Vadose zone

Cross-section of a hillslope depicting the vadose zone, capillary fringe, water table, and phreatic or saturated zone. (Source: United States Geological Survey.)

The vadose zone, also termed the unsaturated zone, is the part of Earth between the land surface and the top of the phreatic zone i.e. the position at which the groundwater (the water in the soil's pores) is at atmospheric pressure ("vadose" is from the Latin 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 (Freeze & Cherry, 1979).

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 hydrology

The unsaturated zone is the portion of the subsurface above the groundwater table. The soil and rock in this zone contains air as well as water in its pores. In some places the unsaturated 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 saturated zone below, the unsaturated zone is not a source of readily available water for human consumption. But it is of great importance in providing water and nutrients that are vital to the biosphere, and is intensively used for the cultivation of plants, construction of buildings, and disposal of waste.

Hydrologically, the unsaturated 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, critical for the use and management of groundwater. It is often regarded as a filter that removes undesirable substances. To some extent this is true, but a more general fact is that flow rates and chemical reactions in the unsaturated zone control whether, where, and how fast contaminants enter groundwater supplies. Understanding of unsaturated-zone processes is therefore crucial in determining the amount and quality of groundwater that is available for human use.

In speleology

In speleology, cave passages formed in the vadose zone tend to be canyon-like in shape, as the water dissolves bedrock on the floor of the passage. Passages created in completely water-filled conditions are called phreatic passages and tend to be circular in cross-section.

See also

• Aquifer
• Epiphreatic zone
• Groundwater
• Water retention curve