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

Abstract

The time precipitation needs to travel through a catchment to its outlet is an important descriptor of a catchment's susceptibility to pollutant contamination, nutrient loss, and hydrological functioning. The fast component of total water flow can be estimated by the fraction of young water (Fyw), which is the percentage of streamflow younger than 3 months. Fyw is calculated by comparing the amplitudes of sine waves fitted to seasonal precipitation and streamflow tracer signals. This is usually done for the complete tracer time series available, neglecting annual differences in the amplitudes of longer time series. Considering inter-annual amplitude differences, we employed a moving time window of 1 year in weekly time steps over a 4.5-year δ 18 O tracer time series to calculate 189 Fyw estimates and their uncertainty. They were then tested against the following null hypotheses: (1) at least 90 % of Fyw results do not deviate more than ±0.04 (4 %) from the mean of all Fyw results, indicating long-term invariance. Larger deviations would 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 high sensitivity of Fyw to a 1-week to 4-week shift in the start of a 1-year sampling campaign;(3) the Fyw results of 1-year sampling campaigns started in a given calendar month do not change more than ±0.04, indicating seasonal invariance. In our study, all three null hypotheses were rejected. Thus, the Fyw results were timevariable, showed variability in the chosen sampling time, and had no pronounced seasonality. We furthermore found evidence that the 2015 European heat wave and including two winters into a 1-year sampling campaign increased the uncer-tainty of Fyw. Based on an increase in Fyw uncertainty when the mean adjusted R 2 was below 0.2, we recommend further investigations into the dependence of Fyw and its uncertainty to goodness-of-fit measures. Furthermore, while investigated individual meteorological factors did not sufficiently explain variations of Fyw, the runoff coefficient showed a moderate negative correlation of r = −0.50 with Fyw. The results of this study suggest that care must be taken when comparing Fyw of catchments that were based on different calculation periods and that the influence of extreme events and snow must be considered.

Effects of Deforestation on Water Flow in the Vadose Zone

“…This, in turn, can affect the transport of Dissolved Organic Carbon (DOC) through the vadose zone (e.g., [58]). Clearly, the transport of DOC is closely linked to its transit time as it reflects the potential of soils to buffer DOC [59,60]. The results of this study suggested that DOC export at the catchment scale will also be influenced by the direct effect of deforestation on preferential flow processes.…”

Tandem use of transit time distribution and fraction of young water reveals the dynamic flow paths supporting streamflow at a mountain headwater catchment

“…Both KGE' and the characteristics of the criterion response surface were utilized to search for the optimum model parameters (Dwivedi et al, 2021). The following ranges of model parameters were considered: Mean transit time: 1 to 50 years when using tritium, which serves as a groundwater age tracer at a time scale of 1 to 50 years (Suckow, 2014;Aggarwal, 2013;Gleeson et al, 2015). Shape parameter (α) for the Gamma TTD: 0.1 to 15 (Stewart et al (2017).…”

“…With respect to δ 18 O, method-and period-based * estimates were coupled to previously established TTD parameters (Dwivedi et al, 2021), and multi-year average values of * were used to compare between different methods (Stockinger et al (2019); Gallart et al (2020a). The * was estimated as a function of period using * estimates obtained from application of each of the three methods.…”

“…Stable water isotopes are generally considered applicable to determine groundwater ages up to 5 years (DeWalle et al, 1997;Dwivedi et al, 2021) and are therefore appropriate for subsurface storages with faster flow (Stewart et al, 2010). In contrast, 3 H is generally considered applicable up to a period of 50 years (Suckow, 2014;Aggarwal, 2013) and is thus appropriate for estimating "hidden" or deep groundwater contributions to streamflow (Stewart et al, 2012). The current study that uses both tracers can separate the contributions of both quick and slow groundwater flow to streamflow in a headwater mountain catchment.…”

Long‐term stable water isotope and runoff data for the investigation of deforestation effects on the hydrological system of the Wüstebach catchment, Germany

“…The separation of precipitation into drainage, groundwater recharge and evapotranspiration on its way through a catchment and along the various water flow paths can be strongly influenced by deforestation-induced changes. One method of studying these changes is to use the stable isotopes of water (δ 18 O and δ 2 H) as tracers of water flow paths (Stockinger et al, 2019). In previous decades, several studies analysed the response of water balance components to deforestation, but studies using longterm stable isotope data to analyse the effects on the travel time of runoff components, for example, subsurface water flow, are still lacking.…”