Runoff generation in a steep, soil-mantled landscape

[1] Scale and slope dependence of hydrologic response are investigated for two channel network source areas (unchanneled valleys) in the Oregon Coast Range. Observations of response to both natural and applied precipitation reveal that runoff occurred as subsurface flow in which water passed through partially saturated soil, into the shallow fractured bedrock, to emerge as subsurface partial source areas near the channel head. The two dominant approaches to modeling subsurface flow in steep topography, routing of Darcy or fracture flow and the hydrologic similarity approximation of TOPMODEL, respectively predict either a strong slope dependence or no slope dependence to timescales of subsurface runoff generation. Compilation of data from our Coos Bay study sites with observations reported previously elsewhere indicates weak area dependence but no slope dependence in the lag-to-peak and discharge recession constants. This finding supports the interpretation that patterns of antecedent soil moisture and vadose zone characteristics control response times of runoff generation by subsurface storm flow. As slope should influence lateral flow routing once subsurface saturation develops, we conclude that the hydrologic response of steep catchments appears to be insensitive to slope because the controlling timescale is that of the vertical unsaturated flow.