Soil properties, topography, and meteorological forcing control preferential flow and streamflow generation in a Mediterranean forested catchment

Preferential flow (PF) is a key hydrological process that influences water infiltration, soil moisture redistribution, and streamflow generation. In Mediterranean forested catchments, the dynamics of PF and its controls remain largely underexplored. Here, we investigated PF mechanisms and their impact on hydrological response in the Re della Pietra experimental catchment (2 km²) in the Tuscan Apennines, central Italy. Two hillslope transects with soil moisture sensors at shallow (15 cm) and deep (35 cm) layers were monitored for 34 and 18 months, respectively. A supervised Random Forest (RF) classification model was employed to identify the dominant controls on PF initiation across varying hydrological, topographical, and soil conditions.

Results showed that antecedent soil moisture was the primary driver of PF in one of the hillslopes, while dry bulk density dominated in the other, highlighting spatial heterogeneity in PF controls. Precipitation characteristics played a secondary role, with PF more likely during dry periods at both sites. PF events altered streamflow dynamics, producing early hydrograph peaks and sustaining flow during recession phases. Mixed flow events, combining sequential and non-sequential soil moisture responses, generated the highest total streamflow volumes, emphasizing the contribution of PF to catchment connectivity.

These findings underscore the importance of PF in Mediterranean hydrology, where alternating wet and dry periods intensify its effects on water redistribution and streamflow generation. By combining extensive field monitoring with machine learning, this study offers new insights into the interplay between PF and catchment hydrological responses.