Hillslope subsurface flow is driven by vegetation more than soil properties in colonized valley moraines along a humid mountain elevation

Abstract. Valley moraines along an elevation gradient are colonized by different climax vegetation, where preferential flow paths (PFPs) and ground layer, as important hillslope structures, significantly influence hillslope flow. However, the roles of these hillslope structures in flow dynamics and the underlying mechanisms in contrasting ultimate forests within moraines remains enigmatic. To this end, we conceptualized PFPs and the ground layer as explicit elements in the HYDRUS 2D model, constructing set-ups of hillslope internal structures that incorporate an optional ground layer and varying intensities of PFPs using the random placement method to represent the shallow root zone (0–50 cm) of vegetated moraines. The results showed that, out of the 50 set-ups in each forest type, only 3 set-ups in the coniferous forest and 2 set-ups in the broadleaf forest successfully predicted the dynamics and water balance of the hydrological response at the event scale. Notably, all 5 successful set-ups featured below-average vertically connected PFPs that were only 5% of total spatial area in both forests, following the principle of maximum free energy dissipation, which is achieved when flow passes through a network composed of partial PFPs and steepened soil matrix gradient. The similar percentage of PFPs between forest types is attributed to similar coarse-textured soils, which resulted from frequent precipitation and clay washout, as well as comparable fine root in both forests. In addition, a linear relationship between vertical PFPs and hillslope flow was observed in both forests, with the coniferous forest being more sensitive to changes in the vertical PFPs due to lower soil organic matter. The presence of ground layer caused the PFPs to be buried, reduced the exchange between PFPs and soil matrix due to fast lateral flow towards downslope within the ground layer, leading to earlier peak flow timing and increased peak flow. This study highlights the roles of the ground layer and the fine root from vegetation in influencing subsurface flow, advancing our understanding of hillslope structures and runoff evolution over time in humid valley moraines.