water table

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For an architectural feature, see Water table (architecture).

The water table is the level at which the groundwater pressure is equal to atmospheric pressure. It may be conveniently visualized as the 'surface' of the groundwater in a given vicinity. It usually coincides with the phreatic surface, but can be many feet above it. As water infiltrates through pore spaces in the soil, it first passes through the zone of aeration, where the soil is unsaturated. At increasing depths water fills in more spaces, until the zone of saturation is reached. The relatively horizontal plane atop this zone constitutes the water table.

Cross section showing the water table varying with surface topography as well as a perched water table

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 of the rock or soil.

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. When water reaches the zone of saturation the movement of the water is no longer vertical, it is horizontal in the direction of the slope of the water table. The slope of the water table stand for the hydraulic gradient, which then depends on the rate at which water is added to the system and the permeability of the material. 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 of the land. If a perched aquifer's flow intersects the Earth's dry surface, at a valley wall for example, the water is discharged as a spring.

Fluctuations

Tidal fluctuations

On low-lying oceanic islands with porous soil, fresh water tends to collect in lenticular pools on top of the denser seawater intruding from the sides of the islands. Such an island's freshwater lens, and thus the water table, rises and falls with the tides.

Seasonal fluctuations in the water table. During the dry season, river beds may dry up.

Seasonal fluctuations

In some regions, for example, Great Britain or Ireland or Carrigaline, winter precipitation is often higher than summer precipitation and so the groundwater storage is not fully 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.

See also

• Groundwater
• Groundwater recharge
• Aquifer
• Hydrogeology
• Watertable control
• Origin of water on Earth
• Salinity
• Salinity in Australia