A Correlated Random-Field Ising Model for Pore-Scale Hysteresis in Soil-Water Characteristic Curves

Soil–Water Characteristic Curve Unsaturated Soils Pore-Scale Modeling Monte Carlo Simulation Hysteresis Microstructural Heterogeneity Hydraulic Behavior Pore Connectivity

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The soil–water characteristic curve (SWCC) plays a central role in the behavior of unsaturated soils, yet explaining its hysteresis directly from pore-scale mechanisms remains challenging. The objective of this study is to investigate how pore-size heterogeneity, spatial correlations, and cooperative dynamics contribute to hysteresis in SWCCs. In this study, a correlated Random-Field Ising Model (RFIM) combined with Monte Carlo simulations is developed to represent the pore space as a two-dimensional lattice with a bimodal distribution of pore volumes and a spatially correlated disorder field. Drainage processes are simulated without parametric curve fitting, enabling direct analysis of pore-scale switching dynamics. The results show that macropore fraction, pore-size heterogeneity, and the activation parameter \beta exert a strong control on drainage behavior. Low \beta values produce smooth and nearly reversible drainage, whereas higher \beta stabilizes metastable pore configurations and yields abrupt transitions accompanied by hysteresis. The divergence between number-based and volume-based saturation serves as a useful indicator of size-selective drainage and cooperative pore-scale events. The novelty of this work lies in providing a physically grounded and statistically dynamics to macroscopic hysteresis in SWCCs, offering insights beyond traditional phenomenological or uncorrelated pore-network approaches.