Editing Aquifer (section)

=== Saturated versus unsaturated ===
{{See also|Water content|Soil moisture}}
Groundwater can be found at nearly every point in the Earth's shallow subsurface to some degree, although aquifers do not necessarily contain [[fresh water]]. The Earth's crust can be divided into two regions: the ''[[water content|saturated]] zone'' or ''[[phreatic]] zone'' (e.g., aquifers, aquitards, etc.), where all available spaces are filled with water, and the ''unsaturated zone'' (also called the [[vadose zone]]), where there are still pockets of air that contain some water, but can be filled with more water.

''Saturated'' means the pressure head of the water is greater than [[atmospheric pressure]] (it has a gauge pressure &gt; 0). The definition of the water table is the surface where the [[Hydraulic head|pressure head]] is equal to atmospheric pressure (where gauge pressure = 0).

''Unsaturated'' conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water that incompletely fills the pores of the aquifer material is under [[suction]]. The [[Hydrogeology#Water content|water content]] in the unsaturated zone is held in place by surface [[Adhesion|adhesive forces]] and it rises above the water table (the zero-[[Hydrogeology#Hydraulic head|gauge-pressure]] [[Contour line#Barometric pressure|isobar]]) by [[capillary action]] to saturate a small zone above the phreatic surface (the [[capillary fringe]]) at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water-content basis. Water content in a capillary fringe decreases with increasing distance from the phreatic surface. The capillary head depends on soil pore size. In [[sand]]y soils with larger pores, the head will be less than in clay soils with very small pores. The normal capillary rise in a clayey soil is less than {{convert|1.8|m|ft|0|abbr=on}} but can range between {{convert|0.3|and|10|m|ft|0|abbr=on}}.<ref>{{cite web |url=http://www.ces.ncsu.edu/plymouth/programs/vepras.html |title=Morphological Features of Soil Wetness |publisher=Ces.ncsu.edu |access-date=6 September 2010 |url-status=dead |archive-url=https://web.archive.org/web/20100809084433/http://www.ces.ncsu.edu/plymouth/programs/vepras.html |archive-date=9 August 2010 }}</ref>

The capillary rise of water in a small-[[diameter]] tube involves the same physical process. The water table is the level to which water will rise in a large-diameter pipe (e.g., a well) that goes down into the aquifer and is open to the atmosphere.