A conceptual model of preferential flow systems in forested hillslopes: evidence of self‐organization

Abstract

Abstract:Preferential flow paths are known to be important conduits of subsurface stormflow in forest hillslopes. Earlier research on preferential flow paths focused on vertical transport; however, lateral transport is also evident in steep forested slopes underlain by bedrock or till. Macropores consisting of decayed and live roots, subsurface erosion, surface bedrock fractures, and animal burrows form the basis of a 'backbone' for lateral preferential flow in such sites. Evidence from field studies in Japan indicates that although individual macropore segments are generally <0Ð5 m in length, they have a tendency to self-organize into larger preferential flow systems as sites become wetter. Staining tests show clear evidence of interconnected macropore flow segments, including: flow within decayed root channels and subsurface erosion cavities; flow in small depressions of the bedrock substrate; fracture flow in weathered bedrock; exchange between macropores and mesopores; and flow at the organic horizon-mineral soil interface and in buried pockets of organic material and loose soil. Here we develop a three-dimensional model for preferential flow systems based on distributed attributes of macropores and potential connecting nodes (e.g. zones of loose soil and buried organic matter). We postulate that the spatially variable and non-linear preferential flow response observed at our Japan field site, as well as at other sites, is attributed to discrete segments of macropores connecting at various nodes within the regolith. Each node is activated by local soil water conditions and is influenced strongly by soil depth, permeability, pore size, organic matter distribution, surface and substrate topography, and possibly momentum dissipation. This study represents the first attempt to characterize the spatially distributed nature of preferential flow paths at the hillslope scale and presents strong evidence that these networks exhibit complex system behaviour.

A review of model applications for structured soils: a) Water flow and tracer transport

“…Lateral PF is a complex process that can take a variety of forms including flow in 'pipes' (i.e., large macropores oriented parallel to the soil surface) along the base of the soil profile or in the topsoil (e.g., Uchida et al, 2001), or flow through a dynamic network of PF paths embedded in the soil matrix, movement in a thin saturated layer or along micro-channels above bedrock, and transport in exfoliation fractures in bedrock (Buttle and McDonald, 2002). Evidence from hillslope studies suggests that although individual macropore segments are often smaller than approximately 0.5 m in length, they have a tendency to selforganize into larger PF systems as sites become wetter (Noguchi et al, 1999;Sidle et al, 2001). There is evidence that a threshold of total storm event precipitation has to be surpassed in order to activate the lateral PF system and trigger subsurface pipe flow (e.g., Tromp-van Meerveld and McDonnell, 2006a,b).…”

“…Different conceptual models were proposed, including interconnected, self-organizing PF systems in forested hillslopes (Sidle et al, 2001), hillslope pipe flow (Uchida et al, 2001), and flow through a subsoil macropore network as one of four main flow pathways .…”

Soil pipe distribution and hydrological functioning within the humid tropics: a synthesis

“…With rapid saturation, the network of small macropores within these wedges may fill connecting near-surface macropores with those in the lower soil profile (cf. Sidle et al, 2001), which are themselves connected with the soil pipes.…”

Modelling subsurface storm flow with the Representative Elementary Watershed (REW) approach: application to the Alzette River Basin

“…In the unsaturated domain, only vertical flows are considered, as described in earlier publications on the REW approach. Although earlier research on preferential flow paths focused on vertical transport only, lateral flow is evident in steep forested slopes underlain by bedrock as discussed by Sidle et al (2001). Therefore, both downward and lateral flows are accounted for in the macropore domain.…”