Hydrological feedback of evaporation-driven salt crystallization in soils: The effect of pore structure and ion composition

Evaporation-induced salt accumulation leads to progressive soil salinization, which has significant implications for numerous environmental processes and applications. In this study, we investigate how pore structure influence evaporation-driven crystallization and its feedback on evaporation dynamics and hydrological functions in packed soil. The pore network model, extracted from high-resolution X-ray microtomography images, was applied to quantify internal pore structure and simulate hydraulic properties in the absence/presence of salt crystallization. Experimental results indicated a significantly higher evaporation rate for NaCl solution compared to Na 2 SO 4 solution across all textures (up to ∼30% during the high-rate period). NaCl predominantly formed surface efflorescence, transitioning from a compact crust in fine-textured soils to patchy “cauliflower-like” deposits in coarse soils, which act as an additional porous layer that restricts brine supply and impedes vapor exchange. In contrast, Na 2 SO 4 precipitation occurred mainly as near-surface subflorescence, with the main crystallization zone progressively detaching from the surface as sand content increased. Crystallization reduced near-surface porosity by 2.12–5.23% for NaCl and 4.86–10.93% for Na 2 SO 4 , accompanied by pore-size redistribution and increased tortuosity. These structural changes led to substantial reductions in transport properties: intrinsic permeability decreased by 15.79–31.97% (NaCl) and 28.07–51.98% (Na 2 SO 4 ), while relative vapor diffusivity declined by 33.34–42.11% and 44.45–62.50%, respectively. The results demonstrate that crystallization-induced evolution of pore structure, permeability, and vapor diffusivity should be explicitly incorporated into hydrological models to improve predictions of evaporation and salinization, particularly in arid and semi-arid regions.