The word water cycle

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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.

Description

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
₂O has smaller molecular mass than the major components of the atmosphere, nitrogen (N
₂) and oxygen (O
₂) 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

The water cycle involves the following processes:

Advection

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]

Condensation

The transformation of water vapor to liquid water droplets in the air, creating clouds and fog.^[5]

Deposition

This refers to changing of water vapor directly to ice.

Evaporation

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]

Infiltration

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]

Interception

The precipitation that is intercepted by plant foliage eventually evaporates back to the atmosphere rather than falling to the ground.

Percolation

Water flows vertically through the soil and rocks under the influence of gravity.

Precipitation

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]

Runoff

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.

Sublimation

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.

Transpiration

The release of water vapor from plants and soil into the air.

Residence times

Average reservoir residence times^[14]

Reservoir	Average residence time
Antarctica	20,000 years
Oceans	3,200 years
Glaciers	20 to 100 years
Seasonal snow cover	2 to 6 months
Soil moisture	1 to 2 months
Groundwater: shallow	100 to 200 years
Groundwater: deep	10,000 years
Lakes (see lake retention time)	50 to 100 years
Rivers	2 to 6 months
Atmosphere	9 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

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

Changes due to other human activities

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

• Bioprecipitation
• Deep water cycle – Movement of water in the deep Earth
• Ecohydrology – interdisciplinary field studying the interactions between water and ecosystems
• Global meteoric water line
• Moisture advection
• Moisture recycling
• Planetary boundaries – Limits not to be exceeded if humanity wants to survive in a safe ecosystem
• Water resources – Sources of water that are potentially useful
• Biotic pump

References

External links

• The Water Cycle, United States Geological Survey
• The Water Cycle for Kids, United States Geological Survey
• The Water Cycle: Following The Water (NASA Visualization Explorer with videos)

The movement of water is all around us. Rain, snow, and the clouds they fall from are part of it. So are rivers, lakes, and the world’s oceans, which contain over ninety percent of all water. It is known as the Water Cycle.

The Water Cycle is the constant movement of water in its different forms on a regional and global scale as it flows and transitions over land, in the oceans, and atmosphere. It includes all related atmospheric occurrences like precipitation, evaporation, condensation, sublimation, and transpiration.

All three forms of fresh water eventually find their way back to the ocean in some way, where the cycle starts all over again. And that is the water cycle in a nutshell.

The water cycle is the path that all water follows as it moves around Earth in different states. Water vapor—a gas—is found in Earth’s atmosphere. Water can be found all over Earth in the ocean, on land and in the atmosphere. The water cycle is the path that all water follows as it moves around our planet.

What is the meaning of water cycle in science?

The water cycle shows the continuous movement of water within the Earth and atmosphere. It is a complex system that includes many different processes. Liquid water evaporates into water vapor, condenses to form clouds, and precipitates back to earth in the form of rain and snow.

What is a word for water cycle?

water cycle, also called hydrologic cycle, cycle that involves the continuous circulation of water in the Earth-atmosphere system. Of the many processes involved in the water cycle, the most important are evaporation, transpiration, condensation, precipitation, and runoff.

What is water cycle process?

The water cycle consists of three major processes: evaporation, condensation, and precipitation. Evaporation is the process of a liquid’s surface changing to a gas. In the water cycle, liquid water (in the ocean, lakes, or rivers) evaporates and becomes water vapor.

What is meant by water cycle short answer?

The Short Answer: The water cycle is the path that all water follows as it moves around Earth in different states. Liquid water is found in oceans, rivers, lakes—and even underground. Water can be found all over Earth in the ocean, on land and in the atmosphere.

What is water cycle for Class 3?

Water vapor evaporates from water bodies in the air, it condenses and changes into tiny droplets and then clouds are formed. When these droplets become so dense and heavy, they come back to the earth in the form of rain, hail, sleet or snow. Precipitation is responsible for bringing back the fresh water on the earth.

