Abstract
Abstract. A popular parameterized soil water retention curve (SWRC) has a hydraulic conductivity curve associated with it that can have a physically unacceptable 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 midsection 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.
Highlights
The soil water retention function introduced by van Genuchten (1980) has been the most popular parameterization to describe the soil water retention curve (SWRC) in numerical models for unsaturated flow for the past few decades (e.g., Kroes et al, 2017; Šimůnek and Bradford, 2008; Šimůnek et al, 2016): θθ(h) = θθr +(1 + |ααh|nn)n1n−1, h ≤ 0 (1)
RIA offers a wider range of shapes for the conductivity curve than any other parameterization that does not lead to the unphysical behavior near saturation that was revealed by Durner (1994) and Ippisch et al (2006) for VGN and by Madi et al (2018) for 14 other parameterizations
RIA proved to be more robust during numerical simulations than VGN itself as well as its modification VGA, which still has a non–physical asymptote at a non–zero residual water content
Summary
The soil water retention function introduced by van Genuchten (1980) has been the most popular parameterization (denoted VGN below; these and other abbreviations are listed in Appendix A) to describe the SWRC in numerical models for unsaturated flow for the past few decades (e.g., Kroes et al, 2017; Šimůnek and Bradford, 2008; Šimůnek et al., 2016): θθ(h) = θθr + (θθs − θθr)(1 + |ααh|nn)n1n−1, h ≤ 0 (1). Van Genuchten (1980) combined Eq (1) with Mualem’s (1976) conductivity model and derived an analytical expression for the unsaturated hydraulic conductivity curve: 1−|ααh|nn−1(1+|ααh|nn)n1n−1 2. Apart from the convenience of having analytical expressions for the retention as well as the conductivity curve, the function’s popularity derives from its 3
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