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Volume 22, issue 2 | Copyright
Hydrol. Earth Syst. Sci., 22, 1193-1219, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 12 Feb 2018

Research article | 12 Feb 2018

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 Mai3,a Raneem Madi et al.
  • 1Dept. Soil System Science, 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
  • acurrently at: Dept. Civil & Environmental Engineering, Univ. of Waterloo, Waterloo, Canada

Abstract. Few parametric expressions for the soil water retention curve are suitable for dry conditions. Furthermore, expressions for the soil hydraulic conductivity curves associated with parametric retention functions can behave unrealistically near saturation. We developed a general criterion for water retention parameterizations that ensures physically plausible conductivity curves. Only 3 of the 18 tested parameterizations met this criterion without restrictions on the parameters of a popular conductivity curve parameterization. A fourth required one parameter to be fixed.

We estimated parameters by shuffled complex evolution (SCE) with the objective function tailored to various observation methods used to obtain retention curve data. We fitted the four parameterizations with physically plausible conductivities as well as the most widely used parameterization. The performance of the resulting 12 combinations of retention and conductivity curves was assessed in a numerical study with 751 days of semiarid atmospheric forcing applied to unvegetated, uniform, 1m 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 of the soil hydraulic parameterization that ideally does not only rely on goodness of fit to static soil water retention data but also on hydraulic conductivity measurements.

Parameter fits for 21 soils showed that extrapolations into the dry range of the retention curve often became physically more realistic when the parameterization had a logarithmic dry branch, particularly in fine-textured soils where high residual water contents would otherwise be fitted.

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Short summary
Water flows through soils with more difficulty when the soil is dried out. Scant rainfall in deserts may therefore result in a seemingly wet soil, but the water will often not penetrate deeply enough to replenish the groundwater. We compared the mathematical functions that describe how well different soils hold their water and found that only a few of them are realistic. The function one chooses to model the soil can have a large impact on the estimate of groundwater recharge.
Water flows through soils with more difficulty when the soil is dried out. Scant rainfall in...