The hydrologic role of the unsaturated zone of a forested colluvium-mantled hollow, Redwood National Park, California

Amen, Bruce

This study examines the hydrologic behavior of some soils in the Redwood Creek drainage. It investigates unsaturated zones, which are important in the effects storms have on forested areas, particularly with regard to landslides. The author found, however, that unsaturated soil in the hollows filled with soil and rock debris from higher elevations does not react differently from any other forest soil. The hydrologic behavior of the unsaturated zone of a forested colluvium-mantled hollow was examined using an array of 11 tensiometers and 3 groundwater wells to record the response to rainfall events during the 1987-88 winter. Erratic tensiometer responses, including inverted soil profile wetting, indicate that at the onset of the rainfall season, when the soil moisture content is relatively low and unsaturated hydraulic conductivity is low, the dominant mode of transport in the unsaturated zone is flow in macropores. Later in the rainfall season, when the soil mass is more uniformly wetted, a uniform tensiometer response indicates that translatory flow is the dominant transport mechanism. Rapid and disproportionate rises in the water table result when small amounts of infiltrating water encounter a thick capillary fringe, where water is held in the soil pores above field capacity. The hydrologic behavior of the unsaturated zone of a forested soil plays a significant role in drainage basin response time to storms. No evidence was found to indicate that the hydrology of the unsaturated zone of a colluvium-mantled hollow differs from that of any other forest soil environment.

The hydrologic role of the unsaturated zone of a forested colluvium-mantled hollow, Redwood National Park, California

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