water cycle
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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 is an Inorganic compound, inorganic, Transparency and translucency, 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 ...

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 harbor life. About 29% of Earth's surface is land consisting of continent A continent is one of several large landmasses. Generally identified by con ...
. 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 is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere an ...

ice
,
fresh water Fresh water (or freshwater) is any naturally occurring water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is ...

fresh water
, saline water (Salt Water) and 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 or the world ocean) is the body of salt water which covers approximately 71% of the surface of the Earth.
ocean
, or from the ocean to the atmosphere, by the physical processes of
evaporation 280px, Demonstration of evaporative cooling. When the sensor is dipped in ethanol and then taken out to evaporate, the instrument shows progressively lower temperature as the ethanol evaporates. Evaporation is a type of vaporization that occu ...

evaporation
,
condensation Condensation is the change of the physical state of matter from the gas, gas phase into the liquid, liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the st ...
,
precipitation In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant p ...
,
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 is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which ...
, and subsurface flow. In doing so, the water goes through different forms: liquid, solid (
ice Ice is 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 constituent of Earth's hydrosphere an ...

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 varieties of the English language native to the United States. Currently, America ...
. 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, present in all matter, which is the source of the occurrence of heat, a flow of energy, when a body is in contact with another that is ...

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 average of weather, typically averaged over a period of 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologi ...
. 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, erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust, and then sedime ...

erosion
and
sedimentation Sedimentation is the tendency for particle (ecology), particles in Suspension (chemistry), suspension to settle out of the fluid in which they are Entrainment (engineering), entrained and come to rest against a barrier. This is due to their motio ...
. 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 File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. Lower pie represents trace gases that together compose about 0.043391% of the atmosphere (0.04402961% at April 2019 concentration ). Number ...
. 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. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and water bodies. Tra ...

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 (unit), daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The r ...
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 force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...
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, volume Volume is the quantity of three-dimensional space enclosed by a closed surface, for example, the space ...
). 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 Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significan ...
, while condensation near ground level is referred to as
fog , covers the entire city as it begins to rise and join the cloud In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecast ...

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 atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant p ...
. Some precipitation falls as snow, hail, or sleet, and can accumulate in
ice cap upright=1.35, Vatnajökull, Iceland ">Iceland.html" ;"title="Vatnajökull, Iceland">Vatnajökull, Iceland In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses coveri ...
s and
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. Glaciers slowl ...

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 is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which ...
. 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 present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usabl ...

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 water-bearing permeability (Earth sciences), permeable rock, rock fractures or unconsolidated materials (gravel, sand, or silt). Groundwater can be extracted using a water well. The study of water flow in aqu ...

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 File:Flood plain 7991.JPG, Riparian vegetation on the floodplain of the Lynches River, close to Johnsonville, South Carolina. These tupelo and Taxodium, cypress trees show the ordinary high water mark, high-water mark of flooding. A floodpl ...
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 atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmosphere by volume, excluding water vapor. L ...
: Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as
rain Rain is liquid water in the form of drop (liquid), droplets that have condensation, condensed from atmosphere, atmospheric water vapor and then precipitation (meteorology), become heavy enough to fall under gravity. Rain is a major component ...

rain
, but also includes
snow Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout i ...

snow
,
hail Hail is a form of solid precipitation In meteorology Meteorology is a branch of the atmospheric sciences Atmospheric science is the study of the Earth's atmosphere File:Atmosphere gas proportions.svg, Composition of Earth's atmo ...

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 which includes atmospheric chemistry and atmospheric physics, with a major f ...

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. On ...

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, the first computer text-formatting program * Runoff or run-off, another name for bleed (printing), bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, ...
: 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 is the 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 constituent of Earth's hyd ...
or for plant transpiration. ;
Subsurface flow Subsurface flow, in hydrology Hydrology (from Greek: wikt:ὕδωρ, ὕδωρ, "hýdōr" meaning "water" and wikt:λόγος, λόγος, "lógos" meaning "study") is the scientific study of the movement, distribution, and management of water o ...
: The flow of water underground, in the
vadose zone Image:Vadose zone.gif, 250px, Cross-section of a hillslope depicting the vadose zone, capillary fringe, water table, and phreatic or saturated zone. ''(Source: United States Geological Survey.)'' The vadose zone, also termed the unsaturated zone, ...

