The young water fraction <i>F</i><sub>yw</sub>, defined as the proportion of catchment outflow younger than ca. 2&ndash;3 months, can be estimated directly from the amplitudes of seasonal cycles of stable water isotopes in precipitation and streamflow. Thus, <i>F</i><sub>yw</sub> may be a useful metric in catchment inter-comparison studies that investigate landscape and hydro-climatic controls on streamflow generation. Here, we explore how <i>F</i><sub>yw</sub> varies with catchment characteristics and climatic forcing, using an extensive isotope data set from 22 small- to medium-sized (0.7&ndash;352&thinsp;km<sup>2</sup>) Swiss catchments. We find that flow-weighting the tracer concentrations in streamwater resulted in roughly 26&thinsp;% larger young water fractions compared to the corresponding un-weighted 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 un-weighted young water fractions are strongly correlated with each other among the catchments. They also correlate with terrain, soil and land use parameters, as well as with measures of hydrologic response. Within individual catchments, young water fractions increase with discharge, indicating an increase in the proportional contribution of faster flowpaths at higher flows. We present a new method to estimate the linear slope of this relationship, which we call the discharge sensitivity of <i>F</i><sub>yw</sub>. Among the 22 catchments, discharge sensitivities of <i>F</i><sub>yw</sub> are highly variable and only weakly correlated with <i>F</i><sub>yw</sub> itself, implying that these two measures reflect catchment behaviour differently. Based on strong correlations between the discharge sensitivity of <i>F</i><sub>yw</sub> and several catchment characteristics, we suggest that low discharge sensitivities imply greater persistence in the proportions of fast and slow runoff flowpaths as catchment wetness changes. High discharge sensitivities, on the other hand, imply the activation of different dominant flowpaths during precipitation events, such as when subsurface water tables rise into more permeable layers and/or the river network expands further into the landscape.
von Freyberg, J., Allen, S. T., Seeger, S., Weiler, M., & Kirchner, J. W. (2017). Sensitivity of young water fractions to hydro-climatic forcing and landscape properties across 22 Swiss catchments. Hydrology and Earth System Sciences Discussions, 1–34. https://doi.org/10.5194/hess-2017-720