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Title (Primary) Spatial patterns of water age: using young water fractions to improve the characterization of transit times in contrasting catchments
Author Lutz, S.R.; Krieg, R.; Müller, C.; Zink, M.; Knöller, K.; Samaniego, L.; Merz, R.;
Journal Water Resources Research
Year 2018
Department CHS; CATHYD;
Volume 54
Issue 7
Language englisch;
POF III (all) T34; T53;
Keywords transit time distribution; young water fraction; spatial patterns; isotope modelling
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 (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 sub‐catchments in a mesoscale catchment (3300 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 sub‐catchment. The convolution integral was first calibrated against the isotope data only (i.e., traditional calibration) and, second, using a multi‐objective 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 multi‐objective calibration, thereby improving the assessment of mean transit times (two 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 non‐consistent 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.
ID 20619
Persistent UFZ Identifier https://www.ufz.de/index.php?en=20939&ufzPublicationIdentifier=20619
Lutz, S.R., Krieg, R., Müller, C., Zink, M., Knöller, K., Samaniego, L., Merz, R. (2018):
Spatial patterns of water age: using young water fractions to improve the characterization of transit times in contrasting catchments
Water Resour. Res. 54 (7), 4767 - 4784