CALCULATION OF HYDRAULIC CONDUCTIVITY FROM SUCTION PERMEAMETER MEASUREMENTS

Suction permeameters are now commonly used for measuring unsaturated hydraulic properties of soil. The hydraulic conductivity (k) is calculated from the steady state flow rate of water through a core of soil which has a permeameter on the top and which is open to the atmosphere at the bottom. This arrangement means that a matric potential gradient exists across the core during steady state flow. An exact equation is derived to describe the steady state flow of water through the soil core, using k = Ks exp(alpha-psi) as the relationship between-psi and k, where psi is matric potential, Ks is saturated hydraulic conductivity and alpha is a soil parameter. The exact equation is rearranged to solve for k and alpha. The equation for alpha is not a closed form equation but can be solved by iteration. An iterative scheme is presented, and an example of its use given. k is calculated for a range of values of alpha and matric potential and compared with an approximate equation for calculating k. The results show that the approximate equation gives large errors if alpha > 20 m-1.