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. The time precipitation needs to travel through acatchment to its outlet is an important descriptor of a catchment'ssusceptibility to pollutant contamination, nutrient loss, and hydrologicalfunctioning. The fast component of total water flow can be estimated by thefraction of young water (Fyw), which is the percentage of streamflow youngerthan 3 months. Fyw is calculated by comparing the amplitudes of sinewaves fitted to seasonal precipitation and streamflow tracer signals. Thisis usually done for the complete tracer time series available, neglectingannual differences in the amplitudes of longer time series. Consideringinter-annual amplitude differences, we employed a moving time window of1 year in weekly time steps over a 4.5-year δ18O tracer time series to calculate 189 Fyw estimates and their uncertainty.They were then tested against the following null hypotheses: (1) at least90 % of Fyw results do not deviate more than ±0.04 (4 %) from themean of all Fyw results, indicating long-term invariance. Larger deviationswould indicate changes in the relative contribution of different flow paths;(2) for any 4-week window, Fyw does not change more than ±0.04 ,indicating short-term invariance. Larger deviations would indicate a highsensitivity of Fyw to a 1-week to 4-week shift in the start of a 1-year samplingcampaign; (3) the Fyw results of 1-year sampling campaigns started in agiven calendar month do not change more than ±0.04 , indicatingseasonal invariance. In our study, all three null hypotheses were rejected.Thus, the Fyw results were time-variable, showed variability in the chosensampling time, and had no pronounced seasonality. We furthermore foundevidence that the 2015 European heat wave and including two winters into a1-year sampling campaign increased the uncertainty of Fyw. Based on anincrease in Fyw uncertainty when the mean adjusted R2 wasbelow 0.2, we recommend further investigations into the dependence of Fyw andits uncertainty to goodness-of-fit measures. Furthermore, while investigatedindividual meteorological factors did not sufficiently explain variations ofFyw, the runoff coefficient showed a moderate negative correlation of r = - 0.50 with Fyw. The results of this study suggest that care must be takenwhen comparing Fyw of catchments that were based on different calculationperiods and that the influence of extreme events and snow must beconsidered.

Time variability and uncertainty in the fraction of young water in a small headwater catchment