Storativity or the storage coefficient is the volume of water released from storage per unit decline in hydraulic head in the aquifer, per unit area of the aquifer, or:
Storativity is the vertically integrated specific storage value for a confined aquifer or aquitard. For a confined homogeneous aquifer or aquitard they are simply related by:
where is the thickness of aquifer. Storativity is a dimensionless quantity, and ranges between 0 and the effective porosity of the aquifer; although for confined aquifers, this number is usually much less than 0.01.
The storativity or storage coefficient of an unconfined aquifer is approximately equal to the specific yield, since the release from specific storage, is typically orders of magnitude less.
Read more about this topic: Specific Storage