Unraveling trapped air-suction interplay in hydraulic hysteresis: experimental insights and a state-dependent water retention model

Hydraulic hysteresis in unsaturated soils is, primarily governed by the interplay between trapped air dynamics and the path dependency of matric suction, facing characterization challenges from experimental complexity and model parameter redundancy. This study establishes that trapped air content is primarily governed by the maximum historically experienced matric suction through experiments on silt, clay and eluvial soils. A novel suction-dependent formulation was proposed achieving high-fidelity predictions (R² = 0.984–0.998, RMSE < 5 %). Validation reveals intensified nonlinearity in fine-textured soils due to pore heterogeneity, while sandy soils show quadratic superiority at high suction. These insights enable a state-dependent soil–water retention model reducing the number of fitting parameters from nine to five without sacrificing fidelity. Crucially, omitting air entrapment in modeling efforts induces 8–10 % saturation errors from unrealistic rewetting curves. The proposed framework has been validated across diverse soil textures and hydraulic conditions, demonstrating its robustness and effectiveness in capturing the hysteretic behavior of unsaturated soils.