The Water Cycle Explained

The Water Cycle Explained

The term “water cycle” describes the continuous movement of water within the ecosphere. A vital element in the climate system, it consists of three basic mechanisms: (1) Evaporation of liquid water into water vapor. (2) Condensation of this gaseous vapor into liquid or semi-solid forms of precipitation. (3) Transportation of this precipitation back to the ocean, from where the cycle restarts. No new water is ever created because it’s a completely closed system, which is why it’s recycled so efficiently.

This continuous hydrological process operates at varying speeds. The water cycle in the Amazon Rainforest, for example, can be especially fast. Ocean water evaporating from the South Atlantic can be recycled half a dozen times, via rainfall and evapotranspiration, as the air mass moves across the continent until it meets the Andes, when the uplift causes major rainfall, filling up the Amazon river system, which then drains away into the Atlantic, where the whole process restarts. 1

One Of The Major Biogeochemical Cycles

The water cycle (also called the hydrologic cycle) is one of the big five biogeochemical cycles whose collective purpose is to recycle essential materials around the planet. The others are: the oxygen cycle, the carbon cycle, the nitrogen cycle, the phosphorus cycle and the sulfur cycle. 2

The water cycle also firmly connects Earth’s five major sub-systems: the hydrosphere (water), atmosphere (air), cryosphere (frozen water), lithosphere (soil, rocks) and biosphere (all living things).

Essential To Life

Like the other biogeochemicals, water is essential to life on Earth. Roughly 70 percent of the planet’s surface is covered with water. 3 The human body is composed of 60 percent water (brain 73 percent); adult men need to ingest about 3 litres (3.2 quarts) of water per day, adult women about 2.2 litres (2.3 quarts) per day. 4 Water is also essential for plants – who need water, carbon dioxide and sunlight for photosynthesis – and thus for all other living things that need plants.

Even if you don’t like drinking water, chances are you depend upon it more than you think. For example, it takes about 3,785 litres (1,000 gallons) of water to grow the wheat to make a two-pound loaf of bread. Shared between six people, that’s about 630 litres (166 gallons) each. And before you add a boiled egg, be aware that it takes about 450 litres (120 gallons) to produce one egg. 5

For some fascinating facts about the timeline of our planet, and global warming, see: History of Earth in One Year (Cosmic Calendar).

Do The Oceans Influence The Water Cycle?

Yes. The Earth contains roughly 1.4 billion cubic km (326 million cubic mi) of water, 96 percent of which is found in the ocean. What’s more, 86 percent of global evaporation takes place in the ocean. 6 See also our in-depth article: How Do Oceans Influence Climate?

Location of Earth’s Water 7

Water SourceWater Volume (cubic miles)% of Total
Oceans 317,000,000 97.24
Glaciers & ice 7,000,000 2.14
Groundwater 2,000,000 0.61
Fresh-water lakes30,0000.009
Inland seas 25,0000.008
Soil moisture 16,000 0.005
Atmosphere 3,100 0.001
Rivers 3000.0001
Total water volume 326,000,000 100.0000

What Are The 3 Main Stages Of The Water Cycle?

Water Cycle: 3 Stages Diagram
The various stages of the water cycle (also called the hydrologic cycle): evaporation, condensation, precipitation and transportation, is the pathway taken by H2O around the planet, moving from gas, to water to solid. As the word ‘cycle’ suggests there is no starting point. Image: © NASA

Evaporation

The first step of the hydrologic cycle is evaporation. This occurs when sunlight (which drives the whole process) heats water in the oceans. Some of it evaporates as vapor into the air. This chemical change occurs when water molecules acquire sufficient kinetic energy to eject themselves out of the water, into the air.

Roughly 86 percent of water vapor in the air comes from water evaporating from the ocean. 8 About another 10 percent comes from the exhalation of water by plants (transpiration) through minute pores in their leaves. The rest comes from the evaporation of water from lakes, streams, puddles and the soil. The evaporation of snow directly into vapor bypassing the liquid stage is known as “sublimation”, while the general term for all categories of evaporation from land sources is “evapotranspiration”. 9

What exactly is transpiration?

Transpiration is the evaporation of water from plants through small openings on the underside of leaves, known as stomata. A completely passive process, transpiration is regulated by the humidity of the atmosphere and the moisture content of the soil.

Almost all water vapour in the atmosphere is found in the troposphere, the layer closest to the surface of the Earth. It is here that all greenhouse gases accumulate and trap the radiant heat emitted by the planet.

