Porosity

The ratio of the volume of void space to the total volume of an undisturbed sample. A measure of the ratio of open space within a rock or soil to its total volume. A nondimensional value that expresses the ratio of the volume of pores to the total volume of a porous material and is usually expressed as a percentage. Porosity ranges from less than 1 percent to as much as 80 percent in some recently deposited clays, but in most granular materials it falls between about 5 and 40 percent. In free aquifers the porosity is equal to the specific retention plus the specific yield. The capacity of soil or rock to hold water.

In hydrologic terms,

(1) The ratio of pore volume to total volume of the formation. Sandy soils have large pores and a higher porosity than clays and other fine-grained soils.

(2) An index of the void characteristics of a soil or stream as pertaining to percolation; degree of previousness.