Unsaturated hydraulic conductivity is often predicted from soil water retention data and a saturated or near-saturated hydraulic conductivity value. One of the commonly used predictive equations (Campbell, 1974) is written as:

K is hydraulic conductivity, is volumetric water content, subscript s indicates saturation and b is a constant determined from fitting soil water retention data. The term 2b+3 is derived from 2b+2+p where p is the pore interaction term, which is empirically derived and assumed to equal one. Here the objective was to evaluate the pore interaction term using unsaturated hydraulic conductivity data measured on 47 soil samples from 18 different B-horizons in the south-eastern Australian wheat belt. Water retention, near-saturated hydraulic conductivity (‑30 mm matric potential), and diffusivity were measured for each sample. The Campbell (1974) water retention equation was fitted to the measured data to determine the value of b.  Subsequent fitting to the diffusivity data enabled calculation of the pore interaction term.  The diffusivity fitting was done with the values of fixed at the measured values. K_s was allowed to optimise. The observed pore interaction term values for these heavy textured B-horizons varied substantially, ranging from 0 to 23.8 with a mean of 6.7 and standard error of 0.70. Assuming a pore interaction term value of one to predict unsaturated hydraulic conductivity in these soil B-horizons in the south-eastern Australian wheat belt thus appears unsatisfactory.  Better estimates are needed in order to predict unsaturated hydraulic conductivity from soil water retention data.

Campbell, G.S. 1974.  A simple model for determining unsaturated conductivity from moisture retention data.  Soil Sci. 117: 311-314.