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water-table
water-table The water-table is the surface that divides the zone that is permanently saturated with water (the phreatic zone) from the zone above in which the pore spaces are not completely filled with water (the vadose zone). The water-table is, in other words, the upper surface of the groundwater or the level below which an unconfined aquifer (a body of permeable rock) is permanently saturated with water. In practical terms, the water-table represents the shallowest depth at which water will flow into an open well.
The word ‘table’ can be misleading here: the water-table is rarely flat, and it can change its shape and position. The water-table is usually below ground, but where there are streams, lakes, or wetlands, parts of the water-table will coincide with the land surface. In general, the water-table tends to follow the contours of the ground above, but its relief is less marked than that of the ground. The level of the water-table is usually at its highest in the spring and at its lowest in the autumn. The water-table can also be described as the surface at which the pressure of groundwater is equal to atmospheric pressure. Below the water-table, the pore spaces in the rock are completely filled with water at a pressure that is greater than atmospheric pressure. Between the water-table and the land surface, some water at less than atmospheric pressure is held in pore spaces by capillary forces. Immediately above the water-table is the capillary fringe or capillary zone, a shallow region in which water can be drawn upon by capillary action. The capillary fringe is usually not more than a few metres deep, and may be only a few centimetres deep. A perched water-table can exist where there is an impervious layer at a relatively high level and water is reaching its upper surface faster that it is being lost by seeping through the layer and by lateral movement. It is thus possible to have two water-tables at the same point, one above the other. Springs usually mark the contact between the upper surface of the impervious layer and the surface of the ground. In coastal regions, sea water can infiltrate the rocks, and a salt-water wedge will then form below sea level under the fresh groundwater. (The fresh water remains above the salt water because of the difference between their densities.) If too much water is pumped from the ground in these circumstances, the fresh water will become contaminated with salt and will be unfit for drinking. The rate of extraction will then need to be reduced to allow the supply of fresh water from the landward side to restore the balance. J. Bahr |
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Cite this article
PAUL HANCOCK and BRIAN J. SKINNER. "water-table." The Oxford Companion to the Earth. 2000. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. PAUL HANCOCK and BRIAN J. SKINNER. "water-table." The Oxford Companion to the Earth. 2000. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O112-watertable.html PAUL HANCOCK and BRIAN J. SKINNER. "water-table." The Oxford Companion to the Earth. 2000. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O112-watertable.html |
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water-table
water-table The upper surface of groundwater or the level below which the material is permanently saturated with water. The region below the water table is the phreatic or vadose zone.
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Cite this article
MICHAEL ALLABY. "water-table." A Dictionary of Ecology. 2004. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. MICHAEL ALLABY. "water-table." A Dictionary of Ecology. 2004. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O14-watertable.html MICHAEL ALLABY. "water-table." A Dictionary of Ecology. 2004. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O14-watertable.html |
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water-table
water-table The upper surface of groundwater, or the level below which an unconfined aquifer is permanently saturated with water. See also VADOSE ZONE.
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Cite this article
AILSA ALLABY and MICHAEL ALLABY. "water-table." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. AILSA ALLABY and MICHAEL ALLABY. "water-table." A Dictionary of Earth Sciences. 1999. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O13-watertable.html AILSA ALLABY and MICHAEL ALLABY. "water-table." A Dictionary of Earth Sciences. 1999. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O13-watertable.html |
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water-table
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JAMES STEVENS CURL. "water-table." A Dictionary of Architecture and Landscape Architecture. 2000. Encyclopedia.com. 31 May. 2012 <http://www.encyclopedia.com>. JAMES STEVENS CURL. "water-table." A Dictionary of Architecture and Landscape Architecture. 2000. Encyclopedia.com. (May 31, 2012). http://www.encyclopedia.com/doc/1O1-watertable.html JAMES STEVENS CURL. "water-table." A Dictionary of Architecture and Landscape Architecture. 2000. Retrieved May 31, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O1-watertable.html |
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