Specific storage
From Biocrawler, the free encyclopedia.
Specific storage, storativity and specific yield (Ss, S and Sy) are aquifer properties; they are measures of the ability of an aquifer to release groundwater from storage, due to a unit decline in hydraulic head. These properties are often determined in hydrogeology using an aquifer test.
Specific storage the amount of water which a given volume of aquifer will produce, provided a unit change in hydraulic head is applied to it (while it still remains fully saturated); it has units of inverse length (1/m). Storativity is the vertically averaged specific storage value for an aquifer or aquitard. For a homogeneous aquifer or aquitard they are simply related by S = Ssb. Storativity is a dimensionless quantity.
In terms of measurable physical properties, Specific Storage can be expressed as
- Ss = γ(cb + ncw),
where γ is the specific weight of water (units of force per volume, N/m³ or lbf/ft³), n is the porosity of the material (a ratio between 0 and 1), cb is the compressibility of the bulk aquifer material and cw is the compressibility of water (compressibility has units of length squared per force, e.g., m²/N). The compressibility terms relate a given change in stress to a change in volume (a strain). These two terms can be defined as:
These equations relate a change in total or water volume (Vt or Vw) per change in applied stress (effective stress — σe or pore pressure - p) 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).
Specific yield is a ratio between 0 and 1 indicating the volumetric fraction of the bulk aquifer volume that a given an aquifer will yield when all the water is allowed to drain out of it under the forces of gravity. Roughly, this is the effective porosity, but there are several subtle things which make this value more complicated than it seems. Some water always remains in the formation, even after drainage; it clings to the grains of sand and clay in the formation. Also, the value of specific yield may not be fully realized until very large times, due to complications caused by unsaturated flow. Tables can be found in the references which give typical ranges for Sy, Ss and S.
Specific storage is the primary mechanism for releasing water in confined aquifers, while specific yield is the primary mechanism for releasing water in unconfined (water table) aquifers.
See also
| Aquifer properties used in hydrogeology |
|---|
| hydraulic head | hydraulic conductivity | storativity | porosity | water content |
