Water cycle

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Diagram depicting the global water cycle. USGS WaterCycle English ONLINE 20221013.png
Diagram depicting the global water cycle.

The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, 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, or from the ocean to the atmosphere, by the physical processes of evaporation, transpiration, condensation, precipitation, infiltration, surface runoff, and subsurface flow. In doing so, the water goes through different forms: liquid, solid (ice) and vapor. The ocean plays a key role in the water cycle as it is the source of 86% of global evaporation. [1]


The water cycle involves the exchange of energy, which leads to 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.

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 and sedimentation. The water cycle is also essential for the maintenance of most life and ecosystems on the planet.


Diagram of the water cycle Diagram of the Water Cycle.jpg
Diagram of the water cycle
Video of the Earth's water cycle (NASA) [2]

Overall process

The water cycle is powered from the energy emitted by the sun. This energy heats water in the ocean and seas. Water evaporates as water vapor into the air. Some ice and snow sublimates directly into water vapor. Evapotranspiration is water transpired from plants and evaporated from the soil. The water molecule H
has smaller molecular mass than the major components of the atmosphere, nitrogen (N
) and oxygen (O
) and hence is less dense. Due to the significant difference in density, buoyancy drives humid air higher. As altitude increases, air pressure decreases and the temperature drops (see Gas laws). 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, while condensation near ground level is referred to as fog.

Atmospheric circulation moves water vapor around the globe; cloud particles collide, grow, and fall out of the upper atmospheric layers as precipitation. Some precipitation falls as snow, hail, or sleet, and can accumulate in ice caps and glaciers, 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. A portion of this runoff enters rivers, with streamflow moving water towards the oceans. Runoff and water emerging from the ground (groundwater) may be stored as freshwater in lakes. Not all runoff flows into rivers; much of it soaks into the ground as infiltration. Some water infiltrates deep into the ground and replenishes aquifers, 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 or be taken up by plants and transferred back to the atmosphere as water vapor by transpiration. Some groundwater finds openings in the land surface and emerges as freshwater springs. In river valleys and floodplains, there is often continuous water exchange between surface water and ground water in the hyporheic zone. Over time, the water returns to the ocean, to continue the water cycle.

The ocean plays a key role in the water cycle. The ocean holds "97% of the total water on the planet; 78% of global precipitation occurs over the ocean, and it is the source of 86% of global evaporation". [1]

Physical processes

Processes leading to movements and phase changes in water HydrologicalCycle1.png
Processes leading to movements and phase changes in water

The water cycle involves the following processes:

The movement of water through the atmosphere. [3] Without advection, water that evaporated over the oceans could not precipitate over land. Atmospheric rivers that move large volumes of water vapor over long distances are an example of advection. [4]
Canopy interception
The precipitation that is intercepted by plant foliage eventually evaporates back to the atmosphere rather than falling to the ground.
The transformation of water vapor to liquid water droplets in the air, creating clouds and fog. [5]
This refers to changing of water vapor directly to ice.
The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere. [6] The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration. Total annual evapotranspiration amounts to approximately 505,000 km3 (121,000 cu mi) of water, 434,000 km3 (104,000 cu mi) of which evaporates from the oceans. [7] 86% of global evaporation occurs over the ocean. [8]
The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater. [9] A recent global study using water stable isotopes, however, shows that not all soil moisture is equally available for groundwater recharge or for plant transpiration. [10]
Water flows vertically through the soil and rocks under the influence of gravity.
Condensed water vapor that falls to the Earth's surface. Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet. [11] Approximately 505,000 km3 (121,000 cu mi) of water falls as precipitation each year, 398,000 km3 (95,000 cu mi) of it over the oceans. [7] [12] The rain on land contains 107,000 km3 (26,000 cu mi) of water per year and a snowing only 1,000 km3 (240 cu mi). [12] 78% of global precipitation occurs over the ocean. [8]
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.
Snow melt
The runoff produced by melting snow.
The state change directly from solid water (snow or ice) to water vapor by passing the liquid state. [13]
Subsurface flow
The flow of water underground, in the vadose zone and 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 or gravity induced pressures. Groundwater tends to move slowly and is replenished slowly, so it can remain in aquifers for thousands of years.
The release of water vapor from plants and soil into the air.

