# Specific Storage

The specific storage is the amount of water that a portion of an aquifer releases from storage, per unit mass or volume of aquifer, per unit change in hydraulic head, while remaining fully saturated.

Mass specific storage is the mass of water that an aquifer releases from storage, per mass of aquifer, per unit decline in hydraulic head:

where

is the mass specific storage ;
is the mass of that portion of the aquifer from which the water is released ;
is the mass of water released from storage ; and
is the decline in hydraulic head .

Volumetric specific storage (or volume specific storage) is the volume of water that an aquifer releases from storage, per volume of aquifer, per unit decline in hydraulic head (Freeze and Cherry, 1979):

where

is the volumetric specific storage ;
is the bulk volume of that portion of the aquifer from which the water is released ;
is the volume of water released from storage ;
is the decline in pressure(N•m-2 or ) ;
is the decline in hydraulic head and
is the specific weight of water (N•m-3 or ).

In hydrogeology, volumetric specific storage is much more commonly encountered than mass specific storage. Consequently, the term specific storage generally refers to volumetric specific storage.

In terms of measurable physical properties, specific storage can be expressed as

where

is the specific weight of water (N•m-3 or )
is the porosity of the material (dimensionless ratio between 0 and 1)
is the compressibility of the bulk aquifer material (m2N-1 or ), and
is the compressibility of water (m2N-1 or )

The compressibility terms relate a given change in stress to a change in volume (a strain). These two terms can be defined as:

where

is the effective stress (N/m2 or )

These equations relate a change in total or water volume ( or ) per change in applied stress (effective stress — or pore pressure — ) per unit volume. The compressibilities (and therefore also Ss) can be estimated from laboratory consolidation tests (in an apparatus called a consolidometer), using the consolidation theory of soil mechanics (developed by Karl Terzaghi).