Role of subsurface flow in generating surface runoff: 2. Upstream source areas

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Abstract
Runoff simulation for rainfall events on hypothetical upstream source areas, carried out with a deterministic mathematical model that couples channel flow and saturated‐unsaturated subsurface flow, provides theoretical support for the runoff‐generating mechanisms observed in the field by Ragan and Dunne. The simulations show that there are stringent limitations on the occurrence of subsurface storm flow as a quantitatively significant runoff component. Only on convex hillslopes that feed deeply incised channels, and then only when saturated soil conductivities are very large, is subsurface storm flow a feasible mechanism. On concave slopes with lower permeabilities, and on all convex slopes, hydrographs are dominated by direct runoff through very short overland flow paths from precipitation on transient near‐channel wetlands. On these wetlands surface saturation occurs from below because of rising water tables that are fed by vertical infiltration rather than by lateral subsurface flow. These conclusions, when coupled with field observations that show classic Hortonian overland flow to be a rare occurrence in vegetated humid environments, have implications in the planning of field instrumentation networks, and in the designing of hydrologic response models.