Hydrologic Processes in a Low‐Gradient Source Area

Discharge, piezometric, and other field observations from a 3‐year monitoring program indicate that runoff from a low‐gradient source area in Marin County, California, is controlled by the interaction of throughflow, macropore flow, and saturation overland flow. Throughflow response integrates multiple storm events and exhibits both seasonal trends and rapid response to midwinter storms upon saturation of highly conductive near‐surface soils. After saturation of the deeper colluvium along the hollow axis, macropore flow responds rapidly to individual storm events and locally provides a ceiling to piezometric response. Saturation overland flow occurs along the axis of the unchanneled valley only during large storms when both soil matrix and macropore transmissivity are exceeded. During large, runoff‐producing storms, saturation overland flow extends continuously over most of the unchanneled valley axis. During smaller runoff‐producing events, however, ground surface saturation may be discontinuous, reflecting either variations in the conductivity of the underlying soil/bedrock or a variable soil thickness along the hollow axis. Results of this study document a sequence of flow path activation in which the temporal distribution of rainfall events within a season determines both the mechanism and magnitude of runoff generation.