Fractal Residence Time Distribution in the Hyporheic Zone of a 2nd-Order Mountain Stream: Implications for Scaling and Stream Biogeochemistry

Exchange of water between streams and shallow groundwater modifies stream chemistry and temperature, and consequently influences biological processes in both the stream and the hyporheic zone. The volume and residence time distribution of the hyporheic zone determine (in part) the influence, with larger volumes and longer residence times generally increasing its significance. We measured the hyporheic residence time distribution in a 2^nd-order mountain stream at the H. J. Andrews Experimental Forest, Oregon, and found it to exhibit fractal scaling over at least 1.5 orders of magnitude in time. The residence time distribution is nearly uniform, scaling approximately as t^-0.3. Such a fractal residence time distribution indicates that the hyporheic zone does not have a characteristic (mean) residence time within the time-scale of our measurements (1 week), and that conventional descriptions of hyporheic exchange flow in such a watershed will be scale-dependent. The implications are that, in streams with a fractal hyporheic residence time distribution, conventional tracer methods may significantly underestimate both the volume and mean residence time of the hyporheic zone. The hyporheic zone may store significant quantities of water and solutes over time-scales very much longer than the mean advection time within the stream channel. In cases where biogeochemical processes are rate-limited beyond the time-scale of advection in the stream channel, fractal residence time distributions may have a significant effect. The fractal nature of the residence time distribution is believed to be linked to the geomorphic complexity of the stream; further studies are underway to investigate this hypothesis.