Darcy's Law states that groundwater flow through a porous material is proportional to the hydraulic gradient, the cross-section area and the hydraulic conductivity.
Darcy's Law is the basic equation of groundwater flow. Henry Darcy published it in 1856 after experiments with sand filters for the water supply of Dijon in France. It states that flow Q equals hydraulic conductivity K times the cross-section area A times the hydraulic gradient, which is the change in head Δh over the flow distance L. For example, with K of 10 m/day, an area of 100 m² and a gradient of 0.01, the flow is 10 m³/day. Dividing Q by A gives the Darcy flux, sometimes called specific discharge. This is not the real speed of the water, because water moves only through the pores. The average speed is the Darcy flux divided by effective porosity, and this is the figure used to estimate how fast a pollutant travels. The law applies when flow is slow and laminar, which is true of most natural groundwater. It can fail close to pumped wells, in large fractures and in karst conduits, where flow turns turbulent. It also forms the basis of numerical groundwater models such as MODFLOW.