H31K-05
Hyporheic transport in headwater mountain streams is time-invariant in locations where geologic controls dominate hydrologic forcin
Wednesday, 16 December 2015: 09:00
3018 (Moscone West)
Adam S Ward, Indiana University Bloomington, School of Public and Environmental Affairs, Bloomington, IN, United States, Noah Schmadel, Indiana University Bloomington, SPEA, Bloomington, IN, United States, Steven M Wondzell, Pacific Northwest Research Station, Seattle, WA, United States, Ciaran J Harman, Johns Hopkins University, Geography and Environmental Engineering, Baltimore, MD, United States; Johns Hopkins University, Baltimore, MD, United States, Michael N Gooseff, Institute of Arctic and Alpine Research, Boulder, CO, United States and Kamini Singha, Colorado School of Mines, Golden, CO, United States
Abstract:
Transport along riparian and hyporheic flowpaths is generally believed to integrate the responses of streams and aquifers to dynamic hydrological forcing. Although it is generally expected transport along these flow paths is time-variable, such dynamic responses have seldom been demonstrated. Further, we do not understand how hydrological forcing interacts with local geologic setting (i.e., valley and streambed morphology) We conducted a series of four stream solute tracer injections in each of two watersheds with contrasting valley morphology in the H.J. Andrews Experimental Forest, monitoring tracer concentrations in the stream and in a network of shallow wells in each watershed. Time series analyses were used to deconvolve transport along subsurface flowpaths from transport in the stream channel. We found time-invariant hyporheic transport in the narrow, bedrock-constrained valley and near large roughness elements (e.g., steps, logs) in the wider valley bottom despite order of magnitude changes in discharge, suggesting geologic controls dominate hyporheic transport in these locations. In contrast, we observed increases in mean arrival time and temporal variance with decreasing discharge at the riparian-hillslope transition, suggesting hydrological dynamics control transport in these locations. We pose several mechanisms by which dynamic hydrology and geologic setting interact that may explain the observed behavior. We interpret time-invariant transport as an indication that discharge in the surface stream is a poor predictor of exchange along the stream-hyporheic-riparian-hillslope continuum in headwater valleys. As such, models able to account for the transition from geologically-dominated processes in the near-stream subsurface to hydrologically-dominated processes near the hillslope are required to predict transport and fate in valley bottoms of headwater mountain streams.
