Evaluating Unsaturated Hydraulic Conductivity Models for Diverse Soils and Climates: A Functional Comparison of Additive, Junction, and Kosugi Parameterizations

We tested three expressions for the unsaturated soil hydraulic conductivity curve (UHCC): Kosugi’s model (KGV), an additive model (ADV), and a junction model (JUV).  KGV generalizes the Mualem-van Genuchten model and assumes that all liquid soil water flows through capillaries.  ADV adds the hydraulic conductivity of water films adsorbed onto the solid surface to the conductivity of the capillaries. The recently introduced JUV has a junction matric potential at which a wet branch with a capillary conductivity function joins a dry branch with a film conductivity function. All models assume water vapor flow is driven by diffusion. We fitted the three models to hydraulic conductivity measurements for a sandy loam, a silt, and a loamy sand. Akaike’s Information Criterion suggested potential overparameterization in ADV, which has up to seven fitting parameters, whereas KGV and JUV have up to six. From the fitted curves, we generated look-up tables that were then used as input for the Hydrus-1D model for soil water flow.

We evaluated the functional performance of the three models by numerically modeling unsaturated flow in uniform vegetated columns of the three soils exposed to 10 years of generated weather records that represent three climates (monsoon, temperate, and semi-arid). The surface flux, transpiration, and bottom boundary flux were aggregated over 5-day, 10-day, and 30-day time windows, and their extremes and seasonal fluctuations were evaluated. JUV and KGV converged for all nine combinations of soil and climate, while ADV crashed three times, particularly for the sandy loam. In addition to the robustness of the three UHCC models, the presentation will highlight how the calculated fluxes and water balances agree or differ between the models.