Hydrologic cycle
The water cycle (known scientifically as the hydrologic cycle) refers to the change of states between liquid, solid and gas; and to the continuous exchange of water, within the hydrosphere, between the atmosphere, land, surface water, groundwater, and plants. It may be divided into four main phases: evaporation, precipitation, infiltration and runoff.
- Evaporation is generally the transfer of water from bodies of surface water to the atmosphere. Along with strict evaporation, it includes transpiration from plants; thus, it is sometimes called evapotranspiration.
- Atmospheric moisture may condense into clouds, and fall from them as precipitation. This generally occurs as rain, but snow and other forms contribute to the cycle. Precipitation seldom falls in the oceans, because under normal circumstances, mountain ranges are needed to induce condensation and the formation of clouds.
- 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. Groundwater moves very slowly, and may return as surface water or be stored within an aquifer for thousands of years. It generally returns to the surface at lower elevations under the usual force of gravity, but may also rise under pressure, as in the case of an artesian well.
- Runoff is the variety of ways in which surface water travels to the oceans. The water flows through rivers and may be delayed in lakes. Not all water completes the runoff phase; some of it evaporates before it reaches the oceans.
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