Time-variable transit times and derived water fractions during low, intermediate and high flow conditions in large Central European watersheds

Flow conditions strongly influence hydrological transport processes at the catchment scale. Water transit time distributions (TTDs) provide an integrative framework to quantify how water is stored and released from catchments under different flow conditions. Previous studies have primarily focused on small to meso-scale catchments. However, how time-variable TTDs and derived water fractions vary across low-, intermediate-, and high-flow conditions and across seasons remains unclear in large watersheds. Therefore, the objective of this study was to determine and compare time-variable TTDs and derived water fractions of high, intermediate and low flows and across seasons. Here, we applied a semi-distributed tracer-aided conceptual hydrological model combined with StorAge Selection (SAS) functions to nine Central European large watersheds ranging in size from 4,981 km² – 139,549 km². The model was calibrated against streamflow and δ¹⁸O in streamflow using 20 – 32 years of data and is validated with an independent part of the streamflow time series and MODIS Terra snow-cover observations. We defined flow conditions as: high (q > q90​), intermediate (q10 ≤ q ≤ q90) and low (q < q10). We focused on younger water fractions f(t<10 days), f(t<30 days), f(t<90 days), medium aged fractions f(t<1 year), f(t<5 years), and older water f(t>5 years). Results across catchments showed that younger water fractions increased with increasing streamflow, with the median of young and medium aged water fractions being higher under high- and lower under low-flow conditions. While the variability of water fractions remained high across all flow conditions, intermediate flows, showed the highest variability in water ages. Water fractions ratios, e.g., f(t<30 days):f(t<1 year) or f(t<90 days):f(t<1 year), showed high variability across seasons and flow conditions. Next, we will elaborate the interannual and seasonal variability of flow conditions, TTDs, and derived water fractions, and eventually relate the findings to hydroclimatic and physical catchment properties.