Specific storage
Encyclopedia
Specific storage storativity (S), specific yield (Sy) and specific capacity are material physical properties that characterize the capacity of an aquifer
to release groundwater
from storage in response to a decline in hydraulic head
. For that reason they are sometimes referred to as "storage properties". In the field of hydrogeology
, these properties are often determined using some combination of field hydraulic tests (e.g., aquifer test
s) and laboratory tests on aquifer material samples.
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 ([L-1]);
is the mass of that portion of the aquifer from which the water is released ([M]);
is the mass of water released from storage ([M]); and
is the decline in hydraulic head
([L]).
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 ([L-1]);
is the bulk volume of that portion of the aquifer from which the water is released ([L3]);
is the volume of water released from storage ([L3]);
is the decline in pressure
(N•m-2 or [ML-1T-2]) ;
is the decline in hydraulic head
([L]) and
is the specific weight
of water (N•m-3 or [ML-2T-2]).
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 [ML-2T-2])
is the porosity
of the material (dimensionless ratio between 0 and 1)
is the compressibility of the bulk aquifer material (m2N-1 or [LM-1T2]), and
is the compressibility of water (m2N-1 or [LM-1T2])
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 [MLT-2/L2])
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).
of water released from storage per unit decline in hydraulic head
in the aquifer, per unit area
of the aquifer, or:
Storativity is the vertically integrated specific storage value for a confined aquifer or aquitard. For a confined homogeneous aquifer or aquitard they are simply related by:
where
is the thickness of aquifer. Storativity is a dimensionless quantity, and ranges between 0 and the effective porosity
of the aquifer; although for confined aquifers, this number is usually much less than 0.01.
The storage coefficient of an unconfined aquifer is approximately equal to the specific yield,
, since the release from specific storage, 
is typically orders of magnitude less.
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
is the volume of water drained, and
is the total rock or material volume
It is primarily used for unconfined aquifers, since the elastic storage component,
, is relatively small and usually has an insignificant contribution. Specific yield can be close to 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 for a very long time, due to complications caused by unsaturated flow.
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
to release groundwater
Groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...
from storage in response to a decline in hydraulic head
Hydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
. For that reason they are sometimes referred to as "storage properties". In the field of hydrogeology
Hydrogeology
Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, . The term geohydrology is often used interchangeably...
, these properties are often determined using some combination of field hydraulic tests (e.g., aquifer test
Aquifer test
An aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
s) and laboratory tests on aquifer material samples.
Specific storage
The specific storage is the amount of water that a portion of an aquiferAquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
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
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
releases from storage, per mass of aquifer, per unit decline in hydraulic head:
where
is the mass specific storage ([L-1]); is the mass of that portion of the aquifer from which the water is released ([M]); is the mass of water released from storage ([M]); and is the decline in hydraulic headHydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
([L]).
Volumetric specific storage (or volume specific storage) is the volume of water that an aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
releases from storage, per volume of aquifer, per unit decline in hydraulic head (Freeze and Cherry, 1979):
where
is the volumetric specific storage ([L-1]); is the bulk volume of that portion of the aquifer from which the water is released ([L3]); is the volume of water released from storage ([L3]); is the decline in pressurePressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
(N•m-2 or [ML-1T-2]) ;
is the decline in hydraulic headHydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
([L]) and
is the specific weightSpecific weight
The specific weight is the weight per unit volume of a material. The symbol of specific weight is γ ....
of water (N•m-3 or [ML-2T-2]).
In hydrogeology
Hydrogeology
Hydrogeology is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust, . The term geohydrology is often used interchangeably...
, 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 weightSpecific weight
The specific weight is the weight per unit volume of a material. The symbol of specific weight is γ ....
of water (N•m-3 or [ML-2T-2])
is the porosityPorosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
of the material (dimensionless ratio between 0 and 1)
is the compressibility of the bulk aquifer material (m2N-1 or [LM-1T2]), and is the compressibility of water (m2N-1 or [LM-1T2])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 stressEffective stress
Karl von Terzaghi first proposed the relationship for effective stress in 1936. For him, the term ‘effective’ meant the calculated stress that was effective in moving soil, or causing displacements...
(N/m2 or [MLT-2/L2])
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 mechanicsSoil mechanics
Soil mechanics is a branch of engineering mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids, liquids, and gasses and other...
(developed by Karl Terzaghi).
Storativity
Storativity is the volumeVolume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
of water released from storage per unit decline in hydraulic head
Hydraulic head
Hydraulic head or piezometric head is a specific measurement of water pressure above a geodetic datum. It is usually measured as a water surface elevation, expressed in units of length, at the entrance of a piezometer...
in the aquifer, per unit area
Area
Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...
of the aquifer, or:
Storativity is the vertically integrated specific storage value for a confined aquifer or aquitard. For a confined homogeneous aquifer or aquitard they are simply related by:
where
is the thickness of aquifer. Storativity is a dimensionless quantity, and ranges between 0 and the effective porosityPorosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
of the aquifer; although for confined aquifers, this number is usually much less than 0.01.
The storage coefficient of an unconfined aquifer is approximately equal to the specific yield,
, since the release from specific storage, is typically orders of magnitude less.Specific yield
| Material | Specific Yield (%) | ||
|---|---|---|---|
| min | avg | max | |
| Unconsolidated deposits | |||
| Clay | 0 | 2 | 5 |
| Sandy clay (mud) | 3 | 7 | 12 |
| Silt | 3 | 18 | 19 |
| Fine sand | 10 | 21 | 28 |
| Medium sand | 15 | 26 | 32 |
| Coarse sand | 20 | 27 | 35 |
| Gravelly sand | 20 | 25 | 35 |
| Fine gravel | 21 | 25 | 35 |
| Medium gravel | 13 | 23 | 26 |
| Coarse gravel | 12 | 22 | 26 |
| Consolidated deposits | |||
| Fine-grained sandstone | 21 | ||
| Medium-grained sandstone | 27 | ||
| Limestone | 14 | ||
| Schist | 26 | ||
| Siltstone | 12 | ||
| Tuff | 21 | ||
| Other deposits | |||
| Dune sand | 38 | ||
| Loess | 18 | ||
| Peat | 44 | ||
| Till, predominantly silt | 6 | ||
| Till, predominantly sand | 16 | ||
| Till, predominantly gravel | 16 | ||
Specific yield, also known as the drainable porosity, is a ratio, less than or equal to the effective porosity
Effective porosity
The term effective porosity lacks a single or straightforward definition. Even some of the terms used in its mathematical description have multiple definitions. However, it is most commonly considered to represent the porosity of a rock or sediment available to contribute to fluid flow through...
, indicating the volumetric fraction of the bulk aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity:
where
is the volume of water drained, and is the total rock or material volumeIt is primarily used for unconfined aquifers, since the elastic storage component,
, is relatively small and usually has an insignificant contribution. Specific yield can be close to 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 for a very long time, due to complications caused by unsaturated flow.See also
- Aquifer testAquifer testAn aquifer test is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" in observation wells...
- Soil mechanicsSoil mechanicsSoil mechanics is a branch of engineering mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids, liquids, and gasses and other...
- Groundwater flow equationGroundwater flow equationUsed in hydrogeology, the groundwater flow equation is the mathematical relationship which is used to describe the flow of groundwater through an aquifer. The transient flow of groundwater is described by a form of the diffusion equation, similar to that used in heat transfer to describe the flow...
describes how these terms are used in the context of solving groundwater flow problems