The water cycle, also known as the hydrologic cycle or the hydrological cycle, describes the continuous movement of water on, above and below the surface of the Earth. During this process, water changes its state from one phase to another, but the total number of water particles remains the same.

What is the meaning of hydrosphere cycle in science?

Water moves through the hydrosphere in a cycle. Water collects in clouds, then falls to Earth in the form of rain or snow. This water collects in rivers, lakes and oceans. Then it evaporates into the atmosphere to start the cycle all over again. This is called the water cycle.

What is water cycle for Class 6?

The constant movement of water from the Earth to the atmosphere and back to the Earth through the process of evaporation, condensation and precipitation is known as the water cycle.

What is water cycle for Class 9?

The process in which water evaporates and falls on the land as rain and later flows back into the sea via rivers is called water cycle.

Why is water cycle called a cycle?

The water cycle, also known as the hydrologic cycle, describes the continuous movement of water as it makes a circuit from the oceans to the atmosphere to the Earth and on again. The sun, which drives the water cycle, heats water in the oceans. Some of it evaporates as vapor into the air.

What is water cycle for Class 4?

There are four main stages in the water cycle. They are evaporation, condensation, precipitation and collection. Let’s look at each of these stages. Evaporation: This is when warmth from the sun causes water from oceans, lakes, streams, ice and soils to rise into the air and turn into water vapour (gas).

How the water cycle works step by step?

Since that is where about 96% of total water exists on Earth. Step 1: Evaporation. The water cycle begins with evaporation. Step 2: Condensation. As water vaporizes into water vapor, it rises up in the atmosphere. Step 3: Sublimation. Step 4: Precipitation. Step 5: Transpiration. Step 6: Runoff. Step 7: Infiltration.

What is the water cycle explain briefly with a diagram?

The water cycle is defined as a natural process of constantly recycling the water in the atmosphere. It is also known as the hydrological cycle or the hydrologic cycle. During the process of the water cycle between the earth and the atmosphere, water changes into three states of matter – solid, liquid and gas.

What is water cycle in short class 7?

Water of Class 7 The water from the oceans and surface of the earth evaporates and rises up in the air. It cools and condenses to form clouds and then falls back to the earth as rain, snow or hail. This circulation of water between the oceans and land is called water cycle.

What does the water cycle Class 7 mean?

Water cycle is the cyclic movement of water from the atmosphere to the earth and back to the atmosphere through various processes. Condensation: Water vapor rises up and condenses on dust particles to form cloud. Precipitation: Water stored in clouds reaches the ground in the form of rain, hail or snow.

What are oceans the atmosphere and groundwater in the water cycle?

Oceans, the atmosphere, and groundwater are all parts of the water cycle.

What are the 3 different form of water that makes up the hydrosphere?

A planet’s hydrosphere can be liquid, vapor, or ice.

Is the Earth surrounded by water?

The Earth is a watery place. About 71 percent of the Earth’s surface is water-covered, and the oceans hold about 96.5 percent of all Earth’s water. Water also exists in the air as water vapor, in rivers and lakes, in icecaps and glaciers, in the ground as soil moisture and in aquifers, and even in you and your dog.

Why does it rain ks2?

At the beginning of the cycle, sunlight heats up water on Earth’s surface. The heat causes the water to evaporate, or to turn into water vapor. This water vapor rises into the air. They fall to Earth as rain.

How old is the water on Earth?

There is also geological evidence that helps constrain the time frame for liquid water existing on Earth. A sample of pillow basalt (a type of rock formed during an underwater eruption) was recovered from the Isua Greenstone Belt and provides evidence that water existed on Earth 3.8 billion years ago.

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We forget that the water cycle and the life cycle are one.

Jacques Yves Cousteau

PRONUNCIATION OF WATER CYCLE

GRAMMATICAL CATEGORY OF WATER CYCLE

Water cycle is a noun.

WHAT DOES WATER CYCLE MEAN IN ENGLISH?

