Spatiotemporal Controls On Hydrologic Exchange Between Hillslopes, Valley Bottoms, And Streams From The Reach To The Watershed Scale

The hydrologic connections between hillslopes, underlying bedrock, riparian areas, and streams are critical for understanding the source areas of water and associated biogeochemical constituents. However, quantification of the spatiotemporal nature of these connections and the exchange of water among component parts of a watershed, particularly the connection between uplands and in-stream hydrologic exchange remains a challenge. We used dilution gauging, mass recovery, and recording stilling wells to characterize exchange dynamics across 52 stream reaches, from peak snowmelt to baseflow, in the Tenderfoot Creek Experimental forest, Montana, USA. We found that watershed contributing area was only a significant predictor of net changes in streamflow at high moisture states and larger spatial scales. However, at the scale of individual stream reaches, the lateral contributing area in conjunction with underlying lithology was a significant predictor of gross hydrologic gains to the stream across all flow states. Reach lateral contributing areas underlain by more permeable sandstone yielded less water (r2: 0.89-0.21, slope: 78-l l/s km-2) across flow states relative to those with granite gneiss (r2: 0.80-0.49, slope: 126-l8 l/s km-2). Additionally, increases in the frequency of steps across each stream reach contributed to greater hydrologic gross losses (r2: 0.43-0.8, slope: 4.21 0.58 l/s/ normalized step count-2). Together, gross gains and losses of water along individual reaches resulted in net changes of discharge that cumulatively scale to the observed outlet discharge dynamics. Our results provide a framework for understanding how hillslope topography, geology and valley bottom structure contribute to the exchange of water and cumulative increases of stream flow across watersheds of increasing size