Describes the amount of water that can be stored in a rock formation. Porosity is the ratio of solid rock material and void space per unit volume of rock. Porous rock units may contain considerable amounts of water but may be poor aquifers because of low permeability (i.e. water movement is restricted). With respect to water movement, it is not just the total magnitude of porosity that is important, but the size of the voids and the extent to which they are interconnected. For example, clay may have a very high porosity with respect to water content, but it is not useful as an aquifer because the pores are usually so small and will not readily release water.