The coefficient of permeability k is a measure of the rate of flow of water through saturated soil under a given hydraulic gradient i, cm/cm, and is defined in accordance with Darcy's law as:
V = KiA
V rate of flow, cm3/s, and A cross-sectional areaof soil conveying flow, cm2.
Coefficient k is dependent on the grain-size distribution, void ratio, and soil fabric and typically may vary from as much as 10 cm/s for gravel to less than 10-7for clays. For typical soil deposits, k for horizontal flow is greater than k for vertical flow, often by an order of magnitude.