Vadose zone
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The vadose zone, also termed the unsaturated zone, is the
part of Earth between the land surface and the top of the phreatic zone, the
position at which the groundwater (the water in the soil's pores) is at
atmospheric pressure ("vadose" is from the Latin word for
"shallow"). Hence, the vadose zone extends from the top of the ground
surface to the water table.
Water in the vadose zone has a pressure head less than
atmospheric pressure, and is retained by a combination of adhesion (funiculary
groundwater), and capillary action (capillary groundwater). If the vadose zone
envelops soil, the water contained therein is termed 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 capillary fringe.
Movement of water within the vadose zone is studied within
soil physics and hydrology, particularly hydrogeology, and is of importance to
agriculture, contaminant transport, and flood control. The Richards equation is
often used to mathematically describe the flow of water, which is based
partially on Darcy's law. Groundwater recharge, which is an important process
that refills aquifers, generally occurs through the vadose zone from
precipitation.
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 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.
Unlike the aquifers of the underlying water-saturated
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 biosphere. It is intensively
used for the cultivation of plants, construction of buildings, and disposal of
waste.
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.