Water table
The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with groundwater,[1] 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 water table is the surface where the water pressure head is equal to the atmospheric pressure (where gauge pressure = 0). It may be visualized as the "surface" of the subsurface materials that are saturated with groundwater in a given vicinity.[2]
The groundwater may be from precipitation or from 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 phreatic zone (zone of saturation), layers of permeable rock that yield groundwater are called 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.[2]
Formation
The water table may vary due to seasonal changes such as
Surface topography
Within an aquifer, the water table is rarely horizontal, but reflects the surface relief due to the capillary effect (
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 (
Fluctuations
Tidal
On low-lying
Seasonal
In some regions, for example,
Long-term
Effects on crop yield
Most crops need a water table at a minimum depth.[6] For some important food and fiber crops a classification was made[7] because at shallower depths the crop suffers a yield decline.[8]
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 |
- (where DWT = depth to water table in centimetres)
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).[citation needed]
See also
- Artesian aquifer– Confined aquifer containing groundwater under positive pressure
- Groundwater recharge – Groundwater that recharges an aquifer
- Hydrogeology – Study of the distribution and movement of groundwater
- 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.
- ^ OCLC 252025686.[page needed]
- ^ 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.
- ^ 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.
- ^ Software for partial regression with horizontal segment
- ^ 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 Agriculural Science, 6, 174–187. Online: [1] or [2]
- ^ 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]
- ^ K.J.Lenselink et al. Crop tolerance to shallow water tables. Online: [4]