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The water cycle—technically known as the hydrologic cycle—is the circulation of water within the earth's hydrosphere, involving changes in the physical state of water between liquid, solid, and gas phases. The hydrologic cycle refers to the continuous exchange of water between atmosphere, land, surface and subsurface waters, and organisms. In addition to storage in various compartments (the ocean is one such "compartment"), the multiple cycles that make up the earth's water cycle involve five main physical actions: evaporation, precipitation, infiltration, runoff, and subsurface flow:
- Evaporation is the transfer of water from bodies of surface water into the atmosphere. This transfer entails a change in the physical nature of water from liquid to gaseous phases. Along with evaporation can be counted transpiration from plants, as well as, to a lesser degree, perspiration from land mammals and marsupials. Thus, this transfer is sometimes referred to as evapotranspiration. 90% of atmospheric water comes from evaporation, while the remaining 10% is from transpiration.
- Precipitation is atmospheric moisture that has previously condensed to form clouds (changed from the gas phases to a liquid or solid phase), falling to the surface of the earth. This mostly occurs as rainfall, but snow, hail, fog drip, and other forms participate as well.
- Interception is precipitation trapped by vegetation instead of falling directly onto the soil.
- Infiltration into the ground is the transition from surface water to groundwater. The infiltration rate will depend upon soil or rock permeability as well as other factors. Infiltrated water may reach another compartment known as groundwater (i.e., an aquifer). Groundwaters tend to move slowly, so the water may return as surface water after storage within an aquifer for a period of time that can amount to thousands of years in some cases. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures.
- Runoff includes the variety of ways by which land surface water moves down slope to the oceans. Water flowing in streams and rivers may be delayed for a time in lakes. Not all precipitated water returns to the sea as runoff; much of it evaporates before reaching the ocean or reaching an aquifer.
- Subsurface flow incorporates movement of water within the earth, either within the vadose zone or aquifers. After infiltrating, subsurface water may return to the surface or eventually seep into the ocean.
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