SUBSURFACE FLOW PATHS IN A STEEP, UNCHANNELED CATCHMENT

Tracer studies during catchment-scale sprinkler experiments illuminate the pathways of subsurface flow in a small, steep catchment in the Or egon Coast Range. Bromide point injections into saturated materials sh owed rapid flow in bedrock to the catchment outlet. Bedrock flow retur ned to the colluvium, sustaining shallow subsurface flow there. The br omide peak velocity of similar to 10(-3) m s(-1) exceeded the saturate d hydraulic conductivity of intact bedrock. This, and the peak shapes, verify that fractures provide important avenues for saturated flow in the catchment. Deuterium added to the sprinkler water moved through t he vadose zone as plug flow controlled by rainfall rate and water cont ent. Ninety-two percent of the labeled water remained in the vadose zo ne after 3 days (similar to 140 mm) of sprinkling. Preferential flow o f new water was not observed during either low-intensity irrigation or natural storms; however, labeled preevent water was mobile in shallow colluvium during a storm following our spiking experiment. In respons e to rainfall, waters from the deeper bedrock pathway, which have trav eled through the catchment, exfiltrate into the colluvium mantle and m ix with relatively young vadose zone water, derived locally, creating an area of subsurface saturation near the channel head. This effective ly becomes a subsurface variable source area, which, depending on its size and the delivery of water from the vadose zone, dictates the appo rtioning of old and new water in the runoff and, correspondingly, the runoff chemistry. The slow movement of water through the vadose zone a llows for chemical modification and limits the amount of new water in the runoff. Moreover, it suggests that travel time of new rain water d oes not control the timing of runoff generation.