Spatial Patterns of Water Age: Using Young Water Fractions to Improve the Characterization of Transit Times in Contrasting Catchments

Transit time distributions (TTDs) are crucial descriptors of flow and transport processes in catchments, which can be determined from stable water isotope data. Recently, the young water fraction (Fyw) has been introduced as an additional metric derivable from seasonal isotope cycles. In this study, we calculated Fyw and TTDs using monthly isotope data from 24 contrasting subcatchments in a mesoscale catchment (3,300 km2) in Germany. Fyw ranged from 0.01 to 0.27 (mean = 0.11) and was smallest in mountainous catchments. Assuming gamma‐shaped TTDs, we determined stationary TTDs with the convolution integral method for each subcatchment. The convolution integral was first calibrated against the isotope data only (i.e., traditional calibration) and, second, using a multiobjective calibration with the Fyw estimates as an additional constraint. This yielded largely differing TTD parameters even for neighboring catchments, with Fyw values below 0.1 generally involving a delayed peak in TTDs (i.e., gamma‐distribution shape parameter > 1). While the traditional calibration resulted in large uncertainties in TTD parameters, these uncertainties were reduced with the multiobjective calibration, thereby improving the assessment of mean transit times (2 years on average, ranging between 9.6 months and 5.6 years). This highlights the need for uncertainty assessment when using simple isotope models and shows that the traditional calibration might not yield an optimum solution in that it may give a TTD nonconsistent with Fyw. Given the robustness of Fyw estimates, isotope models should thus aim at accurately describing both Fyw and measured isotope data in order to improve the description of flow and transport in catchments. Plain Language Summary Information on the age of river water is crucial for assessing the vulnerability of rivers to weather extremes and pollution. The age of river water is defined as the time that water has spent underground after rainfall infiltration and before ending up in the river. The probability distribution of river water age can be determined using environmental tracers, which are tracers that naturally occur in the system such as stable water isotopes. In this study, we used isotope models to analyze time series of stable water isotopes in rainfall and streamwater measured in 24 subcatchments of the Bode catchment in central Germany. We found that the mean age of river water ranges between 9.6 months and 5.6 years depending on catchment ch