Water table
From Wikipedia, the free encyclopedia
The water table or phreatic surface is the surface where the water pressure is equal to atmospheric pressure.
A large amount of water within a body of sand or rock below the water table is called an aquifer, and the ability of rocks to store such groundwater is dependent on their porosity and permeability.
The practice of drilling wells to extract groundwater is dependent on understanding the water table. Because wells must reach the water table, its depth determines the minimum depth of a viable well, and thus the feasibility of drilling it.
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[edit] Form
The form of a water table may change and vary due to seasonal changes or topography. 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 rainy or dry weather. 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 sometimes surprisingly well-preserved water levels which occur in times of mild drought.
[edit] Surface topography
Within an aquifer, the water table is rarely horizontal, but reflects the surface relief due to the effect of gravity.[citation needed] In hilly regions, the variation in gradient give rise to rivers, springs or oases when the water table intersects the surface.
[edit] Perched water tables
A perched water table (or perched aquifer) is an aquifer that occurs above the main water table. This occurs when there is an impermeable layer of rock (aquiclude) above the main aquifer but below the surface. Water percolating down to the main aquifer gets trapped above this second impermeable rock layer. If a perched aquifer's flow intersects the Earth's surface, at a valley wall for example, the water is discharged as a spring.
[edit] Fluctuations
[edit] Seasonal fluctuations
In some regions (Britain for example), winter precipitation is often higher than summer precipitation. The groundwater storage is not recharged by precipitation in summer, consequently, the water table is lowered in the April-October 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.
[edit] 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 their depth below the surface, and any extraction ('mining') causes a permanent change in the water table in such regions.
