What is it?
The water cycle, also known as the hydrological cycle, is the circulation of water between the different compartments or reservoirs of the Earth's Hydrosphere, involving changes in the physical state of water between liquid, solid, and gaseous phases. The water cycle is powered by the Sun's energy and the Earth's gravity.
The Earth's water cycle involves the following main physical processes:
| Evaporation | Is the transfer of water from bodies of surface water into the atmosphere. This transfer involves a change in the physical state of water from liquid to gaseous phases, powered mainly by the solar radiation. 90% of atmospheric water comes from evaporation. |
| Evapotranspiration | Is the transfer of water from living beings into the atmosphere. This transfer involves a change in the physical state of water from liquid to gaseous phases, powered mainly by the solar radiation and the heat released by the metabolism of the living beings. 10% of atmospheric water comes from evapotranspiration. |
| Condensation | It takes place when water vapour in the air accumulates to form liquid water droplets in clouds and fog. |
| Precipitation | Is atmospheric moisture that has previously condensed (or solidified), falling to the surface of the Earth. This happens mostly as rainfall, but also as snow, hail, or fog. |
| Surface runoff | Includes the variety of ways by which land surface water moves down slope to the oceans: snowmelt runoff to streams, streamflow, riverflow… Water flowing in streams and rivers may be delayed for a time in lakes. Much of the precipitated water evaporates before reaching the ocean or infiltrates into the soil. |
| Infiltration | Is the transition of land surface water into the ground. The infiltration rate depends on soil or rock permeability. Infiltrated water may become part of the soil moisture or accumulate in aquifers: in this case it is called groundwater. |
| Groundwater flow | Includes the movement of groundwater in aquifers. Aquifers tend to move slowly, so the water may return as surface water (into rivers, lagoons, oceans or through springs) after thousands of years in some cases. Water returns to the land surface at lower elevation than where it infiltrated. |
| Absorption or drinking | Are the ways in which soil moisture or surface water is taken in by living beings. |
Volume of water stored in the water cycle's reservoirs:
| Volume (106 km3) | Percent of total | |
|---|---|---|
| Seas and oceans | 1370 | 97.25 |
| Ice caps, glaciers and snow covers | 29 | 2 |
| Groundwater | 9.5 | 0.7 |
| Lakes | 0.125 | 0.01 |
| Soil moisture | 0.065 | 0.005 |
| Atmosphere | 0.013 | 0.001 |
| Streams and rivers | 0.0017 | 0.0001 |
| Living beings | 0.0006 | 0.00004 |
Average reservoir residence times:
| Groundwater: deep | 10,000 years |
| Seas and oceans | 3,200 years |
| Groundwater: shallow | 100 to 200 years |
| Lakes | 50 to 100 years |
| Ice caps and glaciers | 20 to 100 years |
| Streams and rivers | 2 to 6 months |
| Seasonal snow covers | 2 to 6 months |
| Soil moisture | 1 to 2 months |
| Atmosphere | 9 days |
