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permeability
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permeation; diffusion

permeability

physics
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permeability, capacity of a porous material for transmitting a fluid; it is expressed as the velocity with which a fluid of specified viscosity, under the influence of a given pressure, passes through a sample having a certain cross section and thickness. Permeability is largely dependent on the size and shape of the pores in the substance and, in granular materials such as sedimentary rocks, by the size, shape, and packing arrangement of the grains.

The standard unit of permeability is the darcy, equivalent to the passage of one cubic centimetre of fluid (having a viscosity of one centipoise) per second through a sample one square centimetre in cross-sectional area under a pressure of one atmosphere per centimetre of thickness.

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Darcy’s law
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Darcy’s law

hydrology
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Henri-Philibert-Gaspard Darcy

Darcy’s law, mathematical relationship discovered (1856) by the French engineer Henri Darcy that governs the flow of groundwater through granular media or the flow of other fluids through permeable material, such as petroleum through sandstone or limestone. As the basic relationship from which many sophisticated theoretical and practical derivations have been devised, it has become the foundation for quantitative work in the field of groundwater flow. One of the most useful derivations from the formula, which can be used to calculate the amount of water flowing through a given cross-sectional area of an aquifer, equates the discharge to the product of the cross-sectional area through which the discharge occurs, the hydraulic gradient (the change in head for a unit of length), and a coefficient of permeability. Symbolically, Qd = PIA, in which Qd is the discharge water in litres per day; P is the coefficient of permeability in litres per day per square metre; I is the hydraulic gradient in metre of head per metre of length; and A is the cross-sectional area through which the discharge occurs.

This article was most recently revised and updated by William L. Hosch.
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