Roughly 90 percent of all water evaporated from the ocean is returned to the sea as rainfall. The rest falls as precipitation on land, after which it can take a variety of paths back to the ocean.

The highest rates of ocean evaporation occur in winter, notably on the east coasts of continents. This is mainly due to winter storms that move off the east coasts helping to carry water vapor away from its source thereby enabling more evaporation to take place.

Warm ocean currents that move towards the poles along the east coasts of continents also play their part, contrasting significantly with much colder winter-time air masses moving overhead. When these differences in air and sea temperatures are combined with strong winds, evaporation becomes very efficient.

Condensation

The second step of the water cycle is condensation, a process during which water vapor changes from a gas to a liquid. Rising air currents carry the vapor up into the atmosphere. This is because water molecules are smaller than those of nitrogen and oxygen, and therefore less dense, which makes them more buoyant. The vapor rises into the air until cooler temperatures cause it to condense into liquid water droplets. Sometimes this process is facilitated by the presence of aerosols who may act as cloud condensation nuclei (CCN’s), around which liquid cloud droplets are formed.

When a large concentration of droplets comes together in the atmosphere, it is known as a cloud. Once a cloud forms, one of two things can happen. Either it gradually dissipates in the sunshine and the water droplets turn back into vapor, or the cloud grows until it gets so heavy that precipitation occurs. Condensation near ground level is called fog.

Precipitation

The third step of the water cycle is precipitation. Precipitation includes every sort of water that falls from the sky, including rain, sleet, snow or hail. It occurs when the condensed water droplets grow too large for the air to support, and thus fall earthwards. Precipitation is the main way that much needed fresh water is distributed around the globe.

When falling water lands on the ground it usually disperses in one of five ways:

  • Some is re-evaporated up into the atmosphere.
  • Some is intercepted by trees or plants and evaporated from the surface of their leaves. See: Sponge Cities – A Solution To Urban Flooding?
  • Some drains away into the soil where it is either taken up by tree or plant roots, or other living organisms, or percolates into the groundwater flow, ending up in streams and ultimately the ocean. Or it may bubble up to the surface as a spring and evaporate into the air.
  • Some is carried away by surface runoff, usually to the ocean. If runoff water flows into a lake with no outlet to the sea, then evaporation is the only way it can return to the atmosphere. Sometimes, as water evaporates, salts and other evaporites are left behind. As a result, the lake may become extremely salty. Examples of this phenomenon include the Dead Sea in Israel and the Great Salt Lake in Utah, USA.
  • Some may fall as snow in polar regions, or on high altitude areas. If so, it may be compacted and locked up in a glacier or ice sheet for millennia. Or, it may just stay frozen for a day or two, then melt and find its way to the ocean.

For much of the water falling on land, this section of the water cycle is the slowest. It may be thousands of years before it arrives in the ocean, especially if it percolates into a deep aquifer – an underground layer of subsurface, water-bearing permeable rock.

Roughly 505,000 cubic km (121,000 cubic mi) of water falls as precipitation every year – an average of 990 mm (39 inches) over the entire surface of the Earth. Only about one third of precipitation over land, ends up in the sea or ocean. The other two thirds are absorbed into the soil, of which a proportion will evaporate or be transpired into the atmosphere.

What Exactly Is Surface Runoff?

Surface runoff, also known as overland flow, is precipitation (typically stormwater or meltwater) that is unable to evaporate or drain away into the soil. As a result, it pools or runs away along the surface. In built-up areas, surface runoff frequently occurs where impervious materials (roofs, roads, pavements) prevent water from soaking into the ground.

Surface runoff is more likely to occur: (1) if the soil is already saturated due to excessive precipitation; (2) if it belongs to a type of soil, like clay, that does not absorb water very well; (3) if the terrain is sloped; (4) if vegetation is scarce.

Water runoff has positive and negative impacts on the local environment and ecosystem. To begin with, runoff is one of the most important means of recycling other biogeochemicals, such as carbon and phosphorus, following chemical weathering. 1

Unfortunately, the balanced distribution system set up by nature has been totally upset by the excessive use of fertilizers in the agricultural industry, and the waste disposal practices of the mining and chemical industries. As a result, surface runoff has become a source of increasing water pollution. For example, the dead zone at the outlet of the Mississippi River has been caused by surface run off transporting fertilizer nitrates from agricultural fields down the river system to the Gulf of Mexico. 10 11

In addition, surface runoff has become one of the main causes of soil erosion. 12

The Slow Water Cycle

Scientists sometimes say that much more water is “in storage” for long periods of time than is actually moving through the hydrological cycle – the storehouses in this case being the oceans, who hold a massive 97 percent of the planet’s water. 13

That said, huge amounts of warm and cool ocean water are continuously transported around the globe by the thermohaline circulation (also known as “the ocean conveyor belt”). This carefully calibrated serpentine network of ocean currents move energy around the globe, and have a significant effect on the Earth’s climate and weather.

