Scaling Phenomena Related to the Transfer of Storm Water from Forested Hillslopes to Headwater Channels

Overland flow that occurs on humid forested hillslopes is generally believed to initiate via return flow, a type of saturation overland flow. Saturation overland flow more typically occurs where water tables approach the soil surface. Additionally, while preferential flow occurs in soils, it is unclear whether these pathways are continuous over longer slopes and if they affect the timing of catchment runoff. To consider scaling phenomena related to stormflow generation in forests, data are presented from several nested catchments in Japan. In most hillslopes, individual macropores were relatively short or invisible, but preferential flow systems tended to self-organize when soils became wet. Spatially variable and non-linear preferential flow responses were attributed to discrete segments of macropores connecting at ‘nodes' within the soil where each node is activated by local soil water conditions and affected by soil depth, permeability, pore size, organic matter, topography, and momentum dissipation. Biomat flow, shallow subsurface flow in the litter and root-permeated zone, was observed to feed other preferential flow systems based on fluorescent staining tests. In contrast, Hortonian overland flow may be significant at smaller scales and during relatively dry antecedent conditions, but is a less important storm runoff component as sites become wetter. Storm runoff coefficients from small (1 x 2 m) plots were up to an order of magnitude higher than from hillslope-scale plots. Such overland flow often infiltrates into the soil as it progresses downslope, partly controlled by preferential flow paths. During the highest storm intensities, some Hortonian overland flow occurs, but its extent appears limited, constituting up to only 10-15% of rainfall at the small scale and a fraction of this at the hillslope scale. The interactions of spatially and temporally variable Hortonian overland flow and subsurface flow (including preferential flow) are presented in a conceptual model of stormflow generation processes.