The hydraulic gradient indicates which direction groundwater will flow, and how rapidly. Perched water tables occur when a low permeability material, located above the water table, blocks or intercepts the downward flow of water from the land surface. Permeability is a property of porous soils indicating how rapidly water will be transmitted through soils toward the groundwater. Sand and gravel have high permeabilities while clay has low permeability. Porosity refers to the percentage of the formation that consists of open spaces or voids that could contain air or water.
Porosity determines the amount of water that can be stored in a saturated zone.
For example, a saturated zone 100 feet thick with a porosity of 20 percent could store an equivalent water depth of approximately 20 feet. Pumping water level is the water level in a well when the pump is operating and water is being removed. Saturated zones are the portions of a soil profile or geologic formation where all voids, spaces or cracks are filled with water. No air is present. Figure 3 locates the saturated zone within a typical geologic formation.