Influence of multiple wetting–drying cycles on water retention and pore structure of silt loam from reservoir hydro-fluctuation belt: Experiments and modeling

Soils in the reservoir hydro-fluctuation belt experience irreversible changes due to repeated wetting–drying (W-D) cycles associated with fluctuating reservoir water levels. This study investigates the pore-scale structure and water retention properties of soils exposed to multiple pressure-driven W-D cycles. The soil water retention curve (SWRC) was determined across a broad range of matric suction using the filter paper method. Microstructural changes were observed using scanning electron microscopy (SEM), while nuclear magnetic resonance (NMR) was employed to analyze the evolution of pore size distribution (PSD). The results highlight the presence of isolated pores, micropores within aggregates, and macropores between aggregates, as well as the bimodal features of SWRC curves. Based on the experimental observations, a bimodal SWRC model was developed to capture the two-step behavior of the SWRC curves by accounting for water retention in both intra- and inter-aggregate pores, along with residual water in isolated pores. The cyclic W-D processes affect the SWRC behavior at multiple scales. The disintegration of clay aggregates reduces the volume of isolated pores, leading to a lower residual water content. The enlargement of intra- and inter-aggregate pores diminishes the air entry value and the soil’s water retention capacity, causing a leftward shift in the SWRC curves. After the first cycle, a slight increase in the air entry value within the intra-aggregate region is observed due to the compaction or partial closure of smaller pores within aggregates. The findings have significant implications for soil management in reservoir regions, particularly for addressing soil erosion and slope instability risks associated with cyclic water level fluctuations.