Parametric soil water retention models: a critical evaluation of expressions for the full moisture range
Raneem Madi1, Gerrit Huibert de Rooij1, Henrike Mielenz2, and Juliane Mai31Dept. Soil Physics, Helmholtz Centre for Environmental Research – UFZ, Halle, Germany 2Institute for Crop and Soil Science, Julius Kühn-Institut – JKI, Braunschweig, Germany 3Dept. Computational Hydrosystems, Helmholtz Centre for Environmental Research – UFZ, Leipzig, Germany
Abstract. Of the many parametric expressions for the soil water retention curve, only a few are suitable for the dry 15 range. Furthermore, expressions for the soil hydraulic conductivity curves associated with these retention functions can exhibit non-physical behavior near saturation. We developed a general criterion that needs to be met by soil water retention parameterizations to ensure physically plausible hydraulic conductivity curves. Only three of the 18 tested parameterizations did not impose any restrictions on the parameters of the most popular conductivity curve parameterization, which includes three functions as special cases. One other retention function required one 20 conductivity parameter to be fixed.
We employed the Shuffled Complex Evolution parameter estimation method with the objective function tailored to various observation methods normally used to obtain retention curve data. We fitted the four parameterizations with physically plausible conductivities as well as the most widely used parameterization. We then compared the performance of the resulting 12 combinations of retention curve and conductivity curve in a 25 numerical study with 999 days of semi-arid atmospheric forcing applied to unvegetated, uniform, 1-m freely draining columns for four textures.
Choosing different parameterizations had a minor effect on evaporation, but cumulative bottom fluxes varied by up to an order of magnitude between them. This highlights the need for a careful selection procedure for the parameterization of the soil hydraulic properties that ideally does not only rely on goodness-of-fit to static soil 30 water retention data but also on observations of the hydraulic conductivity curve made during dynamic flow conditions.
Madi, R., de Rooij, G. H., Mielenz, H., and Mai, J.: Parametric soil water retention models: a critical evaluation of expressions for the full moisture range, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-168, 2016.