New water fractions and their relationships to climate and catchment properties across Alpine rivers

Abstract

Abstract. The Alps are a key water resource for central Europe, providing water for drinking, agriculture, and hydropower production. Thus, understanding runoff generation processes of Alpine streams is important for sustainable water management. It is currently unclear how much streamflow is derived from old water stored in the subsurface, versus more recent precipitation that reaches the stream via near-surface quick flow processes. It is also unclear how this partitioning varies across different Alpine catchments in response to hydroclimatic forcing and catchment characteristics. Here, we use stable water isotope time series in precipitation and streamflow to quantify the young water fractions Fyw (i.e., the fraction of water younger than approximately 2–3 months) and new water fractions Fnew (here, the fraction of water younger than one month) in streamflow from 32 Alpine catchments. We contrast these measures of water age between summer and winter and between wet and dry periods, and correlate them with hydroclimatic variables and physical catchment properties. New water fractions varied from 9.6 % in rainfall-dominated catchments to 3.5 % in snow-dominated catchments (mean across all catchments = 7.1 %). Young water fractions were approximately twice as large (reflecting their longer time scale), varying from 17.6 % in rainfall-dominated catchments to 10.1 % in snow-dominated catchments (mean across all catchments = 14.3 %). New water fractions were negatively correlated with catchment size (Spearman rank correlation rS = 0.38), q95 baseflow (rS = -0.36), catchment elevation (rS = 0.37), total catchment relief (rS = -0.59), and the fraction of slopes steeper 40° (rS = -0.48). Large new water fractions, implying faster transmission of precipitation to streamflow, are more prevalent in small catchments, at low elevations, with small elevation gradients, and with large forest cover (rS = 0.36). New water fractions averaged 3.3 % following dry antecedent conditions, compared to 9.3 % after wet antecedent conditions. Our results quantify how hydroclimatic and physical drivers shape the partitioning of old and new waters across the Alps, thus indicating which landscapes transmit recent precipitation more readily to streamflow, and which landscapes tend to retain water over longer periods. Our results further illustrate how new water fractions may find relationships that remained invisible with young water fractions.

Young and new water fractions in soil and hillslope waters

“…Thus, most of the streamflow at our site originated from water that is stored in the subsurface for longer than 2-3 months. This is not only true for our site but also in line with findings from across many other rivers across the globe (Jasechko et al, 2016;von Freyberg et al, 2018;Floriancic et al, 2023b). However, streamflow young water fractions by themselves cannot indicate where in the surface-to-stream transit pathway this damping of seasonal cycles occurs.…”

“…4). Streams with similarly small fractions of young and new waters have been described elsewhere in the Alps (Floriancic et al, 2023b), in an Andean floodplain (Burt et al, 2023), at Plynlimon in Wales (Knapp et al, 2019); such values are also common globally (Jasechko et al, 2016). The effect of antecedent wetness on shorter transit times that we observed has also been quantified and observed in some of those studies (Floriancic et al, 2023;Knapp et al, 2019).…”

“…separation (Kirchner 2019;Knapp et al, 2019;Kirchner and Knapp 2020) of tracer timeseries. Both methods have been widely applied to streamflow timeseries (e.g., Ceperley et al, 2020;Jasechko et al, 2016;Gentile et al, 2023;von Freyberg et al, 2017;von Freyberg et al, 2018;Knapp et al, 2019;Floriancic et al, 2023b) but few studies have applied them on timeseries of soil waters (Gallart et al, 2020;Burt et al, 2023). Both of these stable isotope tools can reveal how precipitation mixes with older storages as it travels down the soil profile and toward streams.…”

Lessons learned from the spatiotemporal analysis of long‐term and time‐variable young water fractions of large central European river basins

“…In their conceptual model, catchments with low Fyw and low discharge sensitivity (low CV in the present study; also see Figure 10, arrow number 3 of the von Freyberg et al [2018] study) were explained by a dominance of slow subsurface flow paths (old water) in catchments with high infiltration capacity and large subsurface storage. Further, Floriancic et al (2023) suggested that the large storage for 14 out of 32 Alpine catchments is damping tracer fluctuations in streamflow under high monthly precipitation rates, thus not strongly impacting new water contributions to runoff. The high infiltration capacity also fits the explanation of the negative relationship between altitude and Fyw due to deep vertical infiltration (Jasechko et al, 2016), because the basins with low CV and low Fyw in the present study were high elevation ones.…”