Definition of water cycle in the English dictionary

The definition of water cycle in the dictionary is the circulation of the earth’s water, in which water evaporates from the sea into the atmosphere, where it condenses and falls as rain or snow, returning to the sea by rivers or returning to the atmosphere by evapotranspiration Also called: hydrologic cycle.

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REFERENCE

« EDUCALINGO. Water cycle [online]. Available <https://educalingo.com/en/dic-en/water-cycle>. Apr 2023 ».

The water cycle, or the hydrologic cycle, is the continuous circulation of water within the Earth’s hydrosphere. It involves the movement of water into and out of various reservoirs, including the atmosphere, land, surface water, and groundwater. This cycle is driven by radiation from the Sun. The movement of water within the water cycle is the subject of the field of hydrology.

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, condensation, precipitation, infiltration, runoff, and subsurface flow. In so doing, the water goes through different phases: liquid, solid, and gas.

The water cycle also involves the exchange of heat energy, which leads to temperature changes. For instance, in the process of evaporation, water takes up energy from the surroundings and cools the environment. Conversely, in the process of condensation, water releases energy to its surroundings, warming the environment.

The water cycle figures significantly in the maintenance of life and ecosystems on Earth. Even as water in each reservoir plays an important role, the water cycle brings added significance to the presence of water on our planet. By transferring water from one reservoir to another, the water cycle purifies water, replenishes the land with freshwater, and transports minerals to different parts of the globe. It is also involved in reshaping the geological features of the Earth, through such processes as erosion and sedimentation. In addition, as the water cycle involves heat exchange, it exerts an influence on climate as well.

Movement of water within the water cycle

There is no definable start or finish to the water cycle. Water molecules move continuously among different compartments, or reservoirs, of the Earth’s hydrosphere, by different physical processes. Water evaporates from the oceans, forms clouds, which precipitate and the water falls back to Earth. However, water does not necessarily cycle through each compartment in order. Before reaching the ocean, water may have evaporated, condensed, precipitated, and become runoff multiple times.

Explanation of the water cycle

The water cycle is the combination of processes that water goes through in nature. It includes (a) precipitation, which is the falling of water in liquid or solid form to Earth; (b) infiltration, which is the process by which water is absorbed into the soil; (c) surface runoff, in which water flows off the surface; (d) evaporation or transpiration, which occurs when water is heated and vaporizes, or when plants give off water vapor; (e) condensation, which is the process by which water vapor cools and forms clouds. This cycle is repeated over and over again.

The physical processes

The major physical processes involved in the water cycle are the evaporation of water from the oceans and land, the transport of water in the atmosphere, condensation, precipitation over the oceans and land, and the flow of water from land to the oceans.

• Evaporation is the transfer of water from bodies of surface water into the atmosphere. This transfer entails a change in the physical nature of water from liquid to gaseous phases. The source of energy is primarily solar radiation. Evaporation is closely related to transpiration from plants, as well as, to a lesser degree, perspiration from land mammals and marsupials. Thus, this transfer is sometimes referred to as evapotranspiration. About 90 percent of atmospheric water comes from evaporation, while the remaining 10 percent is from transpiration.

• Condensation is the transformation of water vapor to liquid water droplets in the air, producing clouds and fog.

• Advection is the movement of water—in solid, liquid, or vapor states—through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.

• Precipitation is water vapor that has condensed into clouds and falls to the Earth’s surface. This mostly occurs as rainfall, but also includes snow, hail, fog drip, and sleet.

• Runoff includes 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 infiltrate into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.

Less fundamental processes involved in the water cycle are:

• Sublimation is the state change directly from solid water (snow or ice) to water vapor.

• Canopy interception is the precipitation that is intercepted by plant foliage and eventually evaporates back to the atmosphere rather than falling to the ground. The amount of water that it intercepted depends on the duration of the storm, the windspeed and temperature, and the amount of foliage present.

• Infiltration is the flow of water on the ground surface into the ground. The speed of infiltration depends on how moist the ground already is, and on its infiltration capacity. Having infiltrated, water comprises soil moisture within the vadose zone, or groundwater in an aquifer.

