mathematical relationship discovered (1856) by the French engineer Henri Darcy (Darcy, Henri-Philibert-Gaspard) 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, Q_d = PIA, in which Q_d 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.