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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.

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.

Abstract:Headwater catchments are sources of sediments, nutrients, and biota for larger streams, yet the hydrologic pathways that transport these materials remain unclear. Dynamics of storm¯ow generation related to landform attributes and antecedent rainfall were investigated in a steep forested headwater catchment at Hitachi Ohta Experimental Watershed, Japan. Such headwater catchments are deeply incised: the narrow riparian corridors have limited capacities to store and transmit water to streams. Storm runo was monitored at several nested scales within the catchment: (1) 2 . 48 ha ®rst-order drainage (FB); (2) incipient 0 . 84 ha ®rst-order drainage (FA) comprized of two zero-order basins; (3) 0 . 25 ha zero-order basin (ZB); and (4) 45 m 2 hillslope segment (HS), including subsurface matrix¯ow (MF) and preferential¯ow (PF). Results from applied tracer and staining tests as well as observations of piezometric, tensiometric, and subsurface temperature responses were also employed to elucidate hydrologic pathways during storms. During the driest conditions, water yield from FB was only 1%; runo occurred as saturated overland¯ow from the small riparian zone and direct channel interception. For slightly wetter conditions, subsurface¯ow from the soil matrix augmented storm¯ow. As wetness increased, two signi®cant non-linear hydrologic responses occurred: (1) threshold response in geomorphic hollows (zero-order basins) where runo initiated after an accumulation of shallow groundwater; and (2) selforganization and expansion of preferential¯ow pathways, which facilitate subsurface drainage. Storm¯ow increases observed during periods of increasing antecedent wetness depend upon temporal and spatial linkages and the unique hydrologic behavior of three components: (1) narrow riparian corridors; (2) linear hillslopes; and (3) geomorphic hollows. These linkages form the basis for an emerging hydrogeomorphic concept of storm¯ow generation for steep forested headwaters. Knowledge of storm¯ow response is critical to the assessment of management practices in these headwater areas as well as the routing of water and materials to larger stream systems.

Subsurface flow from various portions of a soil profile on a steep, forested hillslope was evaluated by two sets of step‐change miscible displacement tests at different application rates and antecedent hydrologic conditions. Solutions of NaCl (1000 mg L−1 Cl−) were applied at steady state rates (equivalent to 20 and 30 mm h−1 of standing water over the entire plot area) using a line irrigation source located 1.5 m upslope (lateral distance) from an excavated soil pit. Subsurface flow and tracer breakthrough from five portions (the organic‐rich soil layer including macropores, the mineral soil matrix, and three groups of macropores in the mineral soil layer) of the soil profile were individually measured and analyzed using a convective‐dispersive model. Matrix flow dominated discharge from the soil pit during tracer tests (70–93% of total discharge). However, during wet periods with upslope drainage, macropores (including organic‐rich soil) contributed proportionally more flow than during periods when upslope drainage was minimal. Outflow from macropores during the test with wet antecedent conditions had lower Cl− concentrations than drainage from the soil matrix, suggesting dilution in macropores from upslope drainage. Effective pore volumes calculated for the flow‐averaged breakthrough data from the entire profile were much less (<40%) than the estimates (measured by tensiometers) of total volume of pore water, suggesting that preferential flow significantly contributed to subsurface transport of tracer. The pore volume for the entire profile increased only slightly with increasing application rate; however, the relative proportions of pore volumes calculated for individual portions varied proportionally to antecedent hydrologic conditions. These changes are attributed to the expansion of individual macropores with surrounding soil and the lateral extension of macropore networks during wetter conditions.