Characterizing solute transport with transient storage across a range of flow rates: The evidence of repeated tracer experiments in Austrian and Italian streams

Solute transport in rivers and streams with hyporheic zone exchange and/or in-stream storage is typically affected by the prevailing flow rate. The research reported here focuses on stream tracer experiments repeated many times along the same Austrian (Mödlingbach) and Italian (Torrente Lura) channel reaches to characterize parameter dependency on flow rate. Both groups of data sets showed an increase of storage zone area and main stream area with discharge. In either case, a strong negative correlation was obtained between storage zone residence time and flow rate. From the Mödlingbach data, no clear relationship with Q emerged for the dispersion coefficient and the dead zone ratio, whereas Torrente Lura showed a clear positive correlation of the dispersion coefficient with the flow rate and a slightly negative Q-dependency for the dead zone ratio. Mödlingbach and Torrente Lura results are presented against the background of other repeat experiments reported in literature. In practical applications, the computation of peak concentrations frequently rests on the transfer of transport parameters from one flow rate to another. Using the above Austrian and Italian data sets it was shown that the errors in simulated Mödlingbach peak concentrations remain within a 40% margin, if the ratio of flow rates (for calibration and simulation, resp.) does not exceed 2:1. With Torrente Lura, parameter transfer resulted in somewhat lower peak errors.