V = −K grad (φ) (7.1) where V is the rate of flow per unit area, K is the hydraulic conductivity of the porous medium and φ is the hydraulic head or potential. If we
neglect the effects of temperature and osmotic pressure, the potential will be equal to the piezometric head i.e. the sum of the pressure head and the elevation:
φ = h = p γ + z = −S + z (7.2)
where h is the piezometric head, p is the pressure in the soil water, γ is the weight density of the water, S is soil suction and z is the elevation above a fixed horizontal datum.