Abstract
This study evaluated the impact of the rapid change in unsaturated hydraulic conductivity near saturation, as described by the van Genuchten (VG) model, on the results of soil water movement simulations. Free drainage simulations from near-saturated conditions were conducted using HYDRUS-1D for four soil types, including soils containing a large amount of macropores, resulting in a small n value of the VG model parameter and large saturated hydraulic conductivity. The results showed that, for soils having a small n value, the VG model significantly reduced unsaturated hydraulic conductivity near saturation. In contrast, the modified VG model with the conventional air-entry pressure setting (−2 cm) suppressed this reduction. However, comparisons with observed data for soil containing macropores and having a small n value showed that the conventional modified VG model overestimated drainage rates. Conversely, setting he close to 0 cm allowed the model to reproduce the observed data well. A sensitivity analysis revealed that the effect of pore connectivity parameter, l is much less than that of the air-entry pressure, he in modified VG model. Because remarkable improvement on the calculation result was not obtained in spite of the change in l in the VG model, our results indicates the usefulness of modified VG model if we can determine he appropriately.
https://ror.org/00p4k0j84 
