Subsurface flow paths in a steep, unchanneled catchment

Abstract

Tracer studies during catchment‐scale sprinkler experiments illuminate the pathways of subsurface flow in a small, steep catchment in the Oregon Coast Range. Bromide point injections into saturated materials showed rapid flow in bedrock to the catchment outlet. Bedrock flow returned to the colluvium, sustaining shallow subsurface flow there. The bromide peak velocity of ∼10⁻³ m s⁻¹ exceeded the saturated 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 the vadose zone as plug flow controlled by rainfall rate and water content. Ninety‐two percent of the labeled water remained in the vadose zone after 3 days (∼140mm) of sprinkling. Preferential flow of 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 response to rainfall, waters from the deeper bedrock pathway, which have traveled through the catchment, exfiltrate into the colluvium mantle and mix with relatively young vadose zone water, derived locally, creating an area of subsurface saturation near the channel head. This effectively becomes a subsurface variable source area, which, depending on its size and the delivery of water from the vadose zone, dictates the apportioning of old and new water in the runoff and, correspondingly, the runoff chemistry. The slow movement of water through the vadose zone allows for chemical modification and limits the amount of new water in the runoff. Moreover, it suggests that travel time of new rain water does not control the timing of runoff generation.