Morphological Characteristics of Macropores and the Distribution of Preferential Flow Pathways in a Forested Slope Segment

Abstract

Water flow through soil macropores is important in determining hydrologic responses in forested watersheds. Morphological characteristics of macropores and distribution of preferential flow pathways were evaluated in a forest hillslope segment using a combination of staining agents. Almost 80% of described macropores were roughly elliptical with eccentricities ranging from 0.256 to 0.998 (mean of 0.652) and lengths ranging from 2.0 to 61.8 cm (mean of 11.6 cm). Tortuosity of macropores tended to increase with increasing length up to about 30 cm, with a mean value of 1.14. Macropores were aggregated in large clumps within the soil profile. Living and decayed roots and associated vertical zones of loose soil and humus contributed to preferential flow pathways in this soil. Subsurface flow patterns, detected by upslope injection of dilute white paint solution, showed a strong interaction between the soil matrix and macropores. Subsurface flow was lateral along the bedrock and between A and B horizons, with a perched water table occurring on sections of both. Dye tests also showed that flow occurred within surface bedrock fractures. This fracture flow was sometimes connected to macropores through zones of local wetness. Thus, we conclude bedrock topography and fracture characteristics may contribute significantly to preferential flow pathways at the hillslope scale. Even though individual macropores were rather short, the coupling of these flow paths with the soil matrix, bedrock fractures, living and decayed roots, and perched water tables produced complex networks of interconnected preferential flow pathways, all of which help explain the stormflow response observed in the catchment.

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

“…Many recent experimental investigations of hillslope hydrology have considered the role of PF (e.g., Noguchi et al, 1999;Buttle and McDonald, 2002;Tromp-van Meerveld and McDonnell, 2006a,b;Lin et al, 2006).…”

“…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).…”

A model of hydrological and mechanical feedbacks of preferential fissure flow in a slow-moving landslide

“…Hydrological feedback is the mutual dependence between landslide hydrological responses and effectiveness of the fissure network to transport water which increases with soil wetness (Tsuboyama et al, 1994;Noguchi et al, 1999;Sidle et al, 2000). Following the concept presented by Krzeminska et al (2012), the model accounts for dynamic hydrological feedback between fissure connectivity and the degree of saturation of the soil column (Eq.…”

Effects of bedrock groundwater on spatial and temporal variations in soil mantle groundwater in a steep granitic headwater catchment

“…For small granitic catchments in central Japan, Terajima et al [1993] estimated that the ratio exceeded 18.2%. Tracer experiments have further confirmed the existence of bedrock infiltration [e.g., Wilson et al, 1993;Noguchi et al, 1999;Frazier et al, 2002;Legout et al, 2007].…”