Controls on preferential flow and its role on streamflow generation in a Mediterranean forested catchment

The driving mechanisms of preferential flow in soils of Mediterranean forested catchments remain largely unexplored. We used soil moisture sensors to investigate these mechanisms along two hillslope transects in the Re della Pietra catchment (2 km2) in the Tuscan Apennines, central Italy, to understand the drivers of preferential flow and its effect on streamflow generation. The study area has a temperate Mediterranean climate, consists of sandy-loam soils, and is covered almost entirely by a dense deciduous forest. We defined dry and wet periods using an automatic method based on the soil moisture signal's high and low envelope, considering field capacity and potential evapotranspiration thresholds. Among various hydrometeorological, topographic, and soil physical properties, antecedent soil moisture emerged as the primary driver of preferential flow at one hillslope, while dry bulk density dominated at the other hillslope. A supervised classification random forest model was highly effective in classifying the type of soil moisture response based on its timing and identifying the controlling factors. Precipitation events in the headwater subcatchments hosting one of the two hillslopes were classified according to the timing of soil moisture response leading to the identification of three distinct types of hydrographs, revealing the role of preferential flow on sustaining streamflow. Our results shed new light on preferential flow controls and their role in runoff generation, emphasizing the importance of these processes in seasonally dry and wet hydrologic systems, such as Mediterranean catchments, and the need to better understand their spatiotemporal patterns.