Sensitivity of young water fractions to hydro-climatic forcing and landscape properties across 22 Swiss catchments

Abstract

Abstract. The young water fraction F yw , defined as the proportion of catchment outflow younger than approximately 2-3 months, can be estimated directly from the amplitudes of seasonal cycles of stable water isotopes in precipitation and streamflow. Thus, F yw may be a useful metric in catchment inter-comparison studies that investigate landscape and hydro-climatic controls on streamflow generation. Here, we explore how F yw varies with catchment characteristics and climatic forcing, using an extensive isotope data set from 22 small-to medium-sized (0.7-351 km 2 ) Swiss catchments. We find that flow-weighting the tracer concentrations in streamwater resulted in roughly 26 % larger young water fractions compared to the corresponding unweighted values, reflecting the fact that young water fractions tend to be larger when catchments are wet and discharge is correspondingly higher. However, flow-weighted and unweighted young water fractions are strongly correlated with each other among the catchments. They also correlate with terrain, soil, and land-use indices, as well as with mean precipitation and measures of hydrologic response. Within individual catchments, young water fractions increase with discharge, indicating an increase in the proportional contribution of faster flow paths at higher flows. We present a new method to quantify the discharge sensitivity of F yw , which we estimate as the linear slope of the relationship between the young water fraction and flow. Among the 22 catchments, discharge sensitivities of F yw are highly variable and only weakly correlated with F yw itself, implying that these two measures reflect catchment behaviour differently. Based on strong correlations between the discharge sensitivity of F yw and several catchment characteristics, we suggest that low discharge sensitivities imply greater persistence in the proportions of fast and slow runoff flow paths as catchment wetness changes. In contrast, high discharge sensitivities imply the activation of different dominant flow paths during precipitation events, such as when subsurface water tables rise into more permeable layers and/or the river network expands further into the landscape.

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

“…They found that an increase in annual rainfall amounts of 1 mm d −1 led to an increase of 0.03-0.04 in the modeled Fyw (percentage point increase of 3 %-4 %, hereafter written as 0.03-0.04, where the value 1 means that 100 % of streamflow is younger than 3 months). Similar to this, von Freyberg et al (2018) found a positive correlation between Fyw and high-intensity precipitation events. This dependence of Fyw on precipitation characteristics could lead to long-term changes in Fyw due to global warming.…”

“…Streamflow was substantially comprised of groundwater, as its volume-weighed δ 18 O was −8.40 ‰, the precipitation isotope signal was strongly attenuated in streamflow, and Fyw(189) values were generally low, which indicates a strong groundwater influence. The study by Weigand et al (2017) came to the same conclusion for the Wüstebach catchment using wavelet analysis of nitrate and DOC data collected at mainstream and tributary locations. While lower-altitude locations of the catchment near the outlet were dominated by groundwater, higher-altitude areas were less affected.…”

“…The present study aims at answering one aspect of this open research question by focusing on the time variance in Fyw and its associated uncertainty. Past studies fitted one sine wave to the complete time series available, varying from less than a year to several decades (Ogrinc et al, 2008;Song et al, 2017;von Freyberg et al, 2018). To our knowledge, only the study of Stockinger et al (2017) calculated Fyw for two different 1-year periods of a multi-year time series but did not test the temporal variability in Fyw nor influencing factors on it or its uncertainty.…”

“…Their confidence intervals indicated a robustness of Fyw against random errors in input data. The study of von Freyberg et al (2018) focused on three influences on Fyw: (a) spatially interpolating precipitation isotopes, (b) including snowpack, and (c) weighing streamflow in fitting sine waves. They found that weighing streamflow led to significant changes in Fyw, while the other factors had a negligible effect.…”

The Seasonal Origins of Streamwater in Switzerland

“…Agriculture, grassland, and forests are found at elevations up to approximately 2,000 m, above which the ground cover is mostly meadow (and some rock, snow, or ice). The effects of these catchment characteristics on streamflow transit times and runoff processes have been described in previous studies (Seeger & Weiler, 2014;von Freyberg et al, 2018).…”