• 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. 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 water cycle follows. For full treatment, see hydrosphere: The water cycle.

• Evaporation, 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 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 water 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.

• In the natural system, material circulation is driven by energy from the sun and, to a much lesser extent, from radioactive decay of elements in the earth’s interior and motions of its tides. This is a mechanical and inorganic view of the earth. In another and more realistic sense, the earth has a natural metabolism; materials have circulated about its surface for millions of years in a complex, interconnected web of biogeochemical cycles.
• An array of physical, chemical, and biological processes weather and erode rocks and transfer materials in and out of the atmosphere, from the atmosphere to the biota and back again, to the oceans via rivers, and to the continents by uplift. Each element has a natural biogeochemical cycle. It is these cycles and their relationship to the physical climate system that have led to the development of a relatively stable and resilient surface system during geologic time. Life has evolved in this system and plays a strong role in the development and maintenance of the system through processes, and fluxes.
• Human activities have contributed materials to the biogeochemical cycles. Some of these materials enter element cycles already naturally in operation; they are the same chemical species that have circulated for millions of years.
• Other materials are synthetic compounds and are foreign to the natural environment. These anthropogenic fluxes are leading to a number and variety of environmental issues, including the possibility of global climate change. There is no doubt that human activities have interfered in biogeochemical cycles and have modified the composition of the atmosphere. Understanding the consequences of this interference requires better quantitative descriptions of these cycles, their interconnections, and, in particular, their coupled response to perturbations, such as a change in climate.