Comparing transient storage modeling and residence time distribution (RTD) analysis in geomorphically varied reaches in the Lookout Creek basin, Oregon, USA

The stream tracer technique has been widely used as a method of characterizing hyporheic exchange in stream-catchment studies, commonly incorporating the use of the numerical, transient storage model OTIS, which assumes an exponential residence time distribution. In this study, we compare OTIS and, a model that admits a general residence time distribution (RTD), called solute transport and multirate mass transfer-linear coordinates (STAMMT-L). Models were compared using slug-tracer injections of rhodamine WT (RWT) in three geomorphically distinct stream reaches in the Lookout Creek basin, Oregon USA: a second-order reach of a stream in Watershed 3 which is characterized by pool-step morphology; and two fourth-order reaches of Lookout Creek, one characterized by a single-thread, pool-step morphology, the other a morphologically complex reach with braided channels. OTIS modeling results tended to match short time scale concentrations well, including the advective peak, but the simulated late-time RTD of stream RWT concentrations was in error. The STAMMT-L model allowed for more accurate characterization of late-time stream RWT concentrations, and so characterized a larger portion of the entire RTD. Although both models are sensitive to morphologic differences among the studied stream reaches, they are also clearly different in the relative importance placed on short vs. long residence time distributions. Consequently the two models will result in different views of the hyporheic zone and its role in stream ecosystem processes.