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Methods for estimating mean transit times from chemical or isotopic tracers(such as Cl−, δ18O, or δ2H) commonly assumethat catchments are stationary (i.e., time-invariant) and homogeneous. Realcatchments are neither. In a companion paper, I showed that catchment meantransit times estimated from seasonal tracer cycles are highly vulnerable toaggregation error, exhibiting strong bias and large scatter in spatiallyheterogeneous catchments. I proposed the young water fraction, which isvirtually immune to aggregation error under spatial heterogeneity, as abetter measure of transit times. Here I extend this analysis by exploringhow nonstationarity affects mean transit times and young water fractionsestimated from seasonal tracer cycles, using benchmark tests based on asimple two-box model. The model exhibits complex nonstationary behavior,with striking volatility in tracer concentrations, young water fractions,and mean transit times, driven by rapid shifts in the mixing ratios offluxes from the upper and lower boxes. The transit-time distribution instreamflow becomes increasingly skewed at higher discharges, with markedincreases in the young water fraction and decreases in the mean water age,reflecting the increased dominance of the upper box at higher flows. Thissimple two-box model exhibits strong equifinality, which can be partlyresolved by simple parameter transformations. However, transit times areprimarily determined by residual storage, which cannot be constrainedthrough hydrograph calibration and must instead be estimated by tracer behavior. Seasonal tracer cycles in the two-box model are very poor predictors of meantransit times, with typical errors of several hundred percent. However, thesame tracer cycles predict time-averaged young water fractions (Fyw)within a few percent, even in model catchments that are both nonstationaryand spatially heterogeneous (although they may be biased by roughly 0.1–0.2at sites where strong precipitation seasonality is correlated withprecipitation tracer concentrations). Flow-weighted fits to the seasonaltracer cycles accurately predict the flow-weighted average Fyw instreamflow, while unweighted fits to the seasonal tracer cycles accuratelypredict the unweighted average Fyw. Young water fractions can also beestimated separately for individual flow regimes, again with a precision ofa few percent, allowing direct determination of how shifts in a catchment's hydraulicregime alter the fraction of water reaching the stream by fast flowpaths.One can also estimate the chemical composition of idealized "young water"and "old water" end-members, using relationships between young waterfractions and solute concentrations across different flow regimes. Theseresults demonstrate that mean transit times cannot be estimated reliablyfrom seasonal tracer cycles and that, by contrast, the young water fractionis a robust and useful metric of transit times, even in catchments thatexhibit strong nonstationarity and heterogeneity

Aggregation in environmental systems – Part 2: Catchment mean transit times and young water fractions under hydrologic nonstationarity