A Universal Framework to Describe Unimodal and Bimodal Soil Hydraulic
Properties over Full Water Saturation Range
Abstract
Soil hydraulic properties (SHPs) are impacted by various mechanisms such
as soil structure, capillarity, and adsorption forces, often showing a
bimodal shape. Developing soil hydraulic models that describe SHPs over
the entire saturation range often involves balancing the representation
of multiple processes while minimizing the number of free-fitted
parameters. Existing soil hydraulic models rarely capture bimodal soil
hydraulic properties across the full moisture range or introduce too
many free-fitted parameters. In this study, we propose a novel framework
to describe SHPs over the entire moisture range, accounting for the
effects of soil structure, capillarity, adsorption forces, and vapor
diffusion. In its four free-fitted parameters form, the proposed models
can capture unimodal soil water retention curves (SWRC) and bimodal
hydraulic conductivity curves (HCC). With one additional free-fitted
parameter, the proposed models can capture both bimodal SWRC and HCC.
Testing with 107 and 52 soil samples from two public datasets
demonstrated that the proposed models performed exceptionally well in
describing SHPs across the entire moisture range. The reported lowest
root-mean-square error values were 0.005 and 0.009 cm3 cm−3 for fitting
SWRCs, and 0.465 and 0.666 for predicting HCCs, respectively. Due to the
minimal introduction of free-fitted parameters, the proposed framework
showed significant application potential.