View source for Earth:Vadose zone

{{Short description|Unsaturated aquifer above the water table}}
[[Image:Vadose zone.gif|thumb|250px|Cross-section of a hillslope depicting the vadose zone, capillary fringe, water table, and phreatic or saturated zone. ''(Source: United States Geological Survey.)'']]
[[File:Water table.svg|thumb|350px|Cross section showing the water table varying with surface topography as well as a perched water table]]

The '''vadose zone''', also termed the  '''unsaturated zone''', is the part of Earth between the land surface and the top of the [[Earth:Phreatic zone|phreatic zone]], the position at which the groundwater (the water in the soil's pores) is at atmospheric pressure ("vadose" is from the [[Social:Latin|Latin]] word for "shallow"). Hence, the vadose zone extends from the top of the ground surface to the [[Earth:Water table|water table]].

Water in the vadose zone has a [[Physics:Hydraulic head|pressure head]] less than [[Physics:Atmospheric pressure|atmospheric pressure]], and is retained by a combination of [[Chemistry:Adhesion|adhesion]] (''funiculary groundwater''), and [[Physics:Capillary action|capillary action]] (''capillary groundwater''). If the vadose zone envelops [[Chemistry:Soil|soil]], the water contained therein is termed [[Chemistry:Soil moisture|soil moisture]]. In fine grained soils, capillary action can cause the pores of the soil to be fully saturated above the water table at a pressure less than atmospheric. The vadose zone does not include the area that is still saturated above the water table, often referred to as the [[Earth:Capillary fringe|capillary fringe]]. <ref name=Freeze>Freeze, R.A. and Cherry, J.A., 1979. Groundwater. Englewood Cliffs, NJ, Printice-Hall Inc., 604 p.</ref>

Movement of water within the vadose zone is studied within [[Earth:Soil physics|soil physics]] and [[Earth:Hydrology|hydrology]], particularly [[Earth:Hydrogeology|hydrogeology]], and is of importance to [[Earth:Agriculture|agriculture]], contaminant transport, and flood control. The [[Richards equation]] is often used to mathematically describe the flow of water, which is based partially on [[Physics:Darcy's law|Darcy's law]].  [[Earth:Groundwater recharge|Groundwater recharge]], which is an important process that refills aquifers, generally occurs through the vadose zone from precipitation.

==In hydrology==

[[File:Vadose Zone.jpg|thumb|left|The sharp contact between the vadose zone (brown oxidized mudstone) and the underlying phreatic zone (grey unoxidized mudstone) exposed at a construction site.]]
The vadose zone is the undersaturated portion of the subsurface that lies above the groundwater table. The soil and rock in the vadose zone are not fully saturated with water; that is, the pores within them contain air as well as water. The portion of the vadose zone that is inhabited by soil microorganism, fungi and plant roots may sometimes be called the [[Chemistry:Soil carbon sponge|soil carbon sponge]].  

In some places, the vadose zone is absent, as is common where there are lakes and marshes, and in some places, it is hundreds of meters thick, as is common in arid regions.<ref name=USGS />

Unlike the [[Earth:Aquifer|aquifer]]s of the underlying water-saturated [[Earth:Phreatic zone|phreatic zone]], the vadose zone is not a source of readily available water for human consumption. It is of great importance in providing water and nutrients that are vital to the soil carbon sponge and the [[Biology:Biosphere|biosphere]].  It is intensively used for the cultivation of plants, construction of buildings, and disposal of waste.<ref name=USGS />

The vadose zone is often the main factor controlling water movement from the land surface to the aquifer. Thus, it strongly affects the rate of aquifer recharge and is critical for the use and management of groundwater. Flow rates and chemical reactions in the vadose zone also control whether, where, and how fast contaminants enter groundwater supplies. Understanding of vadose-zone processes is therefore crucial in determining the amount and quality of groundwater that is available for human use.<ref name=USGS>[http://water.usgs.gov/ogw/unsaturated.html US Geological Survey, Office of Groundwater]</ref>

==In speleology==

[[File:Mammoth cave canyon.jpg|thumb|An example of a vadose cave passage in [[Earth:Mammoth Cave National Park|Mammoth Cave]], Kentucky]]
In [[Earth:Speleology|speleology]], [[Earth:Cave|cave]] passages formed in the vadose zone tend to be canyon-like in shape, as the water dissolves [[Earth:Bedrock|bedrock]] on the floor of the passage.<ref>[http://www.uppercumberlandcaving.net/glossary.html Caves & Caving: Glossary]</ref>  Passages created in completely water-filled conditions are called [[Earth:Phreatic|phreatic]] passages and tend to be circular in cross-section.<ref>[http://www.nps.gov/history/history/online_books/geology/publications/state/nm/1987-117/sec7-1.htm New Mexico: Bureau of Mines & Mining Bulletin 117 (Part I: Discussion of Deposits and Events)]</ref>

==See also==

* {{annotated link|Earth:Aquifer}}
* {{annotated link|Earth:Capillary fringe}}
* {{annotated link|Earth:Epiphreatic zone}}
* {{annotated link|Earth:Groundwater}}
* {{annotated link|Earth:Infiltration (hydrology)}}
* {{annotated link|Earth:Phreatic zone}}
* {{annotated link|Physics:Water retention curve}}

==References==

{{reflist}}

==Further reading==

* {{cite web | url = http://water.usgs.gov/ogw/unsaturated.html | title = Unsaturated Zone | work=USGS | access-date = 2014-05-01}}
* {{cite web | url = http://water.usgs.gov/edu/earthgwaquifer.html | title = Aquifers | work=USGS | access-date = 2014-05-01}}
* {{cite web | url = http://toxics.usgs.gov/definitions/unsaturated_zone.html | title = Unsaturated Zone Definitions Page | work = USGS | access-date = 2006-07-01 | archive-date = 2015-09-08 | archive-url = https://web.archive.org/web/20150908234849/http://toxics.usgs.gov/definitions/unsaturated_zone.html | url-status = dead }}
* {{cite web | url = http://wwwrcamnl.wr.usgs.gov/uzf/unsatflow.html | title = Unsaturated Zone Flow: Definitions and Details | work = USGS | access-date = 2006-07-01 |archive-url = https://web.archive.org/web/20060524210928/http://wwwrcamnl.wr.usgs.gov/uzf/unsatflow.html <!-- Bot retrieved archive --> |archive-date = 2006-05-24}}
* {{Cite book
  |title=Handbook of Vadose Zone Characterization & Monitoring 
  |editor=Wilson, L.G. 
  |editor2=Everett, Lorne G. 
  |editor3=Cullen, Stephen J. 
  |year=1994
  |publisher=CRC Press
  |isbn=0873716108
}}

{{Soil science topics}}

[[Category:Hydrology]]
[[Category:Hydrogeology]]
[[Category:Soil physics]]

{{Sourceattribution|Vadose zone}}