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phreatic
Phreatic is a term used in hydrology to refer to aquifers, in speleology to refer to cave passages, and in volcanology to refer to eruption type.
==Hydrology==
The term phreatic is used in hydrology and the earth sciences to refer to matters relating to ground water (an aquifer) below the water table (the word originates from the Greek ''phrear'', ''phreat-'' meaning "well" or "spring"). The term 'phreatic surface' indicates the location where the pore water pressure is under atmospheric conditions (i.e. the pressure head is zero). This surface normally coincides with the water table. The slope of the phreatic surface is assumed to indicate the direction of ground water movement in an unconfined aquifer.
The phreatic zone, below the phreatic surface where rock and soil is saturated with water, is the counterpart of the vadose zone, or unsaturated zone, above. Unconfined aquifers are also referred to as phreatic aquifers because their upper boundary is provided by the phreatic surface.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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