Dynamic hyporheic and riparian flow path geometry through base flow recession in two headwater mountain stream corridors

The hydrologic connectivity between streams and their valley bottoms (stream corridor) is a critical determinant of their ecological function. Ecological functions are known to be spatially and temporally variable, but spatial dimensions of the problem are not easily quantified and thus they are usually overlooked. To estimate the spatial patterns of connectivity, and how connectivity varies with changes in discharge, we developed the hyporheic potential model. We used the model to interpret a series of solute tracer injections in two headwater mountain streams with contrasting valley bottom morphologies to estimate connectivity in the stream corridor. The distributions of flow path origination locations and the lengths of hyporheic flow paths appear to vary with base flow recession, even in cases where transport timescales are apparently unchanged. The modeled distribution of origination locations further allowed us to define a spatial analog to the temporal window of detection associated with solute tracer studies, and enables assessment of connectivity dynamics between streams and their corridors. Altogether, the reduced complexity hyporheic potential model provides an easy way to anticipate the spatial distribution and origination locations of hyporheic flow paths from a basic understanding of the valley bottom characteristics and solute transport timescales. Plain Language Summary The manuscript details a simple method to assess the spatial connectivity of streams and their riparian zones. While the timescales of exchange in the river corridor have been broadly studied, the complimentary spatial dimension (i.e., the geometry of exchange flowpaths) remains largely unknown. The major challenge in assessing the spatial dimensions of exchange is the limited information available in the subsurface. Here, we develop a reduced complexity model of valley bottom transport to overcome these information limitations. With this model, relatively simple field site characterization and solute tracer data are combined to assess the spatial distribution of downwelling along a headwater mountain stream. We validate the model with a numerical experiment, and demonstrate its application in two watersheds of contrasting geology, repeated through baseflow recession.