Limits to the Estimation of Old Streamwater in Catchments Using Environmental Tracers

Abstract Water exiting a catchment and flowing in rivers is characterized by a transit time distribution (TTD), which provides essential information on how catchments store and release water. Since TTDs cannot be measured directly, they are typically inferred from tracer time series in precipitation and streamflow. Previous studies suggest that seasonally variable tracers such as water stable isotopes (O) cannot determine water ages beyond 4–5 years, whereas tritium ( 3 H) may extend this limit to several decades. However, the assumptions underlying these estimates have rarely been evaluated, with implications for the reliability of transit time estimations. A key challenge is that streamflow TTDs usually consist of a narrow age range of relatively young waters (a few months) that dominate the tracer signal, and a broad age range of relatively older waters that contribute only marginally and may be masked by modeling and measuring uncertainties. We introduce a framework to systematically assess the “critical” age separating the portion of the TTD that can be identified through tracers from the portion that cannot. We apply this framework across a wide range of TTD shapes and two key tracers: O and 3 H. Our results show that the critical age is often below 1 year for O and rarely exceeds 5–6 years for 3 H in streamflow dominated by younger waters, which is significantly lower than previously assumed. These findings support the systematic evaluation of the critical age alongside TTD results to clarify which portion of the TTD is truly supported by tracer data.