Experimental study on the influence mechanism of freeze–thaw action on the preferential flow pattern in vadose zone

Abstract

The preferential flow in the vadose zone plays a crucial role in accelerating the transport of soil pollutants to groundwater. However, the mechanisms by which freeze–thaw cycles influence preferential flow patterns in this zone remain unclear. This study used dye tracer experiments to visualize preferential flow patterns under varying freeze–thaw conditions. By analyzing the relationships among freeze–thaw cycles (Ftc), antecedent water content (aWc), soil structural parameters, and preferential flow characteristic parameters—including maximum infiltration depth (D_max), actual infiltration depth (D_a), coefficient of variation (CV), length index (LI), fractal dimension (FD), and preferential flow ratio (C_pr)—this study elucidates how freeze–thaw action and initial water content differentially affect preferential flow patterns in the infiltration zone. Experiments were conducted using four levels of aWc (5 %, 10 %, 15 %, 20 %) and four Ftc (0, 1, 3, 5). Results showed that increasing antecedent water content and the number of freeze–thaw cycles inhibited preferential flow development. Freeze-thaw cycles significantly altered soil particle distribution and pore structure, particularly reducing soil porosity and increasing bulk density. Correlation analysis revealed that antecedent water content primarily suppressed vertical preferential flow, while freeze–thaw cycles promoted horizontal water migration by creating stratified soil layers with contrasting permeability. The correlation coefficients between antecedent water content and D_max and D_a were −0.77 and −0.94 (p ≤ 0.01), while the correlation between freeze–thaw cycles and CV and LI were −0.65 and −0.86 (p ≤ 0.01). These findings suggest that freeze–thaw cycles exert a strong influence on preferential flow by modifying soil structure, which gradually stabilizes as the number of cycles increases. Antecedent water content, while influencing vertical preferential flow, had a limited effect on horizontal flow. This study highlights the critical role of freeze–thaw-induced changes in soil structure, particularly the formation of stratified layers, in regulating preferential flow and water migration patterns.