Comparison of the soil hydraulic conductivity predicted from its water content expressed by the equation of Van Genuchten and different capillary models

Knowledge of water retention and conductivity is essential to study water transport in soil. Determination of the conductivity curve is difficult, and it is often predicted by application of a capillary model to the water retention relationship. Three expressions are predicted from the water retention described by the equation of Van Genuchten. Two expressions are obtained in combination with the capillary models of Mualem and Burdine. The third expression is obtained by a combination with the capillary model of Fatt & Dykstra. The three sets of soil properties were applied to clay in order to compute infiltration and infiltration rates according to the series solution of Philip. Comparison with this solution showed that the results of the first two combinations were severely under-estimated, while those of the third were satisfactory. Similar results were obtained for sand by comparison with experimental data. Because conductivity estimated from the capillary model of Fatt & Dykstra is complicated, it was expressed by a power equation, the exponent of which is obtained by applying the Fatt & Dykstra capillary model to the water retention curve expressed according to Brooks & Corey and having the same asymptotic behaviour as the Van Genuchten equation. Application of this procedure to fifty soils selected from a published database gave satisfactory results. It is concluded that the hydraulic conductivity of a soil can be predicted from its water retention as expressed by the equation of Van Genuchten subject to the condition of the capillary model of Fatt & Dykstra and as expressed by the equation of Brooks & Corey, for which the exponent is obtained according to the same capillary model