Abstract
A soil water characteristic curve (SWCC) model named as discrete-continuous multimodal van Genuchten model with a convenient parameter calibration method is developed to describe the relationship between soil suction and the water content of a soil with complex pore structure. The modality number N of the SWCC in the proposed model can be any positive integer (the so-called multimodal or N-modal SWCC). A unique set of parameters is determined by combining curve fitting and a graphical method based on the shape features of the SWCC in the log s–log Se plane. In addition, a modality number reduction method is proposed to minimize the number of parameters and simplify the form of SWCC function. The proposed model is validated using a set of bimodal and trimodal SWCC measurements from different soils, which yield a strong consistency between the fitted curves and the measured SWCC data. The uniqueness in the set of parameters provides the possibility to further improve the proposed model by correlating the parameters to soil properties and state parameters.
Highlights
The soil water characteristic curve (SWCC) describes the relationship between soil suction and water content of a soil
A piecewise N-modal SWCC model named as discrete multimodal van Genuchten model (DMVGM) is derived based on the piecewise bimodal SWCC model proposed by Burger and Shackelford [8], and a continuous N-modal SWCC model named as continuous multimodal van Genuchten model (CMVGM) is extended from the continuous bimodal SWCC model proposed by Ross and Smettem [34]
The determination of all the parameters solely with a curve fitting procedure leads to convergence problems and enhanced uncertainties, due to the noneuniqueness in the parameters of CMVGM
Summary
The soil water characteristic curve (SWCC) describes the relationship between soil suction and water content (e.g., volumetric water content h, gravity water content w or degree of saturation Sr) of a soil. R1 and R2 represent the volumetric fraction of micro- and macroporosity, respectively; Sr; and Sr; represent the sub-SWCC curves for the macro- and microporosity, respectively, which can be described by a unimodal SWCC function Based on this framework, a number of continuous bimodal SWCC model have been developed [9, 11, 12, 28, 34]. For continuous bimodal SWCC, the simple volumetric fraction models show difficulty in parameter determination, while the unique parameter approach requires a rather complex SWCC function This problem is more significant for multimodal SWCCs. Recent studies have reported that the pore structures in unsaturated fine-grained soils are more complex than to be represented by a bimodal pore size distribution [24, 30, 41, 44], resulting in multimodal SWCCs. To describe multimodal SWCCs, the existing bimodal SWCC models can be extended to N-modal SWCC functions. Transfomred:Sr 1⁄4 Sr;res þ Sr;max À Sr;res fR11⁄21 þ ða1sÞn1 Àm1 þR21⁄21 þ ða2sÞn2 Àm2 g
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