Approaches for estimating unsaturated soil hydraulic conductivities at various bulk densities with the extended Mualem-van Genuchten model

The Mualem-van Genuchten model has been widely used for estimating unsaturated soil hydraulic conductivity (K-u) from measured saturated hydraulic conductivity (K-s) and fitted water retention curve (WRC) parameters. Soil bulk density (p(b)) variations affect the accuracy of K-u estimates. In this study, we extend the Mualem-van Genuchten model to account for the p(b) effect with p(b)-related WRC and K-s models. We apply two functions (A and B) that relate the van Genuchten WRC model to p(b) and two models (1 and 2) that estimate K-s with various p(b). By combining the p(b)-related WRC functions and K-s models, we develop four integrated approaches (i.e., A1, A2, B1, and B2) for estimating K-u at various p(b). K-u measurements made on five soils with various textures and p(b) are used to evaluate the accuracy of the four approaches. The results show that all approaches produce reasonable K-u estimates, with average root mean square errors (RMSEs) less than 0.35 (expressed in dimensionless unit because logarithmic K-u values are used). Approach A2, with an average RMSE of 0.25, agrees better with K-u measurements than does Approach A1 that has an average RMSE of 0.28. This is because Model 2 accounts for the WRC shape effect near saturation. Approaches A1 and A2 give more accurate K-u estimates than do Approaches B1 and B2 which both have average RMSEs of 0.35, because Function A performs better in estimating WRCs than does Function B. The proposed approaches could be incorporated into simulation models for improved prediction of water, solute, and gas transport in soils.