Multi-decadal stability of water ages and tracer transport in a temperate-humid river basin

The temporal dynamics of water ages provide crucial insights into hydrological processes and transport mechanisms, yet there remains a significant gap in quantifying water age variability across different temporal scales. This study utilizes a comprehensive dataset spanning 70 years of hydrological observations and tritium records (1953–2022) with a semi-distributed hydrological model with integrated tracer routing routine based on StorageAge Selection functions SAS, to explore the temporal evolution of water ages in the 4000 km2 Upper Neckar River basin, Germany. Our findings indicate a systematic convergence of the variability of young water fractions and other metrics of water age in riverflow and evaporation towards stable values when averaging over increasing time scales. While at daily scales exhibiting considerable variability with young water fractions in riverflow Fwy,Q ∼ 0.01–0.91 and in evaporation Fwy,E ∼ 0.02–0.75, the variability of Fwy,Q and Fwy,E gradually reduces with increasing averaging time scales and converge to 0.45–0.47 and 0.96–0.97, respectively, between individual decades. Liquid water input (PL), comprising rainfall and snow melt, emerges as the dominant driver of Fwy,Q across all time scales. In contrast, Fwy,E shows varying controls with time scale: soil moisture content governs daily fluctuations, whereas PL dominates at the decadal scale. Overall, water ages demonstrate remarkable stability with only minor deviations in response to climatic variability: a 20% fluctuation in average decadal PL results in only ∼4% variation in Fwy,Q and ∼1% in Fwy,E over the study period. These findings suggest a lack of major long-term dynamics in water ages. Consequently, the results suggest that the physical transport dynamics in the Upper Neckar River basin, and potentially in comparable river basins with similar water age characteristics, can be considered near-stationary over multiple decades.