The Critical Zone

Soil Moisture and Groundwater


In most places, an unsaturated zone of soil, sediment, and bedrock exist close to Earth's surface. Although it is unsaturated, the zone still may contain water by capillary action and adhesion to soil particles. This so-called vadose zone exists from the soil surface beneath our feet to the top of the water table. This should be relatively easy to understand if you imagine that if all of the soil and sediment beneath us was saturated, we'd constantly be walking through mud, quicksand, etc., dependent upon the parent material. Instead, as water infiltrates into the subsurface, much of it flows to the water table, while some is retained in the soil and sediment column, the vadose zone.

Reading assignment

  1. To learn more about the vadose zone visit the Toxic Substances Hydrology Program site at USGS and read the short paragraph and view the accompanying figure.
  2. At the previous link, you encountered the term soil moisture as it relates to the water contained within the unsaturated zone. Learn about soil moisture at NASA's Web site.
  3. The term groundwater usually refers to all subsurface water beneath the water table (the interface between the vadose and saturated zones, or the top of the saturated zone) in soils and geologic formations that are fully saturated. This definition excludes soil pore water in the vadose or unsaturated zone. Keeping in mind that the field of hydrogeology or groundwater hydrology is a sub-discipline of its own, learn about groundwater at the Water Encyclopedia site. In addition, learn about groundwater basics at the USGS Groundwater Information site.