The Response of Soil Preferential Flow Characteristics to Vegetation Restoration in the Hilly and Gully Region of the Loess Plateau

ABSTRACT Preferential flow considerably affects soil infiltration in the hilly and gully regions of the Loess Plateau, where large‐scale vegetation restoration under the Grain‐for‐Green Program has substantially altered the soil structure. However, the mechanisms underlying the vegetation restoration‐mediated modification of preferential flow characteristics and the preferential flow‐mediated internal regulation of infiltration partitioning remain insufficiently understood. In this study, we focused on forestlands and grasslands in the Yanhe River Basin, employing the double‐ring infiltration method and dye‐tracing technique to quantify the preferential flow characteristics and their contribution to total infiltration. The results revealed that forestlands exhibited the maximum dye depth (59.33 cm) and highest dye coverage (65.90%). The preferential flow ratios and length indices of forestlands (13.94% and 122.54%, respectively) and grasslands (25.42% and 120.03%, respectively) exceeded those of farmlands (9.53% and 100.01%, respectively). Despite relatively deeper and more continuous preferential pathways in forestlands, grasslands exhibited the highest contribution of preferential flow to total infiltration (46.79%), followed by those in forestlands (21.50%) and farmlands (23.05%); these results indicate that piston flow dominates the infiltration, with the preferential flow serving as a supplementary process. Enhanced preferential flow under vegetation restoration was associated with higher total porosity (54.44%–55.39%), lower bulk density (1.30–1.31 g·cm −3 ), and greater root biomass (105.56–170.22 g·m −2 ), whereas farmland exhibited a more compact soil structure with lower porosity (49.86%), higher bulk density (1.34 g·cm −3 ), and limited root development (18.45 g·m −2 ). Preferential flow acts as an internal regulator, redistributing infiltration pathways rather than directly controlling the total infiltration. In practice, grassland is recommended as the vegetation type for restoration on the Loess Plateau, considering the improvement in soil structure and the balancing effect on deep soil moisture.