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<p>Stable isotopes (<span class="inline-formula"><i>δ</i>¹⁸</span>O) and tritium (<span class="inline-formula">³</span>H) are frequently used as tracers in environmental sciences to estimate age distributions of water. However, it has previously been argued that seasonally variable tracers, such as <span class="inline-formula"><i>δ</i>¹⁸</span>O, generally and systematically fail to detect the tails of water age distributions and therefore substantially underestimate water ages as compared to radioactive tracers such as <span class="inline-formula">³</span>H. In this study for the Neckar River basin in central Europe and based on a <span class="inline-formula"><i>&gt;</i>20</span>-year record of hydrological, <span class="inline-formula"><i>δ</i>¹⁸</span>O and <span class="inline-formula">³</span>H data, we systematically scrutinized the above postulate together with the potential role of spatial aggregation effects in exacerbating the underestimation of water ages. This was done by comparing water age distributions inferred from <span class="inline-formula"><i>δ</i>¹⁸</span>O and <span class="inline-formula">³</span>H with a total of 21 different model implementations, including time-invariant, lumped-parameter sine-wave (SW) and convolution integral (CO) models as well as StorAge Selection (SAS)-function models (P-SAS) and integrated hydrological models in combination with SAS functions (IM-SAS).</p> <p>We found that, indeed, water ages inferred from <span class="inline-formula"><i>δ</i>¹⁸</span>O with commonly used SW and CO models are with mean transit times (MTTs) of <span class="inline-formula">∼</span> 1–2 years substantially lower than those obtained from <span class="inline-formula">³</span>H with the same models, reaching MTTs of <span class="inline-formula">∼10</span> years. In contrast, several implementations of P-SAS and IM-SAS models not only allowed simultaneous representations of storage variations and streamflow as well as <span class="inline-formula"><i>δ</i>¹⁸</span>O and <span class="inline-formula">³</span>H stream signals, but water ages inferred from <span class="inline-formula"><i>δ</i>¹⁸</span>O with these models were, with MTTs of <span class="inline-formula">∼</span> 11–17 years, also much higher and similar to those inferred from <span class="inline-formula">³</span>H, which suggested MTTs of <span class="inline-formula">∼</span> 11–13 years. Characterized by similar parameter posterior distributions, in particular for parameters that control water age, P-SAS and IM-SAS model implementations individually constrained with <span class="inline-formula"><i>δ</i>¹⁸</span>O or <span class="inline-formula">³</span>H observations exhibited only limited differences in the magnitudes of water ages in different parts of the models and in the temporal variability of transit time distributions (TTDs) in response to changing wetness conditions. This suggests that both tracers lead to comparable descriptions of how water is routed through the system. These findings provide evidence that allowed us to reject the hypothesis that <span class="inline-formula"><i>δ</i>¹⁸</span>O as a tracer generally and systematically “cannot see water older than about 4 years” and that it truncates the corresponding tails in water age distributions, leading to underestimations of water ages. Instead, our results provide evidence for a broad equivalence of <span class="inline-formula"><i>δ</i>¹⁸</span>O and <span class="inline-formula">³</span>H as age tracers for systems characterized by MTTs of at least 15–20 years. The question to which degree aggregation of spatial heterogeneity can further adversely affect estimates of water ages remains unresolved as the lumped and distributed implementations of the IM-SAS model provided inconclusive results.</p> <p>Overall, this study demonstrates that previously reported underestimations of water ages are most likely not a result of the use of <span class="inline-formula"><i>δ</i>¹⁸</span>O or other seasonally variable tracers per se. Rather, these underestimations can largely be attributed to choices of model approaches and complexity<span id="page3084"/> not considering transient hydrological conditions next to tracer aspects. Given the additional vulnerability of time-invariant, lumped SW and CO model approaches in combination with <span class="inline-formula"><i>δ</i>¹⁸</span>O to substantially underestimate water ages due to spatial aggregation and potentially other still unknown effects, we therefore advocate avoiding the use of this model type in combination with seasonally variable tracers if possible and instead adopting SAS-based models or time-variant formulations of CO models.</p>