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)₁BC₂-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 R² 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 R² for the widely used Peters model (0.956) when optimizing (K_s, p, a, ω). The proposed equations are useful for practical applications while mathematically being simple and consistent.