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The vadose zone (from the Latin word for "shallow"), 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. Hence, the vadose zone extends from the top of the ground surface to the water table.
Water in the vadose zone has a Hydraulic 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. Freeze, R.A. and Cherry, J.A., 1979. Groundwater. Englewood Cliffs, NJ, Printice-Hall Inc., 604 p.
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.
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 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. US Geological Survey, Office of Groundwater
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