Residence times

Average reservoir residence times [14]
ReservoirAverage residence time
Antarctica20,000 years
Oceans3,200 years
Glaciers20 to 100 years
Seasonal snow cover2 to 6 months
Soil moisture1 to 2 months
Groundwater: shallow100 to 200 years
Groundwater: deep10,000 years
Lakes (see lake retention time)50 to 100 years
Rivers2 to 6 months
Atmosphere9 days

The residence time 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. 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 and 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. [15]

In hydrology, residence times can be estimated in two ways.[ citation needed ] The more common method relies on the principle of conservation of mass (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.

Water in storage

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 1,386,000,000 km3 of the world's water supply, about 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. [16] The Earth's ice caps, glaciers, and permanent snowpack stores another 24,064,000 km3 accounting for only 1.7% of the planet's total water volume. However, this quantity of water is 68.7% of all freshwater on the planet. [17]

Changes caused by humans

Water cycle intensification due to climate change

Extreme weather will be progressively more common as the Earth warms. 20211109 Frequency of extreme weather for different degrees of global warming - bar chart IPCC AR6 WG1 SPM.svg
Extreme weather will be progressively more common as the Earth warms.
The sixth IPCC Assessment Report projects changes in average soil moisture that can disrupt agriculture and ecosystems. A reduction in soil moisture by one standard deviation means that average soil moisture will approximately match the ninth driest year between 1850 and 1900 at that location. Soil moisture and climate change.svg
The sixth IPCC Assessment Report projects changes in average soil moisture that can disrupt agriculture and ecosystems. A reduction in soil moisture by one standard deviation means that average soil moisture will approximately match the ninth driest year between 1850 and 1900 at that location.

Since the middle of the 20th century, human-caused climate change has resulted in observable changes in the global water cycle. [19] :85 The IPCC Sixth Assessment Report in 2021 predicted that these changes will continue to grow significantly at the global and regional level. [19] :85 These findings are a continuation of scientific consensus expressed in the IPCC Fifth Assessment Report from 2007 and other special reports by the Intergovernmental Panel on Climate Change which had already stated that the water cycle will continue to "intensify" throughout the 21st century. [20]

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. Glacial retreat since 1850 due to global warming has been extensive. [21] :1273

Relationship between impervious surfaces and surface runoff Natural & impervious cover diagrams EPA.jpg
Relationship between impervious surfaces and surface runoff

Changes due to other human activities

Water cycle showing human influences and major pools and fluxes. HumanIntegratedWaterCycle (2).jpg
Water cycle showing human influences and major pools and fluxes.

Human activities, other than those that lead to global warming from greenhouse gas emissions, can also alter the water cycle. The IPCC Sixth Assessment Report stated that there is "abundant evidence that changes in land use and land cover alter the water cycle globally, regionally and locally, by changing precipitation, evaporation, flooding, groundwater, and the availability of freshwater for a variety of uses". [23] :1153

Examples for such land use changes are converting fields to urban areas or clearing forests. Such changes can affect the ability of soils to soak up surface water. Deforestation can also "directly reduce soil moisture, evaporation and rainfall locally but can also cause regional temperature changes that affect rainfall patterns". [23] :1153 Aquifer drawdown or overdrafting and the pumping of fossil water increases the total amount of water in the hydrosphere because water that was "previously in the ground is now in direct contact with the atmosphere, being available for evaporation". [23] :1153

Biogeochemical cycling

While the water cycle is itself a biogeochemical cycle, flow of water over and beneath the Earth is a key component of the cycling of other biogeochemicals. [24] Runoff is responsible for almost all of the transport of eroded sediment and phosphorus from land to waterbodies. [25] 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 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. [26] The 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. [27]

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, 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. [28] Planets with hot lower atmospheres could result in humid upper atmospheres that accelerate the loss of hydrogen. [29]

Historical interpretations

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). [30] 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, [30] there is some agreement that the time period is 962–922 BCE. [31] 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). [32]

In the Biblical Book of Job, dated between 7th and 2nd centuries BCE, [31] there is a description of precipitation in the hydrologic cycle, [30] "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).

Understanding of 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 evolution of land animals from fish [33] ) and Xenophanes of Colophon (530 BCE). [34] Chinese scholars such as Chi Ni Tzu (320 BCE) and Lu Shih Ch'un Ch'iu (239 BCE) had similar thoughts. [35]

The idea that the water cycle is a closed cycle can be found in the works of 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, 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. [36] [37]

Up to the time of the Renaissance, it was wrongly assumed 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. Bartholomew of England held this view (1240 CE), as did Leonardo da Vinci (1500 CE) and Athanasius Kircher (1644 CE).

