The water cycle is known scientifically as the hydrologic cycle. It refers to the change of states of water between liquid, solid and gas. Also, the water, or hydrologic, cycle refers to the continuous exchange of water within the hydrosphere, for instance: 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.
- Precipitation falls from atmospheric moisture that has previously condensed to form clouds. This generally occurs as rain, but snow and other forms contribute to the cycle.
- 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. However, in the case of an artesian well, the water will rise under pressure since the well itself is below the recharge zone of an underground aquifer.
- 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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