| Abstrakt: |
Transit time distributions (TTDs) of streamflow are useful descriptors for understanding flow and solute transport in catchments. Catchment-scale TTDs can be modeled using tracer data (e.g., δ 18O; oxygen isotopes) in inflow and outflows, with StorAge Selection (SAS) functions. However, tracer data are often sparse in space and time, so they can be interpolated to increase their spatio-temporal resolution. Also, SAS functions can be parameterized with different forms, but there is no general agreement on which one should be used. Both of these aspects induce uncertainty in the simulated TTDs, and the individual uncertainty sources as well as their combined effect have not been fully investigated. This study provides a comprehensive analysis of the TTD uncertainty resulting from twelve model setups obtained by combining different interpolation schemes for δ 18O in precipitation, and distinct SAS functions. Furthermore, we evaluated the value of the young water fraction (Fyw) as an additional constraint for the TTD uncertainty. For each model setup, we found behavioral solutions with satisfactory model performances for instream δ 18 O (Kling-Gupta Efficiency, KGE>0.57). Differences in KGE values were statistically significant, thus showing the relevance of the chosen setup for simulating TTDs. We found a large uncertainty in the simulated TTDs, with a 90% confidence interval varying between 286 and 895 days across all tested setups. Uncertainty in TTDs was mainly associated with the temporal interpolation of δ 18O in precipitation, time-variant SAS function and low flow conditions. The use of Fyw as an additional constraint substantially reduced the uncertainty in the predicted TTDs by up to 49%. We discussed the implications of these results with respect to the study area and the SAS framework, in order to identify ways to improve uncertainty characterization and water age simulations in TTD-based models. [ABSTRACT FROM AUTHOR] |
