Predicting Unsaturated Hydraulic Conductivity from the Soil Water Characteristic

ABSTRACT FOURTEEN methods to predict the unsaturated hydraulic conductivity function, K(h) from the soil water characteristic, 6(h), were examined. Nine of the fourteen methods were found in the literature; the other five were developed by modifying some of the original nine. All methods were tested using measured K(h) data for 23 soils found in the literature. Predictions were compared statistically and graphically to the laboratory-measured K(h) for each soil. Results showed that the best method of predicting K(h) for the sandy and clayey soils examined was a new closed-form equation for K(h) which uses the Campbell (1974) equation for 0(h). This method also worked best for the low K values of the loamy soils. The choice of this method was based on the assumption that a conservation or safe side estimate will be obtained when K(h) is overpredicted. A numerical approximation to this new form for K(h) provided the best estimate of K(h) values near saturation for the loamy soils. When underestimation of K(h) yields the conservative result, the following method gave the best results: (a) A numerical approximation to the Burdine (1953) closed-form equation for sandy soils; (b) The Burdine equation for K(h) combined with the Campbell form for 6(h) for the clayey soils near saturation; (c) A closed-form equation for K(h) developed by Mualem (1976) combined with the Brooks and Corey (1964) form for 0(h) for the low K(h) values for clayey soils.