Groundwater Storage, Porosity, and Specific Yield: Groundwater occupies the cracks and pore spaces between rocks and mineral grains below the land surface. In the saturated zone, essentially all of the pores are filled with water. If a volume of saturated aquifer material is completely dried, the water volume removed reflects the total porosity of the material, or the fraction of pore space within the total volume of solids plus open spaces. This number can be surprisingly large; some minerals and rock formations can have total porosities in excess of 50%. In the unsaturated, or vadose, zone there can be significant amounts of water present, but the voids are not completely filled (see appendix on saturated thickness).
However, some of the pore spaces may be too small or too poorly connected to permit the water they contain to flow out easily. The effective porosity can be thought of as the volume of pore space that will drain in a reasonable period of time under the influence of gravity. Effective porosity is always less than total porosity, sometimes (as in the case of clays) much less. "Good aquifers" tend to have values of effective porosity in the range of 10-30%, although examples of higher and lower values can be found. Figure 1 illustrates the relationship among the types of porosity and the volume of water in storage.