• Snowmelt refers to the runoff produced by melting snow.

• Subsurface flow is the flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (for instance, 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.

Conservation of mass

Average annual water transport[1]

Water flux	Average rate (10³ km³/year)
Precipitation over land	107
Evaporation from land	71
Runoff & groundwater from land	36
Precipitation over oceans	398
Evaporation from oceans	434

The total amount, or mass, of water in the water cycle remains essentially constant, as does the amount of water in each reservoir of the water cycle. This means that the rate of water added to one reservoir must equal, on average over time, the rate of water leaving the same reservoir.

The adjacent table contains the amount of water that falls as precipitation or rises as evaporation, for both the land and oceans. The runoff and groundwater discharge from the land to the oceans is also included. From the law of conservation of mass, whatever water moves into a reservoir, on average, the same volume must leave. For example, 107 thousand cubic kilometers (107 × 10³ km³) of water falls on land each year as precipitation. This is equal to the sum of the evaporation (71 × 10³ km³/year) and runoff (36 × 10³ km³/year) of water from the land.

Water that cycles between the land and the atmosphere in a fixed area is referred to as moisture recycling.

Reservoirs

Volume of water stored in
the water cycle’s reservoirs[2]

Reservoir	Volume of water (106 km³)	Percent of total
Oceans	1370	97.25
Ice caps & glaciers	29	2.05
Groundwater	9.5	0.68
Lakes	0.125	0.01
Soil moisture	0.065	0.005
Atmosphere	0.013	0.001
Streams & rivers	0.0017	0.0001
Biosphere	0.0006	0.00004

In the context of the water cycle, a reservoir represents a region or zone where water is stored at a certain stage of the water cycle. The largest reservoir is the collection of oceans, accounting for 97 percent of the Earth’s water. The next largest quantity (2 percent) is stored in solid form in the ice caps and glaciers. The water contained within all living organisms represents the smallest reservoir. Freshwater reservoirs, particularly those available for human use, are important water resources.

Residence times

Average reservoir residence times[3]

Reservoir	Average residence time
Oceans	3,200 years
Glaciers	20 to 100 years
Seasonal snow cover	2 to 6 months
Soil moisture	1 to 2 months
Groundwater: shallow	100 to 200 years
Groundwater: deep	10,000 years
Lakes	50 to 100 years
Rivers	2 to 6 months
Atmosphere	9 days

The residence time is a measure of the average time that water will spend in a reservoir. It needs to be understood that some of the water will spend much less time than average, and some, much more. 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, water remains in the atmosphere for an average of about nine days before condensing and falling to the Earth as precipitation.

(See the adjacent table for residence times for other reservoirs.)

Residence times can be estimated in two ways. The more common method relies on the principle of conservation of mass, and may be expressed by the following equation:

An alternative method, gaining in popularity particularly for dating groundwater, is the use of isotopic techniques. This is done in the subfield of isotope hydrology.

Example: Calculating the residence time of the oceans

As an example of how the residence time is calculated, consider the oceans. The volume of the oceans is roughly 1,370×10⁶ km³. Precipitation over the oceans is about 0.398×10⁶ km³/year and the flow of water to the oceans from rivers and groundwater is about 0.036×10⁶ km³/year. By dividing the total volume of the oceans by the rate of water added (in units of volume over time), the calculated residence time is 3,200 years—the average time it takes a water molecule that reaches an ocean to evaporate.

Climate regulation

The water cycle is powered by solar energy. About 86 percent of global evaporation occurs from the oceans, reducing their temperature through the process of evaporation. Without the cooling effect of evaporation, the greenhouse effect would lead to a much higher surface temperature—an estimated 67° C—and a hotter planet [4].

Most of the solar energy warms tropical seas. After evaporating, water vapor rises into the atmosphere and is carried away by winds. Most of the water vapor condenses as rain in what is called the intertropical convergence zone (ITCZ), a low-pressure belt around the equator. This condensation releases latent heat that warms the air. This process, in turn, drives atmospheric circulation.

