How does rapidly changing discharge during storm events affect transient storage and channel water balance in a headwater mountain stream?

Measurements of transient storage in coupled surface-water and groundwater systems
are widely made during base flow periods and rarely made during storm flow periods. We
completed 24 sets of slug injections in three contiguous study reaches during a 1.25 year
return interval storm event (discharge ranging from 21.5 to 434 L s-1) in a net gaining
headwater stream within a steep, constrained valley. Repeated studies over a 9 day period
characterize transient storage and channel water from prestorm conditions through storm
discharge recession. Although the valley floor was always gaining from the hillslopes based
on hydraulic gradients, we observed exchange of water from the stream to the valley floor
throughout the study and flow conditions. Interpretations of transient storage and channel
water balance are complicated by dynamic in-stream and near-stream processes. Metrics of
transient storage and channel water balance were significantly different (95% confidence
level) between the three study reaches and could be identified independently of stream
discharge via analysis of normalized breakthrough curves. These differences suggest that
the morphology of each study reach was the primary control on solute tracer transport.
Unlike discharge, metrics of transient storage and channel water balance did not return to
the prestorm values. We conclude that discharge alone is a poor predictor of tracer transport
in stream networks during storm events. Finally, we propose a perceptual model for our
study site that links hydrologic dynamics in 3-D along the hillslope-riparian-hyporheicstream
continuum, including down-valley subsurface transport.