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The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure. A sustainable amount of water within a unit of sediment or rock, below the water table, in the phreatic zone is called an aquifer. The ability of the aquifer to store groundwater is dependent on the primary and secondary porosity and permeability.
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Form
The form of a water table may change and vary due to seasonal changes, topography and structural geology. In undeveloped regions, or areas with high amounts of precipitation, the water table roughly follows the contour of the overlying land surface, and rises and falls with increases or decreases in infiltration. Springs and oases occur when the water table reaches the surface. Springs commonly form on hillsides, where the earth's slanting surface may "intersect" with the water table. Other, unseen springs are found under rivers and lakes, and account for the base-flow water levels in water bodies.
Surface topography
Within an aquifer, the water table is rarely horizontal, but reflects the surface relief due to the capillary effect in soils, sediments and other porous media. In hilly regions, the variation in gradient give rise to rivers, springs or oases when the water table intersects the surface. It should be noted that the water table does not always mimic the topography due to variations in the underlying geologic structure (i.e. - folded, faulted, fractured bedrock).
Perched water tables
A perched water table (or perched aquifer) is an aquifer that occurs above the regional water table, in the vadose zone. This occurs when there is an impermeable layer of rock or sediment (aquiclude) or relatively impermeable layer (aquitard) above the main aquifer but below the surface. If a perched aquifer's flow intersects the Earth's surface, at a valley wall for example, the water is discharged as a spring.
Fluctuations
Seasonal fluctuations
In some regions (Great Britain for example), winter precipitation is often higher than summer precipitation and so the groundwater storage is not recharged in summer. Consequently, the water table is lower in the summer period yearly. This disparity between the level of the winter and summer water table is known as the zone of intermittent saturation, wherein the water table will fluctuate in response to climatic conditions.
Long term fluctuations
Fossil water is groundwater that has remained in an aquifer for millennia, and occurs mainly in deserts. Fossil water is non-renewable by present day rainfall due to its depth below the surface, and any extraction (such as mining) causes a permanent change in the water table in such regions.
