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

Abstract

The young water fraction F yw , defined as the proportion of catchment outflow younger than ca. 2-3 10 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 -352 km 2 ) Swiss catchments. We find that flow-weighting the tracer 15 concentrations in streamwater resulted in roughly 26 % 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. 20Within 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 F yw . 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 25 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 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. 30Hydrol. Earth Syst. Sci. Discuss., https://doi

The Demographics of Water: A Review of Water Ages in the Critical Zone

“…Accounting for spatial variations in precipitation inputs is especially important when comparing water sample isotopic compositions (e.g., from rivers, soil, plants, and aquifers) that are collected across areas with spatially variable precipitation regimes (Allen et al, ; Bowen, ; Bowen & Wilkinson, ). Comparing those values to location‐specific inputs allows for more accurately inferring how those precipitation patterns propagate through the system of interest (von Freyberg et al, ). However, mixing of spatial and temporal signals complicates interpreting tracer values in subsurface pools that integrate over large areas and durations (Kirchner, ).…”

Global Isotope Hydrogeology―Review

“…The top panel presents the δ 18 O cycle amplitude describing annual variability in catchment precipitation ( y axis) and stream water ( x axis) for global catchments (all n =260 catchments reported by Jasechko et al, , are shown, even the n =6 catchments with the implausible scenario: streamflow δ 18 O cycle amplitude > precipitation δ 18 O amplitude). Reported data are derived from previously published works that report amplitudes (e.g., data from Table 3 of von Freyberg et al, ; Table 4 of Song et al, ; Table S1 of Lutz et al, ; personal communications with S. Jacobs for data in Jacobs et al, ; and personal communication with D. Clow for data in Clow et al, ). The middle panel presents a histogram of young streamflow fractions reported for n =351 catchments.…”

“…Young streamflow comprises ~1/3 of global streamflow (Jasechko et al, ) but varies widely among rivers (Clow et al, ; Jacobs et al, ; Jasechko, Wassenaar, & Mayer, ; Lutz et al, ; Song et al, ; Stockinger et al, , ; von Freyberg et al, ). δ 18 O‐based young streamflow fractions have been reported for n =351 catchments ( n =254 catchments by Jasechko et al, ; n =30 by Jasechko, Wassenaar, & Mayer, ; n =24 by Lutz et al, ; n =22 by von Freyberg et al, ; n =5 by Song et al, ; n =11 by Clow et al, ; n =4 by Jacobs et al, ; n =1 by Stockinger et al, ). The average young streamflow among these n =351 catchments is 22% with a median of 18%, a lower‐upper quartile range of 10‐30%, and a 10th–90th percentile range of 4‐44%.…”

“…Other, partly overlapping approaches are available that use precipitation and streamflow isotopic compositions to evaluate streamflow ages (e.g., flow‐weighted young streamflow fractions—von Freyberg et al, —and storage selection functions—Harman, ). Each approach has advantages and disadvantages; there may be a place for each looking forward.…”

The Seasonal Origins of Streamwater in Switzerland

“…We use measurements from 12 Swiss catchments (Figure S1), composing all of the sites previously analyzed by Seeger and Weiler () and von Freyberg et al () that have at least 4 years of streamwater isotope measurements. Catchment areas ranged from 0.7 to 261 km 2 , with mean elevations from 584 to 2,369 m above sea level (Table S1).…”