (Lysefjord) in Norway Norway, officially the Kingdom of Norway,Names in the official and recognised languages: Bokmål Bokmål (, ; literally "book tongue") is an official written standard for the Norwegian language, alongside Nynorsk. ...

waterbodies
. The
salinity Salinity () is the saltiness or amount of salt Salt is a mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific ...

salinity
of the oceans is derived from erosion and transport of dissolved salts from the land. Cultural
eutrophication Eutrophication (from Greek ''eutrophos'', "well-nourished") is the process by which an entire body of water (Lysefjord) in Norway Norway, officially the Kingdom of Norway,Names in the official and recognised languages: Bokmål Bokm ...

eutrophication
of lakes is primarily due to phosphorus, applied in excess to agricultural fields in
fertilizer A fertilizer (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American E ...

fertilizer
s, and then transported overland and down rivers. Both runoff and groundwater flow play significant roles in transporting nitrogen from the land to waterbodies. The dead zone at the outlet of the
Mississippi River The Mississippi River is the second-longest river and chief river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and b ...

Mississippi River
is a consequence of
nitrate Nitrate is a polyatomic ion A polyatomic ion, also known as a molecular ion, is a covalently bonded A covalent bond is a chemical bond A chemical bond is a lasting attraction between atoms, ions or molecules that enables the format ...

nitrate
s from fertilizer being carried off agricultural fields and funnelled down the
river system In geomorphology incised into shale at the foot of the North Caineville Plateau, Utah, within the pass carved by the Fremont River (Utah), Fremont River and known as the Blue Gate. Grove Karl Gilbert, GK Gilbert studied the landscapes of this ar ...
to the
Gulf of Mexico The Gulf of Mexico ( es, Golfo de México) is an ocean basin 400px, Diagrammatic cross-section of an ocean basin, showing the various geographic features In hydrology Hydrology (from Greek: wikt:ὕδωρ, ὕδωρ, "hýdōr" meaning ...

Gulf of Mexico
. Runoff also plays a part in the
carbon cycle The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and Earth's atmosphere, atmosphere of the Earth. Carbon is the main component of biological compounds as well as ...

carbon cycle
, again through the transport of eroded rock and soil.


Slow loss over geologic time

The hydrodynamic wind within the upper portion of a planet's atmosphere allows light chemical elements such as
Hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

Hydrogen
to move up to the
exobase The thermopause is the Earth's atmosphere, atmospheric boundary of Earth's energy system, located at the top of the thermosphere. The temperature of the thermopause could range from nearly absolute zero to . Below this, the atmosphere is defined to ...
, the lower limit of the
exosphere The exosphere ( grc, ἔξω "outside, external, beyond", grc, σφαῖρα "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density ...
, where the gases can then reach
escape velocity #REDIRECT Escape velocity#REDIRECT Escape velocity In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matt ...
, entering
outer space Outer space, commonly shortened to space, is the expanse that exists beyond Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting ...
without impacting other particles of gas. This type of gas loss from a planet into space is known as
planetary wind Atmospheric escape is the loss of planetary Atmosphere, atmospheric gases to outer space. A number of different mechanisms can be responsible for atmospheric escape; these processes can be divided into thermal escape, non-thermal (or suprathermal) ...
. Planets with hot lower atmospheres could result in humid upper atmospheres that accelerate the loss of hydrogen.


History of hydrologic cycle theory


Floating land mass

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 Homer (; grc, Ὅμηρος , ''Hómēros'') was an ancient Greek Ancient Greek includes the forms of the Greek language Greek ( el, label=Modern Greek Modern Greek (, , or , ''Kiní Neoellinikí Glóssa''), generally re ...

Homer
(circa 800 BCE).


Hebrew Bible

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"
Ecclesiastes 1:6-7
. Scholars are not in agreement as to the date of Ecclesiastes, though most scholars point to a date during the time of
King Solomon of the King of the Romans (variant used in the early modern period) File:Nezahualpiltzintli.jpg, Aztec King Nezahualpiltzintli of Texcoco (altepetl), Texcoco King is the title given to a male monarch in a variety of contexts. The female eq ...

King Solomon
, son of David and Bathsheba, "three thousand years ago, there is some agreement that the time period is 962–922 BCE. Furthermore, it was also observed that when the clouds were full, they emptied rain on the earth
Ecclesiastes 11:3
. In addition, during 793–740 BCE a Hebrew prophet, Amos, stated that water comes from the sea and is poured out on the earth
Amos 5:8
. In the Biblical
Book of Job The Book of Job (; he, אִיּוֹב – ''ʾIyyōḇ'') is a book of the Hebrew Bible. It addresses the problem of theodicy, meaning why God permits evil in the world, through the experiences of the eponymous protagonist. Job (biblic ...
, dated between 7th and 2nd centuries BCE, there is a description of precipitation in the hydrologic cycle, "For he maketh small the drops of water: they pour down rain according to the vapour thereof; which the clouds do drop and distil upon man abundantly"
Job 36:27-28
.


Precipitation and percolation

In the Adityahridayam (a devotional hymn to the Sun God) of
Ramayana ''Rāmāyana'' (; sa, रामायणम्, ) is one of the two major Sanskrit literature, Sanskrit Indian epic poetry, epics of ancient India and important text of Hinduism, the other being the ''Mahabharata, Mahābhārata''. The epi ...

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 Anaximander (; grc-gre, Ἀναξίμανδρος ''Anaximandros''; ) was a who lived in ,"Anaximander" in '. London: , 1961, Vol. 1, p. 403. a city of (in modern-day Turkey). He belonged to the and learned the teachings of his master . He s ...

Anaximander
(570 BCE) (who also speculated about the evolution of land animals from fish) and Xenophanes of Colophon (530 BCE). Chinese scholars such as Chi Ni Tzu (320 BCE) and Lu Shih Ch'un Ch'iu (239 BCE) had similar thoughts. The idea that the water cycle is a closed cycle can be found in the works of (460 BCE) and
Diogenes of Apollonia Diogenes of Apollonia ( ; grc, Διογένης ὁ Ἀπολλωνιάτης, Diogénēs ho Apollōniátēs; 5th century BC) was an ancient Greek philosopher, and was a native of the Milesian colony Apollonia in Thrace Map of Ancient Thr ...
(460 BCE). Both
Plato Plato ( ; grc-gre, Πλάτων ; 428/427 or 424/423 – 348/347 BC) was an Classical Athens, Athenian philosopher during the Classical Greece, Classical period in Ancient Greece, founder of the Platonist school of thought and the Platoni ...

Plato
(390 BCE) and
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...

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 Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorology dates back , though significant progress in meteorology did not begin until the 18th century. The 19th century saw modest progress in the f ...
,'' writing "By it he sun'sagency 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.


Precipitation alone

Up to the time of the Renaissance, it was thought 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. Bartholomeus Anglicus, 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 (scientist), Pierre Perrault. Even then, these beliefs were not accepted in mainstream science until the early nineteenth century.


See also

* * * * * * * * * *


References


Further reading

*


External links


The Water Cycle
United States Geological Survey

United States Geological Survey

from ''Dr. Art's Guide to the Planet''.

1 Mb Flash multilingual animation highlighting the often-overlooked evaporation from bare soil, from managingwholes.com.

– Climate research summary from NOAA Geophysical Fluid Dynamics Laboratory including text, graphics, and animations {{DEFAULTSORT:Water Cycle Biogeochemical cycle Forms of water Hydrology Soil physics Water Articles containing video clips Limnology Oceanography