The Hydraulic Conductivity, K
The hydraulic conductivity, K, is a constant for a given porous medium. A very porous medium with little resistance to flow will have a high value for K, while a tightly packed medium with high resistance to flow will have a low value for K. The approximate range of K values for several types of soil are given in the table at the left.
From the units of the other parameters in the Darcy's Law equation, one can see that K must have units of volumetric flow rate over area. In the table at the left the units are gpd/ft2. Other commonly used units for K are: ft/day (ft3/day/ft2) or m/day (m3/day/m2).
The values of K in the table are for the flow of water through the indicated porous media. The effect of the medium and the properties of the liquid flowing through it can be separated by the use of the specific pemeability, k, as shown in the following equation:
K = kγ/μ, where
k = specific permeability, ft2 (a property of the porous medium only),
γ = specific weight of flowing liquid, lb/ft3,
m = viscosity of flowing liquid, lb-sec/ft2,
K = hydraulic conductivity, ft/sec (ft3/sec/ft2).
Values of specific permeability are sometimes given with the darcy as the unit, where 1 darcy = 1.062 x 10-11 ft2.