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

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.