Evaluation of a general model for multimodal unsaturated soil hydraulic properties

Many soils and other porous media exhibit dual- or multi-porosity type features. In a previous study (Seki et al., 2022) we presented multimodal water retention and closed-form hydraulic conductivity equations for such media. The objective of this study is to show that the proposed equations are practically useful. Specifically, dual-BC (Brooks and Corey)-CH (common head) (DBC), dual-VG (van Genuchten)-CH (DVC), and KO (Kosugi)(1)BC2-CH (KBC) models were evaluated for a broad range of soil types. The three models showed good agreement with measured water retention and hydraulic conductivity data over a wide range of pressure heads. Results were obtained by first optimizing water retention parameters and then optimizing the saturated hydraulic conductivity (K-s) and two parameters (p, q) or (p, r) in the general hydraulic conductivity equation. Although conventionally the tortuosity factor p is optimized and (q, r) fixed, sensitivity analyses showed that optimization of two parameters (p + r, qr) is required for the multimodal models. For 20 soils from the UNSODA database, the average R-2 for log (hydraulic conductivity) was highest (0.985) for the KBC model with r = 1 and optimization of (K-s, p, q). This result was almost equivalent (0.973) to the DVC model with q = 1 and optimization of (K-s, p, r); both were higher than R-2 for the widely used Peters model (0.956) when optimizing (K-s, p, a, omega). The proposed equations are useful for practical applications while mathematically being simple and consistent.

Automated summary

The objective of this study is to demonstrate the practical usefulness of the proposed equations for water retention and hydraulic conductivity in porous media. Three models were evaluated and showed good agreement with measured data for a wide range of pressure heads. The study highlights the simplicity and consistency of the proposed equations for practical applications.