An improved van Genuchten soil water characteristic model to account for surface adsorptive forces

Abstract

Widely used models for the soil water characteristic (SWC), like the van Genuchten model, are primarily based on the assumption that soil pores resemble bundles of cylindrical capillary tubes. While such models effectively describe the wet part of a SWC, they often fail to accurately represent the dry part where water retention is primarily governed by surface adsorptive forces rather than capillary forces. To address this limitation, many have developed alternative models that incorporate additional parameters or mechanisms to better characterize the dry end. Here we propose a novel first-order continuous mathematical expression that modifies the van Genuchten model without adding any additional fitting parameters, covering the entire SWC range from full saturation to oven dry conditions. The new Improved van Genuchten (IvG) function maintains the simplicity of the classical model while significantly improving its ability to represent the dry end of the water content spectrum. We evaluated the new expression using water retention data for a wide range of soil textures from sand to clay. We further integrated the new function with the Mualem hydraulic conductivity model to numerically calculate the unsaturated hydraulic conductivity, yielding reasonable estimates across diverse soil types.