For all injections (sequentially from top to bottom), WS01 has a larger range of observed fractions of stream water, and WS03 always has a larger average fraction of stream water. The highly constrained WS03 exhibits no significant trends with distance from the stream centerline, while WS01 always exhibits significant trends of decreasing fraction of stream water with increasing distance from the stream centerline. Both one-way ANOVA and Kruskal-Wallis tests indicate significant differences in the population mean and median, respectively, for WS01 and WS03 during injections 2 and 3. Notation of m* indicates a slope that is significant (i.e., 0 is not within the 95% confidence interval for linear regression slope). Colorbars map to the y axis values on the two right-most columns, where the boxplots present the distribution of all observations for each flow condition. The scatter plots and best fit trend lines display trends in the peak fraction of stream water as a function of distance from the stream channel. Red and blue data sets on the figures represent WS01 and WS03, respectively. Figures 5–7 are displayed with a similar organization.
Solute transport along riparian and hyporheic flow paths is broadly expected to respond to dynamic hydrologic forcing by streams, aquifers, and hillslopes. However, direct observation of these dynamic responses is lacking, as is the relative control of geologic setting as a control on responses to dynamic hydrologic forcing. We conducted a series of four stream solute tracer injections through base flow recession 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 riparian wells in each watershed. We found hyporheic mean arrival time, temporal variance, and fraction of stream water in the bedrock-constrained valley bottom and near large roughness elements in the wider valley bottom were not variable with discharge, suggesting minimal control by hydrologic forcing. Conversely, we observed increases in mean arrival time and temporal variance and decreasing fraction stream water with decreasing discharge near the hillslopes in the wider valley bottom. This may indicate changes in stream discharge and valley bottom hydrology control transport in less constrained locations. We detail five hydrogeomorphic responses to base flow recession to explain observed spatial and temporal patterns in the interactions between streams and their valley bottoms. Models able to account for the transition from geologically dominated processes in the near-stream subsurface to hydrologically dominated processes near the hillslope will be required to predict solute transport and fate in valley bottoms of headwater mountain streams.
Ward, A.S., Schmadel, N.M., Wondzell, S.M., Harman, C., Gooseff, M.N., and Singha, K. (2016): Hydrogeomorphic controls on hyporheic and riparian transport in two headwater mountain streams during base flow recession. Water Resources Research. DOI: 10.1002/2015WR018225
This Paper/Book acknowledges NSF CZO grant support.