The water table is the upper surface of the zone where soil pores and fractures are fully saturated with groundwater. Above it lies the vadose zone, consisting of unsaturated soil. The water table depth varies with soil type and pressure conditions; in coarse soils, it aligns where the pressure head equals atmospheric pressure, while strong capillary action can pull water upward, forming a capillary fringe. Groundwater originates from precipitation or distant sources, filling permeable zones called aquifers. The term “water table” differs from water level, which in confined aquifers corresponds to the potentiometric surface, affected by subsurface pressure rather than saturation depth.
Formation
The water table may vary due to seasonal changes such as 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. Springs, rivers, lakes and oases occur when the water table reaches the surface. Groundwater entering rivers and lakes accounts for the base-flow water levels in water bodies.4
Surface topography
Within an aquifer, the water table is rarely horizontal, but reflects the surface relief due to the capillary effect (capillary fringe) in soils, sediments 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., 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.
Fluctuations
Tidal
On low-lying oceanic islands with porous soil, freshwater 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
In some regions, for example, Great Britain or California, 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 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 deserts. 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
Most crops need a water table at a minimum depth.5 For some important food and fiber crops a classification was made6 because at shallower depths the crop suffers a yield decline.7
| 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 cm |
| Banana, Surinam | 70 | Slightly sensitive | Yield declines at water tables < 70 cm deep |
| Cotton, Nile Delta | 90 | Sensitive | Cotton needs dry feet, water table should be deep |
Effects on construction
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 (or canals).
See also
- Artesian aquifer – Confined aquifer containing groundwater under positive pressurePages displaying short descriptions of redirect targets
- Groundwater recharge – Groundwater that recharges an aquifer
- Hydrogeology – Study of groundwater's movement and distribution
- Watertable control – Use of drainage to control the groundwater level in an area
References
"What is the Water Table?". imnh.isu.edu. Retrieved 2016-11-25. http://imnh.isu.edu/digitalatlas/hydr/concepts/gwater/wattable.htm ↩
Freeze, R. Allan; Cherry, John A. (1979). Groundwater. Englewood Cliffs, NJ: Prentice-Hall. ISBN 9780133653120. OCLC 252025686.[page needed] 9780133653120 ↩
Freeze, R. Allan; Cherry, John A. (1979). Groundwater. Englewood Cliffs, NJ: Prentice-Hall. ISBN 9780133653120. OCLC 252025686.[page needed] 9780133653120 ↩
Winter, Thomas C; Harvey, Judson W (1998). "Ground Water and Surface Water A Single Resource - U.S. Geological Survey Circular 1139" (PDF). Retrieved 25 August 2018. https://pubs.usgs.gov/circ/circ1139/pdf/part1bb.pdf ↩
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: [1] or [2] https://www.iaras.org/iaras/filedownloads/ijas/2021/014-0023(2021).pdf ↩
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: [3] https://www.waterlog.info/pdf/egypt.pdf ↩
K.J.Lenselink et al. Crop tolerance to shallow water tables. Online: [4] https://www.waterlog.info/cropwat.htm ↩