A Novel Shifting Framework to Describe Bimodal Soil Hydraulic Properties

ABSTRACT Accurately describing bi‐ or multi‐modal soil hydraulic properties (SHPs) is essential for understanding soil water flow processes. Traditional bimodal models, based on capillary theory, often require numerous empirical parameters and fail to adequately represent macropore flow driven by gravity and film flow held by adsorption forces. In this study, we introduce a novel framework to describe bimodal SHPs by shifting from a reference unimodal model. We propose a new bimodal soil hydraulic model based on the widely used unimodal van Genuchten–Mualem (VGM) model and incorporate the additional effects of adsorption forces. The so‐called VGM‐Bi model introduces two additional parameters, which can be easily derived or constrained from the measured soil water retention curve (SWRC), enhancing its usability. We further propose a modified version, the VGM‐Bi‐Macro model, to account for macropore effects by using a hydraulic conductivity value at a slightly negative matric potential as a new matching point. Evaluation with 25 soil samples shows that the VGM‐Bi model accurately captures bimodal SWRCs but struggles with bimodal hydraulic conductivity curves (HCCs) for a few samples. In contrast, the VGM‐Bi‐Macro model effectively captures both. Specifically, the VGM‐Bi‐Macro model achieves a slightly lower mean root‐mean‐square error (RMSE) of 0.200 in estimating HCCs and nearly half the RMSE (0.008 cm 3 cm −3 ) in estimating SWRCs compared to the VGM‐Bi model (0.229 and 0.014 cm 3 cm −3 ) and an existing bimodal model (0.262 and 0.015 cm 3 cm −3 ), underscoring the importance of gravity‐driven macropore flow. These models offer a physically meaningful and efficient tool for simulating water flow in structured soils.