From AMS Glossary
The relationship for movement of fluids through permeable or porous media, such as soil, which states that at low Reynolds numbers the flow velocity V is proportional to hydraulic gradient dh/dl, where the constant of proportionality K is the hydraulic conductivity:
The hydraulic head h is the height of fluid in a manometer, which is proportional to the fluid pressure, while l is the slant length in the medium along the flow streamline. The velocity V is really a specific discharge (volume flow rate of fluid per unit cross-sectional area of medium), and is sometimes called the Darcy velocity or Darcy flux. The hydraulic conductivity depends both on the permeability k of the medium (e.g., sand vs clay), and on the kinematic viscosity ν of the fluid:
where g is gravitational acceleration.
Freeze, R. A., and J. A. Cherry 1979. Groundwater. 15–18.