CONCENTRATION-DISCHARGE RELATIONSHIPS IN RUNOFF FROM A STEEP, UNCHANNELED CATCHMENT

The observation that ''old'' water dominates storm runoff suggests tha t release of low-solute water from soils rather than rainwater must ca use storm runoff dilution. This inference is supported by sprinkling e xperiments in an 860-m(2) catchment in the Oregon Coast Range, in whic h >200 mm of both high and low ionic strength precipitation produced s imilar concentration-discharge trends. Rainwater chemistry was buffere d as it traveled through catchment soils: the amount of sprinkling-der ived water in the runoff increased during long periods of steady disch arge but was not accompanied by a change in runoff solute concentratio ns. Stored water plays a role in runoff dilution as well. Nearly al ru noff from the catchment passes through underlying weathered bedrock ra ther than perching and discharging only through soil. Bedrock water co mposition appears to vary through storm events, as the average contact time of water with rock declines with increasing discharge, a behavio r at odds with the concept of stable end-members.