Changes in the water cycle

Over the past century, the water cycle has become more intense [5], as the rates of evaporation and precipitation have increased. It is thought that this is an outcome global warming, as higher temperatures increase the rate of evaporation.

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 has been extensive.

Human activities that alter the water cycle include:

• agriculture
• alteration of the chemical composition of the atmosphere
• construction of dams
• deforestation and afforestation
• removal of groundwater from wells
• water abstraction from rivers
• urbanization

Biogeochemical cycles

The water cycle is a biogeochemical cycle. Other notable cycles are the carbon cycle and nitrogen cycle.

As water flows over and beneath the Earth, it picks up and transports soil and other sediment, mineral salt and other dissolved chemicals, and pollutants. The oceans are saline because mineral salts are transported from the land by water runoff, but the salts remain in the oceans when water evaporates.

External links

All links retrieved June 7, 2020.

• National Oceanic and Atmospheric Administration (NOAA) website

Biogeochemical cycles
Carbon cycle — Hydrogen cycle — Nitrogen cycle
Oxygen cycle — Phosphorus cycle — Sulfur cycle — Water cycle

What is the Water Cycle

Steps of the Water Cycle: How does it Work

Why is the Water Cycle Important

Human Impact on Water Cycle

FAQs

References

Definition

The water cycle or hydrologic cycle describes the complex systems that allow water to move across the Earth and atmosphere. The most basic step of the water cycle is the change of state of water as a liquid, gas or solid in the atmosphere. However, the water cycle also envelops various methods of water transportation and water types such as plant uptake, transpiration, groundwater, precipitation, and percolation.

Water Cycle Steps

Water cycle steps are becoming less predictable as global warming changes water levels and distribution across the globe. This subcategory of the biogeochemical cycle should also not be discussed as a sequenced number of events as different modes of water uptake, transportation and return occur simultaneously and at different rates according to variances in global or local ecosystems. A mountainous region will experience significantly more sublimation and runoff, for example, when compared to flat, open plains. In fact, when discussing water cycle steps it is easier to look at the movement of water separately: going up and coming down.

Water Goes Up

Water cycle steps in the atmosphere are easy to see wherever a cloud is visible. A cloud is the result of water condensation that is added to the atmosphere by way of water evaporation, water sublimation, and water transpiration. Water can move through the troposphere by way of another water cycle step – water transportation. Water can return to the Earth’s crust through water precipitation and deposition.

The atmospheric water cycle takes place in the lowest layer of our atmosphere or the troposphere. The troposphere extends from the Earth’s surface and reaches heights of 4 miles at the two poles and up to 12 miles at the equator. The layer above – the stratosphere – contains very little water vapor.

Water vapor in the atmosphere is extremely important as these droplets are able to absorb solar energy as well as the heat that radiates from the Earth (thermal radiation). It is water vapor that regulates local climates and air temperatures. Variances in temperature, in turn, cause currents of air known as convection currents that help to create the wind patterns so often typical to a certain region, such as monsoon storms or desert zephyrs.

As already mentioned, the basic water cycle describes the exchange and movement of water between the earth and its troposphere. The powerful heat of the sun evaporates large quantities of water from the Earth’s surface, and this atmospheric moisture is transported to other regions by the wind.

As the air within the troposphere warms and rises, water molecules respond to the heat energy and move further away, expanding the space in which they exist. This leads to a loss of heat energy and cooling, where water vapor condenses and forms clouds. As this vapor further condenses, it forms water droplets which then become heavier and fall to the earth in liquid or solid (rain, hail, sleet, and snow) form, depending on the ambient temperature.

The troposphere gains water molecules through four mechanisms. These are evaporation, sublimation, transpiration, and transportation.

