HOW DO SHORT- AND LONG-TERM STORAGE CHANGE DURING STORM EVENTS IN A HEADWATER MOUNTAIN STREAM?

Conceptual models link riparian hydrology with presumed hyporheic exchange during storm events, yet field data are lacking to substantiate these models. Here, we completed twenty-four sets of slug injections in three contiguous study reaches during a 1.25-yr return interval storm event in a steep headwater stream. Replicate studies over a 10-day period characterize short-term storage (recovered tracer) and long-term storage (unrecovered tracer) from pre-storm conditions through recession. Although common conceptual models predict streams as only gaining during storm events, we found bi-directional exchange between surface and groundwaters throughout rising and falling limbs of the storm hydrograph, including a location where the observed potentiometric surface indicated a losing stream throughout the study period. We found that the tracer study window of detection was a primary control in interpreted short-term storage at most locations. Characteristic metrics of short- and long-term storage were significantly different (95% confidence level) between study reaches and could be identified independently of stream discharge via analysis of normalized breakthrough curves, suggesting morphology of each study segment was the primary control. While stream discharge returned to pre-storm conditions, metrics of short- and long-term storage did not return to the same value, suggesting discharge alone is a poor indicator of short- and long-term storage interpreted from slug tracer injections in stream networks.