Evaluation of the tortuosity parameter for forest soils to predict unsaturated hydraulic conductivity

In order to accurately predict the unsaturated hydraulic conductivity for forest soils, we evaluated the tortuosity parameter for forest soils and investigated the relationships between the tortuosity parameter and parent materials and soil sampling depths. Undisturbed forest soil samples were taken from three parent material types: the granite group, Mesozoic-Paleozoic, and Quaternary. Samples taken from layers corresponding to the A-horizon were categorized as topsoil and others as subsoil. After retention parameters were fitted, the tortuosity parameter was optimized using the Mualem-van Genuchten conductivity model. No significant differences were found in the average values of the tortuosity parameter among depths or among parent materials. Optimized values of the tortuosity parameter varied from 3.20 to −5.23 with a high frequency in the range of about 0 to −2. The average value was −0.94. This means that, for most forest soils, the predicted hydraulic conductivity tends to be underestimated when the most common value (0.5) of the tortuosity parameter is used. Modifying this parameter value to −0.77, as explained in this study, corrected the underestimation tendency and reduced prediction errors. When the unsaturated hydraulic conductivity for forest soils is predicted from water retention, the tortuosity parameter should thus be modified to an appropriate value for more accurate prediction.