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.
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