A Generalized 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 (SHMs) 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 SHMs rarely capture bimodal SHPs across the full moisture range or introduce a higher number of 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 and bimodal hydraulic conductivity curves (HCC). This model is well-suited for situations where small changes in water content near saturation are no longer detectable via measured SWRC, yet soil structure still causes a sharp decline in HCC near saturation. With one additional free-fitted parameter, the proposed models can capture both bimodal SWRC and HCC. Testing with 355 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.