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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/hess-2020-380
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2020-380
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  17 Aug 2020

17 Aug 2020

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This preprint is currently under review for the journal HESS.

Sigmoidal Water Retention Function with Improved Behavior in Dry and Wet Soils

Gerrit H. de Rooij1, Juliane Mai2, and Raneem Madi1,a Gerrit H. de Rooij et al.
  • 1Helmholtz Centre for Environmental Research – UFZ GmbH, Soil System Science Dept., Theodor-Lieser-Strasse 4, 06120 Halle (Saale), Germany
  • 2University of Waterloo, Dept. Civil and Environmental Engineering, 200 University Ave West, Waterloo, ON N2L 3G1, Canada
  • acurrent address: GFI Grundwasser-Consulting-Institut GmbH, Meraner Strasse 10, 01217 Dresden, Germany

Abstract. A popular parameterized soil water retention curve (SWRC) has a hydraulic conductivity curve associated with it that can have an infinite slope at saturation. The problem was eliminated before by giving the SWRC a non–zero air–entry value. This improved version still has an asymptote at the dry end, which limits its usefulness for dry conditions and causes its integral to diverge for commonly occurring parameter values. We therefore joined the parameterizations' sigmoid mid–section to a logarithmic dry section ending at zero water content for a finite matric potential, as was done previously for a power–law type SWRC. We selected five SWRC parameterizations that had been proven to produce unproblematic near–saturation conductivities and fitted these and our new curve to data from 21 soils. The logarithmic dry branch gave more realistic extrapolations into the dry end of both the retention and the conductivity curves than an asymptotic dry branch. We tested the original curve, its first improvement, and our second improvement by feeding them into a numerical model that calculated evapotranspiration and deep drainage for nine combinations of soils and climates. The new curve was more robust than the other two. The new curve was better able to produce a conductivity curve with a substantial drop during the early stages of drying than the earlier improvement. It therefore generated smaller amounts of more evenly distributed deep drainage compared to the spiked response to rainfall produced by the earlier improvement.

Gerrit H. de Rooij et al.

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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. We combined two existing lines of research to improve the behaviour of a popular function for very dry and very wet conditions. Our new function could handle a wider range of conditions than earlier curves. We provide fits to a wide range of soils.
The way soils capture infiltrating water affects crops and natural vegetation as well as...
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