Evaporation as a part of the water cycle describes the change of water from liquid to gas. Our oceans, seas, lakes, and rivers provide approximately 90 percent of the troposphere’s water vapor via evaporation. Water vapor is a gas. When the sun warms the surface of the water, whether this is the ocean or a puddle on the ground, water molecules are given the energy to move at faster and faster rates, splitting off and entering the air as the equivalent of steam. The warmer the heat source, the more the water will evaporate. A saucepan full of water will create large amounts of steam or water vapor.

Sublimation is the forming of water vapor directly from solid water or ice. Sublimation is more likely to occur at high altitudes such as snowy mountain tops where the heat of the sun is hardly obstructed. One cubic centimeter or gram of ice will need 80 calories worth of energy to melt, 100 calories to boil, and a further 540 calories to vaporize. Sublimation requires this total of 720 calories of heat energy but skips the liquid phase, where ice immediately turns to water vapor. However, in high humidity, it is more likely that there will also be an intermediate liquid form of water. In the United States, it has been calculated that solar energy provides an average area of land with approximately 500 calories per square foot per day. This increases in mountainous regions and current renewable energy research is considering mountain-top solar farms for higher energy production. This shows that sublimation provides a large amount of the atmosphere’s water vapor, although in significantly lower quantities than evaporation – around 1%.

Transpiration is the conversion of water by plants into water vapor. In ideal conditions, plants only use around 5% of the water they take up through their root systems. One only has to see pictures of the mist above a rainforest to understand this contribution to water vapor levels in the troposphere. Under the sun’s rays, water escapes through leaf pores as a gas. The combination of evaporation and transpiration is called evapotranspiration. While transpiration is probably responsible for 10% of the troposphere’s water content, combined evapotranspiration provides about 99%.

Transportation does not provide water vapor to the troposphere but describes the movement of water via the wind or the jet streams – strong wind currents at the top of the troposphere or at the tropopause, a level of air between the troposphere and stratosphere. We can see the effects of transportation by watching clouds move across the sky. In addition, winds remove water vapor from the air above sources of water. This lowers the saturation levels (or humidity) of the air and allows even more water vapor to enter the atmosphere.

Water Goes Down

Water cycle steps on the Earth’s crust are highly dependent on the type of ecosystem. These steps are water condensation, precipitation, and deposition.

Water does not fall to earth in the form of water vapor. As water vapor rises, it loses heat energy through continuous motion. In addition, gaseous forms of water experience less pressure as they rise. Where there is less pressure, the air is unable to hold as much water as when pressures are high. Furthermore, other substances in the air such as pollen, pollutants, and dust provide a surface on which water vapor can settle and condense. Condensation is the opposite of evaporation and we have all seen the effect of condensation on windows and bathroom mirrors. As warm water vapor hits a cooler surface, energy levels dramatically drop. The water molecules no longer move at rapid rates and settle as water droplets. This also occurs in the atmosphere in the presence of condensation nuclei – small particles onto which water vapor can settle.

Clouds are the result of condensed water vapor. Eventually, they become saturated and are no longer able to hold liquid water droplets. This leads to precipitation.

Rain is the most common example of water cycle precipitation. Other forms are hailstones, sleet, and snow.

Deposition is the opposite of sublimation. In cases of deposition, water vapor is instantly converted from gas state to solid state (ice) without the intermediate liquid phase. In contrast to sublimation, the process of deposition releases energy. Deposition can be seen in snowfall and in the formation of frost.

Intermediary

Intermediary water cycle steps provide a bridge between water landing on the Earth’s surface and water vapor rising into the troposphere.

Infiltration is the absorption of water by the soil and rock of the upper level of the Earth’s crust and is very much dependent on environmental factors such as soil or rock depth, vegetation levels, saturation levels, and porosity. Percolation describes the flow of this infiltrated water through the soil or rock under the force of gravity. Eventually, percolated water will reach an impenetrable layer of non-porous rock. The water settles here in aquifers. You can make your own scale model of an aquifer by digging a deep pit in the sand when next on the beach. The pools or reservoirs of water that form above non-porous rock are called aquifers, but the water they contain is known as groundwater. Groundwater is another named phase of the water cycle and does not describe a step but the result of precipitation, infiltration and percolation.

