hydrologic cycle


hydrologic cycle
the natural sequence through which water passes into the atmosphere as water vapor, precipitates to earth in liquid or solid form, and ultimately returns to the atmosphere through evaporation.
[1955-60; HYDROLOG(Y) + -IC]

* * *

Cycle that involves the continuous circulation of water in the Earth-atmosphere system.

Water is transferred from the oceans through the atmosphere to the continents and back to the oceans by means of evaporation, transpiration, precipitation, interception, infiltration, subterranean percolation, overland flow, runoff, and other complex processes. Although the total amount of water within the cycle remains essentially constant, its distribution among the various processes is continually changing.

* * *

 cycle that involves the continuous circulation of water in the Earth-atmosphere system. Of the many processes involved in the hydrologic cycle, the most important are evaporation, transpiration, condensation, precipitation, and runoff. Although the total amount of water within the cycle remains essentially constant, its distribution among the various processes is continually changing.

      A brief treatment of the hydrologic cycle follows. For full treatment, see hydrosphere: The hydrologic cycle (hydrosphere).

      Evaporation (vaporization), one of the major processes in the cycle, is the transfer of water from the surface of the Earth to the atmosphere. By evaporation, water in the liquid state is transferred to the gaseous, or vapour, state. This transfer occurs when some molecules in a water mass have attained sufficient kinetic energy to eject themselves from the water surface. The main factors affecting evaporation are temperature, humidity, wind speed, and solar radiation. The direct measurement of evaporation, though desirable, is difficult and possible only at point locations. The principal source of water vapour is the oceans, but evaporation also occurs in soils, snow, and ice. Evaporation from snow and ice, the direct conversion from solid to vapour, is known as sublimation. transpiration is the evaporation of water through minute pores, or stomata, in the leaves of plants. For practical purposes, transpiration and the evaporation from all water, soils, snow, ice, vegetation, and other surfaces are lumped together and called evapotranspiration, or total evaporation.

      Water vapour is the primary form of atmospheric moisture. Although its storage in the atmosphere is comparatively small, water vapour is extremely important in forming the moisture supply for dew, frost, fog, clouds, and precipitation. Practically all water vapour in the atmosphere is confined to the troposphere (the region below 6 to 8 miles [10 to 13 km] altitude).

      The transition process from the vapour state to the liquid state is called condensation. Condensation may take place as soon as the air contains more water vapour than it can receive from a free water surface through evaporation at the prevailing temperature. This condition occurs as the consequence of either cooling or the mixing of air masses of different temperatures. By condensation, water vapour in the atmosphere is released to form precipitation.

      Precipitation that falls to the Earth is distributed in four main ways: some is returned to the atmosphere by evaporation, some may be intercepted by vegetation and then evaporated from the surface of leaves, some percolates into the soil by infiltration, and the remainder flows directly as surface runoff into the sea. Some of the infiltrated precipitation may later percolate into streams as groundwater runoff. Direct measurement of runoff is made by stream gauges and plotted against time on hydrographs.

      Most groundwater is derived from precipitation that has percolated through the soil. Groundwater flow rates, compared with those of surface water, are very slow and variable, ranging from a few millimetres to a few metres a day. Groundwater movement is studied by tracer techniques and remote sensing.

       ice also plays a role in the hydrologic cycle. Ice and snow on the Earth's surface occur in various forms such as frost, sea ice, and glacier ice. When soil moisture freezes, ice also occurs beneath the Earth's surface, forming permafrost in tundra climates. About 18,000 years ago glaciers and ice caps covered approximately one-third of the Earth's land surface. Today, about 12 percent of the land surface remains covered by ice masses.

* * *


Universalium. 2010.

Look at other dictionaries:

  • hydrologic cycle — n. WATER CYCLE …   English World dictionary

  • hydrologic cycle — noun : a complex sequence of conditions through which water naturally passes from water vapor in the atmosphere through precipitation upon land or water surfaces and ultimately back into the atmosphere as a result of evaporation and transpiration …   Useful english dictionary

  • hydrologic cycle — /haɪdrəˈlɒdʒɪk saɪkəl/ (say huydruh lojik suykuhl) noun → water cycle. Also, hydrological cycle …   Australian English dictionary

  • hydrologic cycle — noun Date: 1936 the sequence of conditions through which water passes from vapor in the atmosphere through precipitation upon land or water surfaces and ultimately back into the atmosphere as a result of evaporation and transpiration called also… …   New Collegiate Dictionary

  • hydrologic cycle — hy′drolog′ic cy′cle n. mer the natural sequence through which water passes into the atmosphere as water vapor, precipitates to earth, and returns to the atmosphere through evaporation • Etymology: 1955–60 …   From formal English to slang

  • hydrologic sciences — Introduction       the fields of study concerned with the waters of the Earth. Included are the sciences of hydrology, oceanography, limnology, and glaciology.       In its widest sense hydrology encompasses the study of the occurrence, the… …   Universalium

  • cycle — /suy keuhl/, n., v., cycled, cycling. n. 1. any complete round or series of occurrences that repeats or is repeated. 2. a round of years or a recurring period of time, esp. one in which certain events or phenomena repeat themselves in the same… …   Universalium

  • Water cycle — The Earth s water is always in movement, and the water cycle, also known as the hydrologic cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Since the water cycle is truly a cycle, there is no… …   Wikipedia

  • water cycle — I. noun Etymology: cycle from cycle (as in bicycle) : any of various more or less experimental watercraft propelled by treadles after the manner of a bicycle II. noun : hydrologic cycle …   Useful english dictionary

  • water cycle — noun Date: 1928 hydrologic cycle …   New Collegiate Dictionary


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.