The unsaturated zone is that portion of the subsurface in which the intergranular openings of the geologic medium contain both water and air. The unsaturated zone, also known as the vadose zone or the zone of aeration, extends downward from the land surface to the top of the underlying saturated zone. Water in the pores of this zone is at a pressure that is lower than atmospheric pressure. Most of the water that eventually recharges the saturated zone must first pass through the unsaturated zone.

The movement of water in the unsaturated zone is dominated by capillary action. This tendency of a liquid to be drawn into interstices, is the result of cohesion of water molecules and adhesion of those molecules to the solid material forming the void. Smaller voids produce greater capillary forces frequently great enough to resist the downward force of gravity.

Water originating...