Quantifying pore-scale heterogeneity of drying and wetting soil water retention behavior using X-ray computed tomography

Pore water transport in unsaturated soils has been observed through X-ray computed tomography (CT), but effective methods for determining the local degree of saturation (S_r ) and matric suction (ψ_m ) and explaining hydraulic hysteresis in individual pores with sufficiently high spatial resolution are lacking. This study proposed a novel workflow for extracting pore-scale information based on analyzed X-ray CT images, quantifying local S_r and ψ_m values at individual pores (denoted as S_{r, Pore} and ψ_{m, Pore} ), and exploring hysteretic water retention behavior of unsaturated porous media. Results reveal significant spatial variations in S_{r, Pore} and ψ_{m, Pore} at each equilibrium suction. Their variations first increased in the transition zone of the water retention curve (WRC) when continuous and isolated water phases co-existed, but then reduced in the residual zone, where water existed as isolated clusters constrained by local pore geometry. Variations in ψ_{m, Pore} were primarily governed by pore geometry that controlled interface curvature and hysteresis, whereas variations in S_{r, Pore} were more influenced by pore size and connectivity. These factors collectively regulate the drainage and imbibition behavior. The pore-scale WRC (i.e., relating S_{r, Pore} with ψ_{m, Pore} ) derived from a single CT scan taken at the suction within the transition zone approximated the full-range pore-scale WRC.