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

Abstract. Soil water moves as capillary flow, film flow, and vapour diffusion. The additive model for the unsaturated soil hydraulic conductivity curve adds up conductivities of capillary water, adsorbed water, and an equivalent vapour conductivity. A recently introduced junction model with liquid water in either films or capillaries has one parameter less. We compared calculated water fluxes based on both models and Kosugi’s model (which only considers capillary water) by fitting the RIA soil water retention curve and the three conductivity models to data for three soils. Five subsets of the model parameters were calibrated by fitting, with the other parameters fixed. For all 135 resulting cases, we ran the Hydrus-1D numerical model for uniform columns of these soils subjected to generated weather records for three climates. Hydrus-1D crashed 14 times for the additive model, twice for Kosugi’s model and once for the junction model. The conductivity models and fitting parameter sets only significantly affected the drainage flux at 2 m depth (and, in one case, the transpiration), but the effect was only large in two cases. If the conductivity models disagreed, the additivity model was usually the outlier. An analysis of the water balance terms revealed the impact of soil type to be limited on transpiration or infiltration, but stronger on groundwater recharge. The conductivity models had only a minor effect. The fluxes were insensitive to differences in the dry-range conductivity. Fewer fitted parameters rarely altered the results significantly. This favours the more parsimonious and robust junction and Kosugi models.