Hydrologic response of a steep, unchanneled valley to natural and applied rainfall

Observations from natural rain storms and sprinkling experiments at a steep zero-order catchment in the Oregon Coast Range demonstrate the importance of flow through near-surface bedrock on runoff generation and pore pressure development in shallow colluvial soils. Sprinkling experiments, involving irrigation of the entire 860 m(2) catchment at average intensities of 1.5 and 3.0 mm/h, permitted detailed observation of runoff and the development of subsurface saturation under controlled conditions. A weir installed to collect flow through the colluvium at the base of the catchment recovered runoff equal to one third to one half of the precipitation rate during quasi-steady irrigation. Three key observations demonstrate that a significant proportion of storm runoff flows through near-surface bedrock and illustrate the importance of shallow bedrock flow in pore pressure development in the overlying colluvial soil: (1) greater discharge recovery during both the experiments and natural rainfall at a weir installed approximately 15 m downslope of the weir at the base of the catchment, (2) spatially discontinuous patterns of positive pressure head in the colluvium during steady sprinkling, and (3) local development of upward head gradients associated with flow from weathered rock into the overlying colluvium during high-intensity rainfall. Data from natural storms also show that smaller storms produce no significant runoff or piezometric response and point to a critical intensity-duration rainfall to overcome vadose zone storage. Together these observations highlight the role of interaction between flow in colluvium and nearsurface bedrock in governing patterns of soil saturation, runoff production, and positive pore pressures.