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 list of natural phenomena, natural phenomenon by which all things with mass or energy—including planets, stars, galaxy, galaxies, and even light—are brought toward (or ''gravitate'' toward) one another. ...

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 File:10. Ладење при испарување.ogv, 280px, Demonstration of evaporative cooling. When the sensor is dipped in ethanol and then taken out to evaporate, the instrument shows progressively lower temperature as the ethanol evapor ...

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 per unit area received from the Sun in the form of electromagnetic radiation as measured in the wavelength In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowle ...
. 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 eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions o ...

plant
s, though together they are specifically referred to as
evapotranspiration Evapotranspiration (ET) is the sum of water evaporation 280px, Demonstration of evaporative cooling. When the sensor is dipped in ethanol and then taken out to evaporate, the instrument shows progressively lower temperature as the ethanol eva ...

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, engineering, and earth sciences, advection is the transport of a substance or quantity by bulk motion of a fluid. The properties of that substance are carried with it. Generally the majority of the advected substance is a fl ...
: 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, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and Plasma (physics), plasma. ...

Condensation
: The transformation of water vapor to liquid water droplets in the air, creating
cloud In meteorology Meteorology is a branch of the atmospheric sciences which includes atmospheric chemistry and atmospheric physics, with a major focus on weather forecasting. The study of meteorology dates back millennia, though significan ...
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 list of natural phenomena, natural phenomenon by which all things with mass or energy—including planets, stars, galaxy, galaxies, and even light—are brought toward (or ''gravitate'' toward) one another. ...

gravity
. ;
Plate tectonics File:Earth cutaway schematic-en.svg, 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, tectonicus, from the grc, τεκτον ...
: 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 water-bearing permeability (Earth sciences), permeable rock, rock ...
. 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 Oce ...

Antarctica
and
Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an Autonomous administrative division, autonomous territory* * * within the Danish Realm and the List of islands by area, world's largest island, located between the Arctic Ocean, Arcti ...

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 physics and chemistry, the law of conservation of mass or principle of mass conservation states that for any closed system, system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as the syst ...
(
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 hydrologyIsotope hydrology is a field of geochemistry and 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 S ...
.


Changes over time

Global map of annual mean evaporation minus precipitation by latitude-longitude The water cycle describes the processes that drive the movement of water throughout the hydrosphere. 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 (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. The drying is projected to be strongest near the poleward margins of the subtropics (for example, the Mediterranean Basin, South Africa, southern Australia, and the Southwestern United States). 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 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 (journal), Science'' 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 Aquarius (SAC-D instrument), instrument carried by the SAC-D satellite Aquarius, launched in June, 2011, measured global sea surface salinity. Glacial retreat 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. Retreat of glaciers since 1850, Glacial retreat since 1850 has been extensive. Image:Natural & impervious cover diagrams EPA.jpg, 400px, Relationship between impervious surfaces and
surface runoff Surface runoff (also known as overland flow) is the flow of water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which ...
Human activities that alter the water cycle include: * agriculture * Industry (manufacturing), industry * alteration of the chemical composition of the atmosphere * construction of dams * deforestation and afforestation * removal of groundwater from water well, wells * water abstraction from rivers * urbanization - to counteract its impact, water-sensitive urban design 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. Without the cooling, the effect of evaporation on the greenhouse effect would lead to a much higher surface temperature of , and a warmer planet. Aquifer drawdown (hydrology), drawdown or overdrafting 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 erosion, eroded sediment and phosphorus from land to waterbody, waterbodies. The salinity of the oceans is derived from erosion and transport of dissolved salts from the land. Cultural eutrophication of lakes is primarily due to phosphorus, applied in excess to field (agriculture), agricultural fields in fertilizers, 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 (ecology), dead zone at the outlet of the Mississippi River is a consequence of nitrates from fertilizer being carried off agricultural fields and funnelled down the river system to the Gulf of Mexico. Runoff also plays a part in the 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 to move up to the exobase, 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, entering outer space without impacting other particles of gas. This type of gas loss from a planet into space is known as planetary wind. 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 (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, 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, 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, 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 (570 BCE) (who also speculated about the History of evolutionary thought, evolution of land animals from fish) and Xenophanes, 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 Anaxagoras, Anaxagoras of Clazomenae (460 BCE) and Diogenes of Apollonia (460 BCE). Both Plato (390 BCE) and 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 (Aristotle), Meteorology,'' writing "By it [the sun's] agency 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

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References


Further reading

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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