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hydrosphere
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Earth's environmental spheres hydrologic cycle
Follow water as it cycles through the air, land, lakes and rivers, and oceans
Mekong River suspended soil particles temperate lake circulation diagram illustrating the water table Perito Moreno Glacier, Los Glaciares National Park surface hydrologic cycle Latitudinal variation in precipitation and evaporation
For Students
hydrosphere summary
Quizzes
water glass on white background. (drink; clear; clean water; liquid)
Water and its Varying Forms
rain. Child in the rain, wearing a rain coat, under a yellow umbrella. April Showers weather climate rain storm water drops
April Showers to March’s Lions and Lambs
Related Questions
Read Next
Sea shell of sea snail in close up showing damage and pitting.
Ocean Acidification: How Carbon Dioxide Is Hurting the Seas
World map. Continents. Oceans. Mendel thumb ok
Just How Many Oceans Are There?
Large ocean waves breaking on a stormy day. (wave; sea; whitecaps; storm)
Why Is There Fresh and Salt Water on Earth?
ripped torn flag, pirate, skull
Are There Laws on the High Seas?
A big, powerful wave breaking near the shore.
What Makes a Wave Go Rogue?
Discover
Periodical Cicada, Adult, Magicicada spp. Requires 17 years to complete development. Nymph splits its skin, and transforms into an adult. Feeds on sap of tree roots. Northern Illinois Brood. This brood is the largest emergence of cicadas anywhere
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Albert Einstein in Washington, D.C. c. 1921-1923. Physicist
The Solar Eclipse That Made Albert Einstein a Science Celebrity
Pluto. The Changing Faces of Pluto. Most detailed view to date of the entire surface of the dwarf planet Pluto, as constructed from multiple NASA Hubble Space Telescope photographs taken from 2002 to 2003.
10 Important Dates in Pluto History
Solar Eclipse, Eclipse, Solar Flare, Outer Space, Astronomy
What Causes Lunar and Solar Eclipses?
Solar eclipse of the sun.
The Sun Was Eaten: 6 Ways Cultures Have Explained Eclipses
Two chicks near an egg with a white background (poultry, chick, chickens, birds).
What Do Eggs Have to Do with Easter?
solar eclipse, sun, moon, astronomy, space
How Do You Tell the Difference Between Total, Annular, Solar, and Lunar Eclipses?

The water cycle

General nature of the cycle

surface hydrologic cycle
surface hydrologic cycle
The present-day surface hydrologic cycle, in which water is transferred from the oceans through the atmosphere to the continents and back to the oceans over and beneath the land surface. The values in parentheses following the various forms of water (e.g., ice) refer to volumes in millions of cubic kilometres; those following the processes (e.g., precipitation) refer to their fluxes in millions of cubic kilometres of water per year.

The present-day water cycle at Earth’s surface is made up of several parts. Some 496,000 cubic km (about 119,000 cubic miles) of water evaporates from the land and ocean surface annually, remaining for about 10 days in the atmosphere before falling as rain or snow. The amount of solar radiation necessary to evaporate this water is half of the total solar radiation received at Earth’s surface. About one-third of the precipitation falling on land runs off to the oceans primarily in rivers, while direct groundwater discharge to the oceans accounts for only about 0.6 percent of the total discharge. A small amount of precipitation is temporarily stored in the waters of rivers and lakes. The remaining precipitation over land, 73,000 cubic km (17,500 cubic miles) per year, returns to the atmosphere by evaporation. Over the oceans, evaporation exceeds precipitation, and the net difference represents transport of water vapour over land, where it precipitates as rain or snow and returns to the oceans as river runoff and direct groundwater discharge.

The various reservoirs in the water cycle have different water residence times. Residence time is defined as the amount of water in a reservoir divided by either the rate of addition of water to the reservoir or the rate of loss from it. The oceans have a water residence time of 3,000 to 3,230 years; this long residence time reflects the large amount of water in the oceans. In the atmosphere the residence time of water vapour relative to total evaporation is only about 10 days. Lakes, rivers, ice, and groundwaters have residence times lying between these two extremes and are highly variable.

Latitudinal variation in precipitation and evaporation
Latitudinal variation in precipitation and evaporation
Latitudinal variation in precipitation and evaporation and its relationship to major wind belts and oceanic salinity.

There is considerable variation in evaporation and precipitation over the globe. In order for precipitation to occur, there must be sufficient atmospheric water vapour and enough rising air to carry the vapour to an altitude where it can condense and precipitate. Precipitation and evaporation vary with latitude and their relation to the global wind belts. The trade winds, for example, are initially cool, but they warm up as they blow toward the Equator. These winds pick up moisture from the ocean, increasing ocean surface salinity and causing seawater at the surface to sink. When the trade winds reach the Equator, they rise, and the water vapour in them condenses and forms clouds. Net precipitation is high near the Equator and also in the belts of the prevailing westerlies, where there is frequent storm activity. Evaporation exceeds precipitation in the subtropics, where the air is stable, and near the poles, where the air is both stable and has a low water vapour content because of the cold. The Greenland Ice Sheet and the Antarctic Ice Sheet formed because the very low evaporation rates at the poles resulted in precipitation exceeding evaporation in these local regions.