Plant uptake is another way in which the water provided to the earth’s crust via precipitation and infiltration can be absorbed. Plant root systems take up water, using it as a nutrient source and discharging water vapor through leaf pores in the earlier described transpiration phase.

Where the ground is saturated and unable to deal with high levels of precipitation, another part of the water cycle takes place. This is water runoff. Water runoff is becoming a global problem due to the effects of global warming. Gravity is an extremely important factor when water droplets fall from the clouds. As everyone should know, water moves downhill. Where precipitation is high and the land it falls on is either limited in porosity or already saturated with water, water begins to flow downwards. Runoff may also be the result of snow melts.

Runoff is the combination of surface runoff, interflow, and baseflow. Surface runoff comes in the forms of saturation excess overland flow where the ground is already wet and unable to absorb more water, and overland flow or the runoff from our roofs, sidewalks and roads. As we increase non-porous infrastructures, we simultaneously reduce the globe’s ability to absorb precipitation. Storm runoff also occurs during heavy rainfall.

Interflow but involves water that has already percolated into lower soil levels. With the next heavy rain, this already saturated soil or rock is not given the time to reach the aquifer and water rises upwards to the soil subsurface and pushes upwards to produce increased surface runoff.

Baseflow or fair-weather flow describes how moving bodies of water such as streams and rivers take on infiltrated water over a longer period of time, between precipitation (hence ‘fair weather flow’). This is a delayed response but also contributes to runoff as an already present body of water that can increase dramatically in size in the days that follow precipitation events.

Water Cycle Facts

1. our oceans hold 96.5% of the earth’s water. This means that a single water molecule might not be given the opportunity to move from ocean to troposphere for tens of thousands of years.
2. Approximately 45 inches of water evaporate from the surface of the World Ocean every year; with a total surface area of 139.4 million square miles, that’s a lot of water vapor.
3. The troposphere contains 99% of our atmosphere’s water vapor.
4. The temperature in the troposphere can be as low as -115°F.
5. There is over a thousand times more water under the ground than in all of our Earth’s rivers, streams and lakes.
6. Although water covers 75% of the Earth’s surface, water represents just 0.05 percent of its total mass.
7. The average modern-day European uses 50 gallons of freshwater per day. The average medieval European used less than 5 gallons per day.
8. To grow a single cultivated tomato, you need 2.5 gallons of water. To grow a single grain of rice – just one grain – you need 0.05 gallons of water. If one portion of rice is about 3500 grains, that’s 175 gallons per meal. However, a loaf of factory-produced bread has used around 570 gallons of the Earth’s water before it arrives on the supermarket shelf.
9. A small, thirty-minute thunderstorm can produce around 2000 tons of rain.
10. 21 out of 37 of the world’s major aquifers are receding due to increasing population, and industrial and agricultural demands. This means we need to find alternative freshwater sources. In Israel, 86% of wastewater is recycled. The next-best global performer is Spain – this runner-up manages to recycle just 19% of its wastewater.

Quiz

1. How does the troposphere gain water molecules?
A. Evaporation, percolation, and sublimation
B. Evaporation, precipitation, transportation, and sublimation
C. Evaporation, sublimation, transpiration, and transportation
D. Evaporation, sublimation, and transpiration

2. Where do you find jet streams?
A. In aquafers
B. In the troposphere
C. In the oceans
D. In the stratosphere

3. Which of the following provides the most water vapor?
A. Precipitation
B. Sublimation
C. Evapotranspiration
D. Condensation

4. Which of the following is a delayed response to heavy precipitation?
A. Infiltration
B. Interflow
C. Baseflow
D. Runoff

5. What will you find under an aquifer?
A. Non-porous rock
B. Saltwater
C. Freshwater
D. Sand

Last Updated:
14 Apr 2023
— Updated example sentences