Discovery of the correct theory

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. Even then, these beliefs were not accepted in mainstream science until the early nineteenth century. [38]

See also

Related Research Articles

<span class="mw-page-title-main">Hydrology</span> Science of the movement, distribution, and quality of water on Earth and other planets

Hydrology is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and drainage basin sustainability. A practitioner of hydrology is called a hydrologist. Hydrologists are scientists studying earth or environmental science, civil or environmental engineering, and physical geography. Using various analytical methods and scientific techniques, they collect and analyze data to help solve water related problems such as environmental preservation, natural disasters, and water management.

<span class="mw-page-title-main">Evapotranspiration</span> Process by which water moves into the air from plants and soil.

Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation and transpiration. Evapotranspiration is an important part of the local water cycle and climate, and measurement of it plays a key role in agricultural irrigation and water resource management.

<span class="mw-page-title-main">Hydrosphere</span> Total amount of water on a planet

The hydrosphere is the combined mass of water found on, under, and above the surface of a planet, minor planet, or natural satellite. Although Earth's hydrosphere has been around for about 4 billion years, it continues to change in shape. This is caused by seafloor spreading and continental drift, which rearranges the land and ocean.

Surface-water hydrology is the sub-field of hydrology concerned with above-earth water, in contrast to groundwater hydrology that deals with water below the surface of the Earth. Its applications include rainfall and runoff, the routes that surface water takes, and the occurrence of floods and droughts. Surface-water hydrology is used to predict the effects of water constructions such as dams and canals. It considers the layout of the watershed, geology, soils, vegetation, nutrients, energy and wildlife. Modelled aspects include precipitation, the interception of rain water by vegetation or artificial structures, evaporation, the runoff function and the soil-surface system itself.

<span class="mw-page-title-main">Environmental degradation</span> Any change or disturbance to the environment perceived to be deleterious or undesirable

Environmental degradation is the deterioration of the environment through depletion of resources such as quality of air, water and soil; the destruction of ecosystems; habitat destruction; the extinction of wildlife; and pollution. It is defined as any change or disturbance to the environment perceived to be deleterious or undesirable.

In hydrology, discharge is the volumetric flow rate of water that is transported through a given cross-sectional area. It includes any suspended solids, dissolved chemicals, or biologic material in addition to the water itself. Terms may vary between disciplines. For example, a fluvial hydrologist studying natural river systems may define discharge as streamflow, whereas an engineer operating a reservoir system may equate it with outflow, contrasted with inflow.

Isotope hydrology is a field of geochemistry and hydrology that uses naturally occurring stable and radioactive isotopic techniques to evaluate the age and origins of surface and groundwater and the processes within the atmospheric hydrologic cycle. Isotope hydrology applications are highly diverse, and used for informing water-use policy, mapping aquifers, conserving water supplies, assessing sources of water pollution, and increasingly are used in eco-hydrology to study human impacts on all dimensions of the hydrological cycle and ecosystem services.

<span class="mw-page-title-main">Water balance</span> Looks at how water moves in a closed system

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 system. A system can be one of several hydrological or water domains, such as a column of soil, a drainage basin, an irrigation area or a city. Water balance can also refer to the ways in which an organism maintains water in dry or hot conditions. It is often discussed in reference to plants or arthropods, which have a variety of water retention mechanisms, including a lipid waxy coating that has limited permeability.

Streamflow, or channel runoff, is the flow of water in streams and other channels, and is a major element of the water cycle. It is one component of the movement of water from the land to waterbodies, the other component being surface runoff. Water flowing in channels comes from surface runoff from adjacent hillslopes, from groundwater flow out of the ground, and from water discharged from pipes. The discharge of water flowing in a channel is measured using stream gauges or can be estimated by the Manning equation. The record of flow over time is called a hydrograph. Flooding occurs when the volume of water exceeds the capacity of the channel.

<span class="mw-page-title-main">Climate system</span> Interactions that create Earths climate and may result in climate change

Earth's climate system is a complex system having five interacting components: the atmosphere (air), the hydrosphere (water), the cryosphere, the lithosphere and the biosphere. Climate is the statistical characterization of the climate system, representing the average weather, typically over a period of 30 years, and is determined by a combination of processes in the climate system, such as ocean currents and wind patterns. Circulation in the atmosphere and oceans is primarily driven by solar radiation and transports heat from the tropical regions to regions that receive less energy from the Sun. The water cycle also moves energy throughout the climate system. In addition, different chemical elements, necessary for life, are constantly recycled between the different components.