At the same time, the network plays a vital role in replenishing the warmer, usually nutrient-depleted surface water with more nutrient-rich upwelled water from the depths. The latter has a huge effect on the growth and reproduction of phytoplankton, as well as zooplankton, the slightly larger krill and the rest of the marine food chain. 14

For these reasons one can say that the Thermohaline circulation is an integral part of the water cycle that operates within the ocean itself.

How Long Water Spends In Reservoirs
Source: 15

There are exceptions to these averages. For example, vapor that rises as high as the stratosphere – the layer of the atmosphere which sits above the troposphere where weather usually forms – is likely to stay there for a long time. Also, while water can spend thousands of years moving very slowly through the ocean depths, water in warm, shallow areas may evaporate and leave the ocean very quickly. 6

How Does The Water Cycle Affect Global Climate?

The hydrologic cycle influences global climate in five major ways.

First, water – in the form of water vapor – is a key contributor to the tropospheric greenhouse effect, which prevents some of the planet’s radiant energy from being returned to deep space. This maintains Earth’s temperature at a cosy 15 °C (59 °F), instead of the minus 18°C (0°F) it would otherwise be.

But water vapor is not a cause of today’s global warming. Water vapor has gathered in the atmosphere for thousands of years without raising the temperature of the Earth. The cause of global warming is the rising level of anthropogenic greenhouse gas emissions, chiefly carbon dioxide, resulting from the burning of fossil fuels, like coal, oil and gas. Rather than a cause, water vapor is an amplifier of warming. The process goes like this: as the atmosphere warms, it causes more evaporation from the oceans because warm air holds more moisture. As a result, more water vapor gathers in the troposphere, where it traps more heat, which warms the atmosphere leading to more water vapor, which leads to more heat, and so on. In other words, water vapor is a climate feedback, not a climate forcing.

Second, by locking up large quantities of water in glaciers and polar ice, the water cycle helps to maintain the cooling effect of the cryosphere and to support the thermohaline circulation in its task of absorbing carbon dioxide from the polar air and pulling it down into the slow, deep water carbon cycle.

Thirdly, as water vapor rises it condenses, forming clouds. And clouds can exert both a warming and a cooling effect on temperature and climate. To begin with they help to trap heat rising from the Earth, a mechanism that, for example, prevents overnight frosts in winter. At the same time white clouds reflect sunlight back into space thus cooling the planet. For more on this topic, please see: How Do Clouds Affect Climate?

Fourthly, in tropical rainforests, transpiration (water evaporation from trees and plants) exerts a cooling effect on the surrounding area. The Amazon Basin actually creates its own mini-climate system. This is why the deforestation in the Amazon Rainforest is such a concern. Scientists think that rising temperatures will dry out the rainforest biome, shutting down its unique water cycle and converting it into grassland.

Fifthly, the ocean plays a massive role in storing heat. This is helped by its ability to spread the heat far and wide thanks to its serpentine system of currents and convective movements. Anytime the earth’s surface cools or is heated by the sun, for instance, the effect on temperature is felt more (and more rapidly) over the land, rather than over the sea.

How Is Climate Change Affecting The Water Cycle?

Global warming means hotter extremes and longer droughts. It also means more evaporation and more precipitation – mostly in the form of rainfall.

For example, the IPCC’s Fourth Assessment Report (2007) states that the water cycle is likely to continue to intensify throughout the 21st century, in both rainfall and drought. 16 In dry subtropical areas, for instance, precipitation is likely to decrease during the 21st century, thus increasing the probability of drought. The drying is predicted to be most pronounced in South Africa, southern Australia, the Mediterranean Basin, and the Southwestern United States. Meantime, annual rainfall is predicted to increase in wetter equatorial regions, and also at high latitudes.

Overall, IPCC scientists believe that increased hydrologic variability as well as climate change will continue to have a major influence on the water cycle, water availability and scarcity, at global, regional, and local levels. 17

In its 2019 Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC), the Intergovernmental Panel on Climate Change warned that global warming is unbalancing the hydrological cycle in a wide variety of ways.

