Editing Water table

{{Short description|Top of a saturated aquifer, or where the water pressure head is equal to the atmospheric pressure}}
{{For|the architectural feature|Water table (architecture)}}
[[File:Water table.svg|thumb|350px|Cross section showing the water table varying with surface topography as well as a perched water table]]
[[Image:Groundwater flow and infiltration diagram.svg|thumb|250px|Cross-section of a hillslope depicting the [[vadose zone]], [[capillary fringe]], water table, and the [[phreatic]] or saturated zone. ''(Source: [[United States Geological Survey]].)'']]

The '''water table''' is the upper surface of the [[phreatic zone]] or zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with [[groundwater]],<ref>{{Cite web|url=http://imnh.isu.edu/digitalatlas/hydr/concepts/gwater/wattable.htm|title=What is the Water Table?|website=imnh.isu.edu|access-date=2016-11-25}}</ref> which may be fresh, saline, or brackish, depending on the locality. It can also be simply explained as the depth below which the ground is saturated. The portion above the water table is the [[vadose zone]]. It may be visualized as the "surface" of the subsurface materials that are saturated with groundwater in a given vicinity.<ref name="groundwater">{{cite book |last1=Freeze |first1=R. Allan |first2=John A. |last2=Cherry |year=1979 |title=Groundwater |location=Englewood Cliffs, NJ |publisher=Prentice-Hall |isbn=9780133653120 |oclc=252025686}}{{page needed|date=February 2012}}</ref>

In coarse soils, the water table settles at the surface where the water [[Hydraulic head|pressure head]] is equal to the [[atmospheric pressure]] (where gauge pressure = 0). In soils where [[capillary action]] is strong, the water table is pulled upward, forming a [[capillary fringe]]. 

The groundwater may be from [[precipitation]] or from more distant groundwater flowing into the aquifer. In areas with sufficient precipitation, water infiltrates through pore spaces in the soil, passing through the unsaturated zone. At increasing depths, water fills in more of the pore spaces in the soils, until a zone of saturation is reached. Below the water table, in the zone of saturation, layers of permeable rock that yield groundwater are called [[aquifer|aquifers]]. In less permeable soils, such as tight bedrock formations and historic lakebed deposits, the water table may be more difficult to define.

“Water table” and “[[water level]]” are not synonymous. If a deeper aquifer has a lower permeable unit that confines the upward flow, then the water level in this aquifer may rise to a level that is greater or less than the elevation of the actual water table. The elevation of the water in this deeper well is dependent upon the pressure in the deeper aquifer and is referred to as the [[potentiometric surface]], not the water table.<ref name="groundwater" />

== Formation ==
The water table may vary due to seasonal changes such as [[Precipitation (meteorology)|precipitation]] and [[evapotranspiration]]. In undeveloped regions with permeable soils that receive sufficient amounts of precipitation, the water table typically slopes toward rivers that act to drain the groundwater away and release the pressure in the aquifer. [[Spring (hydrology)|Spring]]s, [[river]]s, [[lake]]s and [[oasis|oases]] occur when the water table reaches the surface. Groundwater entering rivers and lakes accounts for the base-flow water levels in water bodies.<ref>{{Cite web|url=https://pubs.usgs.gov/circ/circ1139/pdf/part1bb.pdf|title=Ground Water and Surface Water A Single Resource - U.S. Geological Survey Circular 1139|last1=Winter|first1=Thomas C|last2=Harvey|first2=Judson W|date=1998|access-date=25 August 2018}}</ref>

=== Surface topography ===
Within an aquifer, the water table is rarely horizontal, but reflects the surface relief due to the capillary effect ([[capillary fringe]]) in [[soil]]s, [[sediment]]s and other [[porous media]]. In the aquifer, groundwater flows from points of higher pressure to points of lower pressure, and the direction of groundwater flow typically has both a horizontal and a vertical component. The slope of the water table is known as the “hydraulic gradient”, which depends on the rate at which water is added to and removed from the aquifer and the permeability of the material. The water table does not always mimic the topography due to variations in the underlying geological structure (e.g.,&nbsp;folded, faulted, fractured bedrock).

=== Perched water tables ===
A perched water table (or perched aquifer) is an aquifer that occurs above the regional water table. This occurs when there is an impermeable layer of rock or sediment ([[aquiclude]]) or relatively impermeable layer ([[aquitard]]) above the main water table/aquifer but below the land surface. If a perched aquifer's flow intersects the surface, at a valley wall, for example, the water is discharged as a [[spring (hydrosphere)|spring]].

