13 May 2022
13 May 2022
Status: this preprint is currently under review for the journal HESS.

Technical note: A sigmoidal soil water retention curve with a logarithmic dry branch that is robust when reliable dry-range data are lacking

Gerrit Huibert de Rooij Gerrit Huibert de Rooij
  • Helmholtz Centre for Environmental Research – UFZ GmbH, Soil System Science Dept., Theodor– Lieser–Strasse 4, 06120 Halle (Saale), Germany

Abstract. In a recently introduced parameterization for the soil water retention curve (SWRC) with a sigmoid wet branch and a logarithmic dry branch, the matric potential at the junction point of the sigmoid and the logarithmic branch (hj) was a fitting parameter, while that at oven–dryness (hd) was derived from the fitting parameters. The latter is undesirable, especially if reliable data in the dry range are limited. Therefore, an alternative is presented in which shape parameter α instead of hd is a derived parameter, and hd can be fitted or fixed. The resulting relationship between α and hj is such that it prevents correct fits for hj. Fortunately, an expression for hj is found that allows it to be replaced by α as a fitting parameter. The corresponding parameter space is well–behaved and has fewer internal bounds defined by restraining relationships between parameters. The few available values of hj in the literature are in line with those according to the new expression. The reformulated SWRC is fitted to data of 21 soils by shuffled complex evolution. The paper gives the main features of an accompanying open–source fitting code. The fits are good, except for some clayey soils. A theoretical value of hd performs well for a wide range of soils. For some soils, α is very large. If this is the case, the new SWRC simplifies to an earlier junction model of the SWRC based on a well–known power–law SWRC.

Gerrit Huibert de Rooij

Status: open (until 08 Jul 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Gerrit Huibert de Rooij

Model code and software

Fitting the parameters of the RIA parameterization of the soil water retention curve (1.0) de Rooij, G.

Gerrit Huibert de Rooij


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Short summary
The way soils capture infiltrating water affects crops and natural vegetation as well as groundwater recharge. This retention of soil water is captured by a mathematical function that covers all water contents from very dry to water-saturated. Unfortunately, data in the dry range are often absent or unreliable. I modified an earlier function to be more robust in the absence of dry-range data, and present a computer program to estimate the parameters of the new function.