The lower portion of the vadose zone hosts less biological activity. Precipitation either runs off sloping land or infiltrates only the upper few inches of soil, then percolates downward and permeates the uppe vadose zone. As water enters soil voids, a variety of physical processes pull it into the vadose zone, replacing air. The water table is defined as the area that separates the vadose and saturated zones. Water below the water table is ground water.
All soils can store water in voids.
A soil's ability to store and transfer water downward in saturated or unsaturated conditions is a function of numerous interrelated processes and features. For example, the nature of soil particulates and the way they aggregate influence features such as porosity and how water is attracted to soils. Soils with small voids can store more water than those with larger voids. Under saturated flow, porosity and the pull of gravity greatly influence water movement. Horizontal flow of ground water generally is slow and is measured in inches per day or feet per year, depending on the porosity and the permeability of the materials making up the aquifer.