== Fluctuations ==
[[File:Water table-season fluctuation.svg|thumb|350px|Seasonal fluctuations in the water table may cause river beds to dry up during the dry season]]

=== Tidal ===
On low-lying [[Island#Oceanic islands|oceanic islands]] with porous soil, [[freshwater]] tends to collect in [[Lens (geology)|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 ===
In some regions, for example, [[Great Britain]] or [[California]], [[winter]] [[Precipitation (meteorology)|precipitation]] is often higher than [[summer]] precipitation and so the groundwater storage is not fully recharged in summer. Consequently, the water table is lower during the summer. 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 ===
[[Fossil water]] is groundwater that has remained in an aquifer for several millennia and occurs mainly in [[desert]]s. It is non-renewable by present-day [[rainfall]] due to its depth below the surface, and any extraction causes a permanent change in the water table in such regions.

==Effects on crop yield==
[[File: Rudd PartReg.png|thumb|upright=1.4|A plot of sugarcane yield versus depth of water table in Australia. The critical depth is 0.6&nbsp;m.<ref>Rudd, A.V. and C.W Chardon 1977. ''The effects of drainage on cane yields as measured by water table height in the Machnade Mill area.'' In: Proceedings of the 44th Conference of the Queensland Society of Sugar Cane Technology, Australia.</ref><ref>[https://www.waterlog.info/partreg.htm Software for partial regression with horizontal segment]</ref>]]

Most crops need a water table at a minimum depth.<ref>''Crop Yield versus Depth of the Ground Water Table, Statistical Analysis of Data Measured in Farm Lands Aiming at the Formulation of Drainage Needs.'' International Journal of Agricultural Science, 6, 174–187. Online: [https://www.iaras.org/iaras/filedownloads/ijas/2021/014-0023(2021).pdf] or [https://www.waterlog.info/pdf/Crop%20yield%20and%20depth%20of%20water%20table.pdf]</ref> For some important food and fiber crops a classification was made<ref>Nijland, H.J. and S. El Guindy 1984.''Crop yields, soil salinity and water table depth in the Nile Delta''. In: ILRI Annual Report 1983, Wageningen, The Netherlands, pp. 19–29. Online: [https://www.waterlog.info/pdf/egypt.pdf]</ref> because at shallower depths the crop suffers a yield decline.<ref>K.J.Lenselink et al. ''Crop tolerance to shallow water tables''. Online: [https://www.waterlog.info/cropwat.htm]</ref>

{| class="wikitable" 
! Crop and location || DWT tolerance  || Classification || Explanation 
|-
| Wheat, Nile Delta, Egypt  || 45 || Very tolerant || Resists shallow water tables
|-
| Sugar cane, Australia     || 60 || Tolerant || The water table should be deeper than 60&nbsp;cm
|-
| Banana, Surinam           || 70 || Slightly sensitive || Yield declines at water tables <&nbsp;70&nbsp;cm deep
|-
| Cotton, Nile Delta        || 90 || Sensitive || Cotton needs dry feet, water table should be deep
|}
:(where DWT = depth to water table in centimetres)

== Effects on construction ==
[[File:Elvis in Berlin.jpg|thumb|Blue pipes to remove groundwater in [[Berlin]]]]

A water table close to the surface affects excavation, drainage, foundations, wells and leach fields (in areas without municipal water and sanitation), and more.

When excavation occurs near enough to the water table to reach its capillary action, groundwater must be removed during construction. This is conspicuous in [[Berlin]], which is built on sandy, marshy ground, and the water table is generally 2 meters below the surface. Pink and blue pipes can often be seen carrying groundwater from construction sites into the [[Spree (river)|Spree]] river (or canals).{{citation needed|date = June 2023}}

== See also ==
* {{annotated link|Artesian aquifer}}
* {{annotated link|Groundwater recharge}}
* {{annotated link|Hydrogeology}}
* {{annotated link|Watertable control}}

==References ==
{{reflist}}

{{Agricultural water management}}
{{Authority control}}

[[Category:Aquifers]]
[[Category:Hydrology]]
[[Category:Hydrogeology]]
[[Category:Irrigation]]
[[Category:Water supply]]
[[Category:Water and the environment]]
[[Category:Karst]]