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Thevadose zone (from theLatin word for "shallow"), also termed theunsaturated zone, is the part of Earth between the land surface and the top of thephreatic zone, the position at which the groundwater (the water in the soil's pores) is at atmospheric pressure. Hence, the vadose zone extends from the top of the ground surface to thewater table.
Water in the vadose zone has apressure head less thanatmospheric pressure, and is retained by a combination ofadhesion (funiculary groundwater), andcapillary action (capillary groundwater). If the vadose zone envelopssoil, the water contained therein is termedsoil 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 thecapillary fringe.[1]
Movement of water within the vadose zone is studied withinsoil physics andhydrology, particularlyhydrogeology, and is of importance toagriculture,contaminant transport, andflood control. TheRichards equation is often used to mathematically describe the flow of water, which is based partially onDarcy's law.Groundwater recharge, which is an important process that refills aquifers, generally occurs through the vadose zone from precipitation.
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 thesoil 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.[2]
Unlike theaquifers of the underlying water-saturatedphreatic 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 thebiosphere. It is intensively used for the cultivation of plants, construction of buildings, and disposal of waste.[2]
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.[2]
Inspeleology,cave passages formed in the vadose zone tend to be canyon-like in shape, as the water dissolvesbedrock on the floor of the passage.[3] Passages created in completely water-filled conditions are calledphreatic passages and tend to be circular in cross-section.[4]
