Fitting the junction model and other parameterizations for the unsaturated soil hydraulic conductivity curve: KRIAfitter version 1.0

Abstract. Several current models for the unsaturated soil hydraulic conductivity curve consider the conductivity of the domains of capillary water in water-filled pores and adsorbed water in films on soil grains, as well as an equivalent conductivity for water vapour diffusion. These models rely on unrealistic configuration of the domains. A junction model is introduced that sidesteps this problem by assigning all liquid water to films (dry range) or to capillaries (wet range). Combined with a sigmoidal junction model for the soil water retention curve, it has up to six fitting parameters, one fewer than the other multi-domain models. Tests on data for 13 soils show that the junction model and an additive model (that adds all domain conductivities) often produce good fits. Models with six or more parameters may be overparameterized for many soils, giving the more parsimonious junction model an advantage, but for some soils, the extra parameter of the additive model is needed to achieve a good fit. This paper and a user manual document a Fortran code (KRIAfitter) that uses the shuffled complex evolution algorithm to fit the junction, additive, and four other conductivity models for any combination of fixed and fitting parameters or their log transforms. KRIAfitter either maps the root mean square error in the entire parameter space in order to then constrain the parameter space around the likely global minimum or generates many fits and uses those to calculate statistics for individual parameters, as well as the covariance and correlation matrices.