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

Abstract

Abstract 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 ( F yw ) has been introduced as an additional metric derivable from seasonal isotope cycles. In this study, we calculated F yw and TTDs using monthly isotope data from 24 contrasting subcatchments in a mesoscale catchment (3,300 km 2 ) in Germany. F yw 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 F yw estimates as an additional constraint. This yielded largely differing TTD parameters even for neighboring catchments, with F yw 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 F yw . Given the robustness of F yw estimates, isotope models should thus aim at accurately describing both F yw and measured isotope data in order to improve the description of flow and transport in catchments.