Storativity, or the storage coefficient, is the volume of water an aquifer releases per unit area for each unit fall in head.
Storativity, also called the storage coefficient, is the volume of water an aquifer takes in or releases from storage per unit surface area for each unit change in head. It has no units. The value depends on the type of aquifer. In a confined aquifer, water comes from slight expansion of the water and compression of the aquifer, so storativity is very small, typically 0.00005 to 0.005. It equals the specific storage multiplied by the aquifer thickness. In an unconfined aquifer the pores drain, so storativity is close to the specific yield, commonly 0.01 to 0.30. The difference is large in practice. If the head falls by 1 m over 1 km², a confined aquifer with a storativity of 0.0002 gives only 200 m³ of water. An unconfined aquifer with a specific yield of 0.15 gives 150,000 m³. Storativity is estimated from pumping tests and needs at least one observation well, since readings in the pumped well alone are unreliable for this purpose. Water budget studies use it to convert a measured change in groundwater level into a change in stored volume.