Root-induced preferential flow in the critical zone: Observations, processes, mechanisms, and modeling

Abstract

Root-induced preferential flow (RPF) is an important form of non-equilibrium water transport in vegetated critical zones, but its identification, controls, and hydrological consequences remain difficult to generalize because root-derived pathways coexist with cracks, faunal burrows, soil pipes, and lithological fractures. This review synthesizes 330 studies to clarify the evidence for root-specific preferential flow and its links to root characteristics, pathway properties, soil structure, and hydrological forcing. Four non-exclusive structural expressions of RPF are identified: shallow root network–dominated preferential flow (SRPF), vertical root-controlled preferential flow (VRPF), interface-guided preferential flow (IRPF), and legacy root-biopore–dominated preferential flow (BRPF). Across the reviewed literature, SRPF was reported most frequently (63.0%), followed by VRPF (56.1%), BRPF (35.8%), and IRPF (17.0%); these frequencies represent literature occurrence rather than ecosystem prevalence. Root architecture, abundance, mortality and decomposition influence pathway formation, whereas pathway connectivity, soil heterogeneity, antecedent wetness, and water-input conditions regulate their hydraulic expression. Consequently, RPF can enhance infiltration and subsurface redistribution, but its effects on storage, groundwater recharge, runoff timing, and peak discharge are context dependent. Apparent threshold-like responses are therefore better interpreted as changes in effective pathway connectivity than as universal rainfall or soil-moisture thresholds. Current understanding remains constrained by limited direct measurements linking root provenance, pathway geometry, connectivity, and flux, and by models that rarely represent the biological origin and temporal evolution of preferential pathways. Stronger root-specific experiments, repeated three-dimensional observations, and dynamic root-informed parameterization are needed to improve representation of RPF across scales.