In the field ofhydrogeology,storage properties are physical properties that characterize the capacity of anaquifer to releasegroundwater. These properties arestorativity (S),specific storage (S_s) andspecific yield (S_y). According toGroundwater, by Freeze and Cherry (1979), specific storage,${\displaystyle S_s}$ [m⁻¹], of a saturated aquifer is defined as the volume of water that a unit volume of the aquifer releases from storage under a unit decline in hydraulic head.^[1]

They are often determined using some combination of field tests (e.g.,aquifer tests) and laboratory tests on aquifer material samples. Recently, these properties have been also determined usingremote sensing data derived fromInterferometric synthetic-aperture radar.^[2][3]

Storativity or thestorage coefficient is thevolume of water released from storage per unit decline inhydraulic head in the aquifer, per unitarea of the aquifer. Storativity is a dimensionless quantity, and is always greater than 0.

${\displaystyle S=\frac{d{V}_w}{dh}\frac{1}{A}=S_{s}b+S_y}$

• ${\displaystyle V_w}$ is the volume of water released from storage ([L³]);
• ${\displaystyle h}$ is thehydraulic head ([L])
• ${\displaystyle S_s}$ is the specific storage
• ${\displaystyle S_y}$ is thespecific yield
• ${\displaystyle b}$ is the thickness of aquifer
• ${\displaystyle A}$ is the area ([L²])

For a confined aquifer or aquitard, storativity is the vertically integrated specific storage value. Specific storage is the volume of water released from one unit volume of the aquifer under one unit decline in head. This is related to both the compressibility of the aquifer and the compressibility of the water itself. Assuming the aquifer or aquitard ishomogeneous:

${\displaystyle S=S_{s}b}$

For an unconfined aquifer, storativity is approximately equal to the specific yield (${\displaystyle S_y}$) since the release from specific storage (${\displaystyle S_s}$) is typically orders of magnitude less (${\displaystyle S_{s}b\ll S_y}$).

${\displaystyle S=S_y}$

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

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

${\displaystyle (S_s{)}_m=\frac{1}{m_a}\frac{d{m}_w}{dh}}$

where

${\displaystyle (S_s{)}_m}$ is the mass specific storage ([L⁻¹]);

${\displaystyle m_a}$ is the mass of that portion of the aquifer from which the water is released ([M]);

${\displaystyle d{m}_w}$ is the mass of water released from storage ([M]); and

${\displaystyle dh}$ is the decline inhydraulic head ([L]).

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

${\displaystyle S_s=\frac{1}{V_a}\frac{d{V}_w}{dh}=\frac{1}{V_a}\frac{d{V}_w}{dp}\frac{dp}{dh}=\frac{1}{V_a}\frac{d{V}_w}{dp}{\gamma }_w}$

where

${\displaystyle S_s}$ is the volumetric specific storage ([L⁻¹]);

${\displaystyle V_a}$ is the bulk volume of that portion of the aquifer from which the water is released ([L³]);

${\displaystyle d{V}_w}$ is the volume of water released from storage ([L³]);

${\displaystyle dp}$ is the decline inpressure(N•m⁻² or [ML⁻¹T⁻²]) ;

${\displaystyle dh}$ is the decline inhydraulic head ([L]) and

${\displaystyle {\gamma }_w}$ is thespecific weight of water (N•m⁻³ or [ML⁻²T⁻²]).

Inhydrogeology,volumetric specific storage is much more commonly encountered thanmass specific storage. Consequently, the term specific storage generally refers tovolumetric specific storage.

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

${\displaystyle S_s={\gamma }_w({\beta }_p+n\cdot {\beta }_w)}$

where

${\displaystyle {\gamma }_w}$ is thespecific weight of water (N•m⁻³ or [ML⁻²T⁻²])

${\displaystyle n}$ is theporosity of the material (dimensionless ratio between 0 and 1)

${\displaystyle {\beta }_p}$ is thecompressibility of the bulk aquifer material (m²N⁻¹ or [LM⁻¹T²]), and

${\displaystyle {\beta }_w}$ 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:

${\displaystyle {\beta }_p=-\frac{d{V}_t}{d{\sigma }_e}\frac{1}{V_t}}$

${\displaystyle {\beta }_w=-\frac{d{V}_w}{dp}\frac{1}{V_w}}$

where

${\displaystyle {\sigma }_e}$ is theeffective stress (N/m² or [MLT⁻²/L²])

These equations relate a change in total or water volume (${\displaystyle V_t}$ or${\displaystyle V_w}$) per change in applied stress (effective stress —${\displaystyle {\sigma }_e}$ or pore pressure —${\displaystyle p}$) per unit volume. The compressibilities (and therefore also S_s) can be estimated from laboratory consolidation tests (in an apparatus called a consolidometer), using the consolidation theory ofsoil mechanics (developed byKarl Terzaghi).

Aquifer-test analyses provide estimates ofaquifer-system storage coefficients by examining the drawdown and recovery responses of water levels in wells to applied stresses, typically induced by pumping from nearby wells.^[4]

Elastic and inelastic skeletal storage coefficients can be estimated through a graphical method developed by Riley.^[5] This method involves plotting the applied stress (hydraulic head) on the y-axis against vertical strain or displacement (compaction) on the x-axis. The inverse slopes of the dominant linear trends in these compaction-head trajectories indicate the skeletal storage coefficients. The displacements used to build the stress-strain curve can be determined byextensometers,^[5][6]InSAR^[7] orlevelling.^[8]

Laboratory consolidation tests yield measurements of the coefficient of consolidation within the inelastic range and provide estimates of verticalhydraulic conductivity.^[9] The inelastic skeletal specific storage of the sample can be determined by calculating the ratio of vertical hydraulic conductivity to the coefficient of consolidation.

Simulations ofland subsidence incorporate data on aquifer-system storage andhydraulic conductivity. Calibrating these models can lead to optimized estimates of storage coefficients and verticalhydraulic conductivity.^[8][10]

Specific yield, also known as the drainable porosity, is a ratio and is the volumetric fraction of the bulkaquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity:

${\displaystyle S_y=\frac{V_{wd}}{V_T}}$

where

${\displaystyle V_{wd}}$ is the volume of water drained, and

${\displaystyle V_T}$ is the total rock or material volume

It is primarily used for unconfined aquifers since the elastic storage component,${\displaystyle S_s}$, is relatively small and usually has an insignificant contribution. Specific yield can be close to effective porosity, but several subtleties 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. Also, the value of a specific yield may not be fully realized for a very long time due to complications caused by unsaturated flow. Problems related to unsaturated flow are simulated using the numerical solution ofRichards Equation, which requires estimation of the specific yield, or the numerical solution of theSoil Moisture Velocity Equation, which does not require estimation of the specific yield.

• Aquifer test
• Soil mechanics
• Groundwater flow equation describes how these terms are used in the context of solving groundwater flow problems

• Freeze, R.A. and J.A. Cherry. 1979.Groundwater. Prentice-Hall, Inc. Englewood Cliffs, NJ. 604 p.
• Morris, D.A. and A.I. Johnson. 1967.Summary of hydrologic and physical properties of rock and soil materials as analyzed by the Hydrologic Laboratory of the U.S. Geological Survey 1948-1960. U.S. Geological Survey Water Supply Paper 1839-D. 42 p.
• De Wiest, R. J. (1966). On the storage coefficient and the equations of groundwater flow. Journal of Geophysical Research, 71(4), 1117–1122.

Specific

• Hydrology
• Aquifers
• Water
• Soil mechanics
• Soil physics