Specific storage

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 (S s ) and specific yield (S y ). According to Groundwater, by Freeze and Cherry (1979), specific storage, 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. 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 using remote sensing data derived from Interferometric synthetic-aperture radar.

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 (S s ) and specific yield (S y ). According to Groundwater, by Freeze and Cherry (1979), specific storage, 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. 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 using remote sensing data derived from Interferometric synthetic-aperture radar. ==Storativity== S = \frac{dV_w}{dh}\frac{1}{A} = S_s b + S_y \, V_w is the volume of water released from storage ([L 3 ]); h is the hydraulic head ([L]) S_s is the specific storage S_y is the specific yield b is the thickness of aquifer A is the area ([L 2 ]) S=S_s b \, S=S_y \, The specific storage is the amount of water that a portion of an aquifer 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 releases from storage, per mass of aquifer, per unit decline in hydraulic head: (S_s)_m = \frac{1}{m_a}\frac{dm_w}{dh} where (S_s)_m is the mass specific storage ([L −1 ]); m_a is the mass of that portion of the aquifer from which the water is released ([M]); dm_w is the mass of water released from storage ([M]); and dh 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 the aquifer, per unit decline in hydraulic head (Freeze and Cherry, 1979): S_s = \frac{1}{V_a}\frac{dV_w}{dh} = \frac{1}{V_a}\frac{dV_w}{dp}\frac{dp}{dh}= \frac{1}{V_a}\frac{dV_w}{dp}\gamma_w where S_s is the volumetric specific storage ([L −1 ]); V_a is the bulk volume of that portion of the aquifer from which the water is released ([L 3 ]); dV_w is the volume of water released from storage ([L 3 ]); dp is the decline in pressure(N•m −2 or [ML −1 T −2 ]) ; dh is the decline in hydraulic head ([L]) and \gamma_w is the specific weight of water (N•m −3 or [ML −2 T −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 S_s = \gamma_w (\beta_p + n \cdot \beta_w) where \gamma_w is the specific weight of water (N•m −3 or [ML −2 T −2 ]) n is the porosity of the material (dimensionless ratio between 0 and 1) \beta_p is the compressibility of the bulk aquifer material (m 2 N −1 or [LM −1 T 2 ]), and \beta_w is the compressibility of water (m 2 N −1 or [LM −1 T 2 ]) 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{dV_t}{d\sigma_e}\frac{1}{V_t} \beta_w = -\frac{dV_w}{dp}\frac{1}{V_w} where \sigma_e is the effective stress (N/m 2 or [MLT −2 /L 2 ]) 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 S s ) 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== Specific yield, also known as the drainable porosity, is a ratio and is 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: S_y = \frac{V_{wd}}{V_T} where V_{wd} 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 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 of Richards Equation, 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 Soil mechanics Groundwater flow equation 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

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