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In the field of
hydrogeology Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust (commonly in aquif ...
, ''storage properties'' are physical properties that characterize the capacity of an
aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
to release
groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidate ...
. These properties are storativity (S), specific storage (Ss) and specific yield (Sy). According to ''Groundwater'', by Freeze and Cherry (1979), specific storage, S_s −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. They are often determined using some combination of field tests (e.g.,
aquifer test An aquifer test (or a pumping test) is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" ( drawdown) in observation wells. Aquifer testing is a common tool that hydroge ...
s) and laboratory tests on aquifer material samples. Recently, these properties have been also determined using remote sensing data derived from
Interferometric synthetic-aperture radar Interferometric synthetic aperture radar, abbreviated InSAR (or deprecated IfSAR), is a radar technique used in geodesy and remote sensing. This geodetic method uses two or more synthetic aperture radar (SAR) images to generate maps of surface defor ...
.


Storativity

Storativity or the storage coefficient is the
volume Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The de ...
of water released from storage per unit decline in
hydraulic head Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22. It is usually measured as a liquid surface elevation, expressed in units of length, ...
in the aquifer, per unit
area Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape A shape or figure is a graphics, graphical representation of an obje ...
of the aquifer. Storativity is a dimensionless quantity, and is always greater than 0. :S = \frac\frac = S_s b + S_y \, * V_w is the volume of water released from storage ( 3; * h is the
hydraulic head Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22. It is usually measured as a liquid surface elevation, expressed in units of length, ...
( * S_s is the specific storage * S_y is the
specific yield 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). According to '' ...
* b is the thickness of aquifer * A is the area ( 2


Confined

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 is
homogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
: :S=S_s b \,


Unconfined

For an unconfined aquifer, storativity is approximately equal to the specific yield (S_y) since the release from specific storage (S_s) is typically orders of magnitude less (S_s b \ll \!\ S_y). :S=S_y \, The specific storage is the amount of water that a portion of an
aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
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 an
aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
releases from storage, per mass of aquifer, per unit decline in hydraulic head: :(S_s)_m = \frac\frac where :(S_s)_m is the mass specific storage ( −1; :m_a is the mass of that portion of the aquifer from which the water is released ( ; :dm_w is the mass of water released from storage ( ; and :dh is the decline in
hydraulic head Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22. It is usually measured as a liquid surface elevation, expressed in units of length, ...
( . Volumetric specific storage (or volume-specific storage) is the volume of water that an
aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
releases from storage, per volume of the aquifer, per unit decline in hydraulic head (Freeze and Cherry, 1979): :S_s = \frac\frac = \frac\frac\frac= \frac\frac\gamma_w where :S_s is the volumetric specific storage ( −1; :V_a is the bulk volume of that portion of the aquifer from which the water is released ( 3; :dV_w is the volume of water released from storage ( 3; :dp is the decline in
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
( N•m−2 or L−1T−2 ; :dh is the decline in
hydraulic head Hydraulic head or piezometric head is a specific measurement of liquid pressure above a vertical datum., 410 pages. See pp. 43–44., 650 pages. See p. 22. It is usually measured as a liquid surface elevation, expressed in units of length, ...
( and :\gamma_w is the
specific weight The specific weight, also known as the unit weight, is the weight per unit volume of a material. A commonly used value is the specific weight of water on Earth at , which is .National Council of Examiners for Engineering and Surveying (2005). ''Fu ...
of water ( N•m−3 or L−2T−2. In
hydrogeology Hydrogeology (''hydro-'' meaning water, and ''-geology'' meaning the study of the Earth) is the area of geology that deals with the distribution and movement of groundwater in the soil and rocks of the Earth's crust (commonly in aquif ...
, 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 :S_s = \gamma_w (\beta_p + n \cdot \beta_w) where :\gamma_w is the
specific weight The specific weight, also known as the unit weight, is the weight per unit volume of a material. A commonly used value is the specific weight of water on Earth at , which is .National Council of Examiners for Engineering and Surveying (2005). ''Fu ...
of water ( N•m−3 or L−2T−2 :n is the
porosity Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
of the material (dimensionless ratio between 0 and 1) :\beta_p is the
compressibility In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...
of the bulk aquifer material (m2N−1 or M−1T2, and :\beta_w is the compressibility of water (m2N−1 or M−1T2 The compressibility terms relate a given change in stress to a change in volume (a strain). These two terms can be defined as: :\beta_p = -\frac\frac :\beta_w = -\frac\frac where :\sigma_e is the
effective stress The effective stress can be defined as the stress, depending on the applied tension \boldsymbol_ and pore pressure p, which controls the strain or strength behaviour of soil and rock (or a generic porous body) for whatever pore pressure value or, ...
(N/m2 or LT−2/L2 These equations relate a change in total or water volume (V_t or V_w) per change in applied stress (effective stress — \sigma_e 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 Soil mechanics is a branch of soil physics and applied 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 (usually air and wat ...
(developed by
Karl Terzaghi Karl von Terzaghi (October 2, 1883 – October 25, 1963) was an Austrian mechanical engineer, geotechnical engineer, and geologist known as the "father of soil mechanics and geotechnical engineering". Early life In 1883, he was born the first c ...
).


Specific yield

Specific yield, also known as the drainable porosity, is a ratio, less than or equal to the
effective porosity Effective porosity is most commonly considered to represent the porosity of a rock or sediment available to contribute to fluid flow through the rock or sediment, or often in terms of "flow to a borehole". Porosity that is not considered "effectiv ...
, indicating the volumetric fraction of the bulk
aquifer An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characterist ...
volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity: :S_y = \frac where :V_ is the volume of water drained, and :V_T is the total rock or material volume It is primarily used for unconfined aquifers, since the elastic storage component, S_s, 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. Problems related to unsaturated flow are simulated using the numerical solution of
Richards Equation The Richards equation represents the movement of water in unsaturated soils, and is attributed to Lorenzo A. Richards who published the equation in 1931. It is a quasilinear partial differential equation; its analytical solution is often limited ...
, which requires estimation of the specific yield, or the numerical solution of the Soil Moisture Velocity Equation, which does not require estimation of the specific yield.


See also

*
Aquifer test An aquifer test (or a pumping test) is conducted to evaluate an aquifer by "stimulating" the aquifer through constant pumping, and observing the aquifer's "response" ( drawdown) in observation wells. Aquifer testing is a common tool that hydroge ...
*
Soil mechanics Soil mechanics is a branch of soil physics and applied 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 (usually air and wat ...
*
Groundwater flow equation Used 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 ...
describes how these terms are used in the context of solving groundwater flow problems


References

* 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 {{Geotechnical engineering, state=collapsed Hydrology Aquifers Water Soil mechanics Soil physics