Mass specific storage is the mass of water that an aquifer releases from storage, per mass of aquifer, per unit decline in hydraulic head:
- 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):
- 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
- 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:
- 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).
Other articles related to "specific storage, specific":
... Values of specific yield, from Johnson (1967) Material Specific Yield (%) min avg max Unconsolidated deposits Clay 5 ... Sandy clay (mud) 12 ... Silt 19 ... Fine sand 28 ... Medium sand 32 ... Coarse sand 20 27 ... Specific yield can be close to effective porosity, but there are several subtle things which make this value more complicated than it seems ... Also, the value of specific yield may not be fully realized for a very long time, due to complications caused by unsaturated flow ...
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