**Darcy’s Law of Permeability**

The law of flow of water through soil was first studied by Darcy (1856) who demonstrated experimentally that for laminer flow conditions in a saturated soil, the rate of flow or the discharge per unit lime is proportional to the hydraulic gradient.

q = k i A

V = q / A = k i

**where**

q = discharge per unit time

A = total cross-sectional area of soil mass, perpendicular to the direction of flow

i = hydraulic gradient

k = Darcy’ s coefficient of permeability

v = velocity of flow, or average discharge velocity.

If a soil sample of length L and cross-sectional area A, is subjected to differential head of water h1 – h2 ,the hydraulic gradient i will be equal to h1 – h2 / L and, we have

q = k (h1 – h2 / L) A

From Equation when hydraulic gradient is unity, k is equal to v. Thus, the coefficient of * permeability*, or simply

*Permeability*is defined as the average velocity of flow that will occur through the total cross-sectional area of soil under unit hydraulic gradient. The dimensions of the coefficient of permeability k are the same as those of velocity. It is usually as cm/sec or m/day or feet/day.