<span class="mw-page-title-main">Groundwater recharge</span> Groundwater that recharges an aquifer

Groundwater recharge or deep drainage or deep percolation is a hydrologic process, where water moves downward from surface water to groundwater. Recharge is the primary method through which water enters an aquifer. This process usually occurs in the vadose zone below plant roots and is often expressed as a flux to the water table surface. Groundwater recharge also encompasses water moving away from the water table farther into the saturated zone. Recharge occurs both naturally and through anthropogenic processes, where rainwater and or reclaimed water is routed to the subsurface.

<span class="mw-page-title-main">Water distribution on Earth</span> Overview of the distribution of water on planet Earth

Most water in Earth's atmosphere and on its crust comes from saline seawater, while fresh water accounts for nearly 1% of the total. The vast bulk of the water on Earth is saline or salt water, with an average salinity of 35‰, though this varies slightly according to the amount of runoff received from surrounding land. In all, water from oceans and marginal seas, saline groundwater and water from saline closed lakes amount to over 97% of the water on Earth, though no closed lake stores a globally significant amount of water. Saline groundwater is seldom considered except when evaluating water quality in arid regions.

<span class="mw-page-title-main">Surface water</span> Water located on top of land forming terrestrial bodies of water

Surface water is water located on top of land forming terrestrial waterbodies, and may also be referred to as blue water, opposed to the seawater and waterbodies like the ocean.

The following outline is provided as an overview of and topical guide to hydrology:

DPHM-RS is a semi-distributed hydrologic model developed at University of Alberta, Canada.

<span class="mw-page-title-main">Hydrology of Switzerland</span>

Hydrology is the science which studies the water cycle as a whole, hence the water exchanges between soil and atmosphere but also between the soil and sub ground (groundwater).

<span class="mw-page-title-main">Biotic pump</span>

The theory of a biotic pump pertains to the importance of forests in the water cycle, specifically, in determining the levels of rainfall a region will receive. It states that an increased amount of evaporation or transpiration will cause a reduction in atmospheric pressure as clouds form, which will subsequently cause moist air to be drawn to regions where evapotranspiration is at its highest. In a desert this will correspond to the sea whereas in a forest, moist air from the sea will be drawn inland. The theory predicts two different types of coast to contentinental rainfall patterns, first in a forested area one can expect no decrease in rainfall as one moves inland in contrast to a deforested region where one observes an exponential decrease in annual rainfall. While current global climate models fit these patterns well, it is argued this is due to parametrization and not the veracity of the theories.

<span class="mw-page-title-main">Fresh water</span> Naturally occurring water with low amounts of dissolved salts

Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include non-salty mineral-rich waters such as chalybeate springs. Fresh water may encompass frozen and meltwater in ice sheets, ice caps, glaciers, snowfields and icebergs, natural precipitations such as rainfall, snowfall, hail/sleet and graupel, and surface runoffs that form inland bodies of water such as wetlands, ponds, lakes, rivers, streams, as well as groundwater contained in aquifers, subterranean rivers and lakes. Fresh water is the water resource that is of the most and immediate use to humans.

<span class="mw-page-title-main">Effects of climate change on the water cycle</span>

The effects of climate change on the water cycle are profound and have been described as an "intensification" or an overall "strengthening" of the water cycle. This effect has been observed since at least 1980. One example is the intensification of heavy precipitation events. This has important knock-on effects on the availability of freshwater resources, as well as other water reservoirs such as oceans, ice sheets, atmosphere and land surface. The water cycle is essential to life on earth and plays a large role in the global climate and the ocean circulation. The warming of the earth is expected to cause changes in the water cycle for various reasons. For example, warmer atmosphere can contain more water vapor which has effects on evaporation and rainfall. Oceans play a large role as well, since they absorb 93% of heat. The increase in ocean heat content since 1971 has a big effect on the ocean as well as the cycle. To avoid further, or more extreme, changes to the water cycle, greenhouse gas emissions must be reduced.

Oceanic freshwater fluxes are defined as the transport of non saline water between the oceans and the other components of the Earth's system. These fluxes have an impact on the local ocean properties, as well as on the large scale circulation patterns.


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