In the field of hydrogeology, "storage properties" are physical properties that characterize the capacity of an aquifer to release groundwater. These properties are Storativity (S), specific storage (Ss) and specific yield (Sy).
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They are often determined using some combination of field tests (e.g., aquifer tests) and laboratory tests on aquifer material samples.
Storativity
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. Storativity is a dimensionless quantity, and ranges between 0 and the effective porosity of the aquifer.
Confined
For a confined aquifer or aquitard, storativity is the vertically integrated specific storage value. Therefore, if the aquitard is homogeneous:
Unconfined
For unconfined aquifer storativity is approximately equal to the specific yield (
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
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
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
The compressibility terms relate a given change in stress to a change in volume (a strain). These two terms can be defined as:
where
These equations relate a change in total or water volume (
Specific yield
Specific yield, also known as the drainable porosity, is a ratio, less than or equal to the effective porosity, indicating the volumetric fraction of the bulk aquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity:
where
It is primarily used for unconfined aquifers, since the elastic storage component,