HYDROLOGIC PROCESSES IN A LOW-GRADIENT SOURCE AREA

Discharge, piezometric, and other field observations from a 3-year mon itoring program indicate that runoff from a low-gradient source area i n Marin County, California, is controlled by the interaction of throug hflow, macropore flow, and saturation overland flow. Throughflow respo nse integrates multiple storm events and exhibits both seasonal trends and rapid response to midwinter storms upon saturation of highly cond uctive near-surface soils. After saturation of the deeper colluvium al ong the hollow axis, macropore flow responds rapidly to individual sto rm events and locally provides a ceiling to piezometric response. Satu ration overland flow occurs along the axis of the unchanneled valley o nly during large storms when both soil matrix and macropore transmissi vity are exceeded. During large, runoff-producing storms, saturation o verland 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 c onductivity of the underlying soil/bedrock or a variable soil thicknes s along the hollow axis. Results of this study document a sequence of flow path activation in which the temporal distribution of rainfall ev ents within a season determines both the mechanism and magnitude of ru noff generation.