  • In general, the widest effects of climate change are being felt by the ocean which, let’s not forget, is by far the most important player in the water cycle. The IPCC’s 1300-page Report listed the following impacts: increased temperatures (virtually certain), greater upper ocean stratification (very likely), further acidification (virtually certain), oxygen decline (medium confidence), and more frequent extreme El Niño-Southern Oscillation events (medium confidence). 18 For an in-depth article on this topic, see: Effects of Global Warming on Oceans.
  • Global warming melts glaciers and polar ice sheets, and at the same time expands the volume of the water in the ocean (thermal expansion). Both factors help to raise sea levels. In its summary of the IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate, Carbon Brief said that the rate of sea level rise is “unprecedented” over the past century. A rise of 2 metres (6.6 ft) by 2100 “cannot be ruled out if greenhouse gas emissions continue to increase strongly.” 19
  • As the area of ice shrinks, so does its albedo effect. As a result, less sunlight is reflected back into space, leading to an increase in solar energy reaching the surface of the planet. According to the IPCC’s Special Report, the Arctic Ocean could be ice free in September “one year in three” if global warming continues to rise to 2 °C. Prior to the Industrial Revolution it was ice free only “once in every hundred years”. 20
  • As global temperature projections continue to rise, so does the amount of sub-surface Arctic permafrost and peat that is thawing, along with the amount of carbon dioxide and methane being emitted.
  • The IPCC report also highlighted the weakening of the Atlantic meridional overturning circulation (AMOC) – part of the global Thermohaline Circulation – caused by global warming. This is likely to affect numerous aspects of global climate. 18

References

  1. Rainforest-initiated wet season onset over the southern Amazon.” Jonathon S. Wright, Rong Fu, John R. Worden, Sudip Chakraborty, Nicholas E. Clinton, Camille Risi, Ying Sun, Lei Yin. PNAS August 8, 2017 114 (32) 8481-8486. [][]
  2. Biogeochemical Cycles.” The Environmental Literacy Council. 2015. []
  3. North Carolina Climate Office. “The Water Cycle.” climate.ncsu.edu/edu/WaterCycle []
  4. “The Water in You: Water and the Human Body.” USGS. []
  5. “Interesting Water Facts.” Oldham County Water District. []
  6. The Water Cycle. UCAR Center for Science Education (UCAR SciEd). [][]
  7.  “Where is Earth’s water located?” National Oceanic and Atmospheric Administration. National Weather Service. JetStream. []
  8. “Salinity. Science Mission Directorate”. science.nasa.gov []
  9. “Sublimation.” National Snow and Ice Data Center”. nsidc.org. []
  10. Nitrogen Input to the Gulf of Mexico”. Journal of Environment Quality. 30 (2): 329–36. Goolsby, Donald A.; Battaglin, William A.; Aulenbach, Brent T.; Hooper, Richard P. (2001). []
  11. “Happening Now: Dead Zone in the Gulf 2019.” Ocean Today. NOAA. []
  12. “Soil Erosion – Causes and Effects. Factsheet.” Ontario Ministry of Agriculture, Food and Rural Affairs. July 25, 2019. []
  13. “The Water Cycle summary”. USGS Water Science School. []
  14. “Marine Fisheries Ecology.” Oxford: Blackwell Science Ltd. ISBN 0-632-05098-5. Jennings, S., Kaiser, M.J., Reynolds, J.D. (2001) []
  15. Pidwirny, M. (2006). “The Hydrologic Cycle”. Fundamentals of Physical Geography, 2nd Edition. []
  16. Climate Change 2007: The Physical Science Basis” (PDF). International Panel on Climate Change. Alley, Richard; et al. (February 2007). []
  17. “Water and climate change: understanding the risks and making climate-smart investment decisions”. Washington, DC: World Bank. pp. 1–174. Vahid, Alavian; Qaddumi, Halla Maher; Dickson, Eric; Diez, Sylvia Michele; Danilenko, Alexander V.; Hirji, Rafik Fatehali; Puz, Gabrielle; Pizarro, Carolina; Jacobsen, Michael (November 1, 2009). []
  18. IPCC’s Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). Summary for Policymakers (PDF) September 25, 2019. [][]
  19. In-depth Q&A: The IPCC’s special report on the ocean and cryosphere”. Carbon Brief. September 25, 2019. []
  20. IPCC (Press release). Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC). September 25, 2019. []
Share on facebook
Share on twitter
Share on linkedin
Share on whatsapp
Share on email