Comparison of Two Physically Capillary-Adsorption Based Models for Estimation of Unsaturated Hydraulic Conductivity at Both Near Dry and Saturated Water Contents

It has been documented that the assumption of capillary movement of water in a bundle of cylindrical tubes often leads to systematically underestimation of soil unsaturated hydraulic conductivity at low potentials which is due to ignoring the contribution of adsorptive forces and liquid films. In this study, the performance of two physical models of (Tuller-Or) and (Lebeau-Konrad) that take into account the contribution of both adsorptive and capillary forces and the well-known capillary-based model of (Van Genuchten-Mualem) for modeling the soil-water retention curve and estimation of the unsaturated hydraulic conductivity were evaluated. To that end, experimental data gathered from literature including six soils that cover a broad range of texture and hydraulic behavior. The results showed the superiority of the two capillary-adsorption models over the capillary-based Van Genuchten-Mualem model in estimating the soil-water characteristic curve and unsaturated hydraulic conductivity. Among the two physically capillary-adsorption based models, the Lebeau-Konrad model performed better. The results showed that the Lebeau-Konrad model underestimates the hydraulic conductivity of clayey soils in the near saturation range and the best performance of the Tuller-Or model was shown to be for loamy soils. Because the Tuller-Or model is the most comprehensive and physics-based model for modeling the unsaturated hydraulic conductivity up to date, future studies should be devoted to improving the flexibility of this model via extending the model to other pore size distribution than the original Gamma distribution, such as lognormal, incomplete gamma and Weibull distributions.