Towards a conceptualization of the hydrological processes behind changes of young water fraction with elevation: a focus on mountainous alpine catchments

Abstract

Abstract. The young water fraction (Fyw*), defined as the fraction of catchment outflow with transit times of less than 2–3 months, is increasingly used in hydrological studies that exploit the potential of isotope tracers. The use of this new metric in catchment intercomparison studies is helpful to understand and conceptualize the relevant processes controlling catchment functioning. Previous studies have shown surprising evidence that mountainous catchments worldwide yield low Fyw*. These low values have been partially explained by isolated hydrological processes, including deep vertical infiltration and long groundwater flow paths. However, a thorough framework illustrating the relevant mechanisms leading to a low Fyw* in mountainous catchments is missing. The main aim of this paper is to give an overview of what drives Fyw* variations according to elevation, thus clarifying why it generally decreases at high elevation. For this purpose, we assembled a data set of 27 study catchments, located in both Switzerland and Italy, for which we calculateFyw*. We assume that this decrease can be explained by the groundwater storage potential, quantified by the areal extent of Quaternary deposits over a catchment (Fqd), and the low-flow duration (LFD) throughout the period of isotope sampling (PoS). In snow-dominated systems, LFD is strictly related to the snowpack persistence, quantified through the mean fractional snow cover area (FSCA). The drivers are related to the catchment storage contribution to the stream that we quantify by applying a cutting-edge baseflow separation method to the discharge time series of the study sites and by estimating the mean baseflow fraction (Fbf) over the PoS. Our results suggest that Quaternary deposits could play a role in modulating Fyw* elevation gradients via their capacity to store groundwater, but subsequent confirmation with further, more detailed geological information is necessary. LFD indicates the proportion of PoS in which the stream is sustained and dominated by stored water coming from the catchment storage. Accordingly, our results reveal that the increase of LFD at high elevations, to a large extent driven by the persistence of winter snowpacks and the simultaneous lack of a liquid water input to the catchments, results in lower Fyw*. In our data set, Fbf reveals a strong complementarity with Fyw*, suggesting that the latter could be estimated as Fyw*≃1-Fbf for catchments without stable water isotope measurements. As a conclusion, we develop a perceptual model that integrates all the results of our analysis into a framework for how hydrological processes control Fyw* according to elevation. This lays the foundations for an improvement of the theory-driven models.

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

“…The relative importance of rain versus snow had a clear effect on Fnew and Fyw, which were lower in hybrid catchments than in rainfall-dominated catchments, and lower still in snow-dominated catchments (Figure 3). Conversely, Gentile et al, (2023) found the highest Fyw in hybrid catchments, and similarly low Fyw in rainfall-and snow-dominated catchments. The reason for discrepancy might be that our dataset also covers much larger catchments (catchment areas in our dataset ranged from 29 km 2 to 103'946 km 2 , versus 0.14 km 2 to 351 km 2 in Gentile et al (2023).…”

“…Catchment area has previously been identified as a major control on catchment mean transit time (DeWalle et al, 1995;Soulsby et al, 2000), but has not been found to be significantly related to young water fractions (von Freyberg et al, 2018). water fractions in the Alpine region have primarily focused on small headwater catchments (Gentile et al, 2023;von Freyberg et al, 2018;Ceperley et al, 2020) have been widely used to assess how hydroclimatic and physiographic properties shape catchment transport, new water fractions have thus far remained under-exploited for this purpose. To date, no studies have systematically linked new water fractions to hydroclimatic drivers and physical catchment properties, using datasets that include both headwater catchments and larger downstream basins.…”

Technical Note: two-component Electrical Conductivity-based hydrograph separaTion employing an EXPonential mixing model (EXPECT) provides reliable high temporal resolution young water fraction estimates in three small Swiss catchments

“…Please, note that Eq. (3.2) was previously presented in Gentile et al (2023). The definition of the fraction of the streamflow younger than a threshold age (varying modestly from 2 to 3 months) at the generic time ti, 𝐹 𝑦𝑤 (𝑡 𝑖 ) , implicitly defines the existence of a complementary fraction of streamflow older than that threshold age at the same time ti, 𝐹 𝑜𝑤 (𝑡 𝑖 ).…”

“…( 8) to estimate 𝐹 𝑦𝑤 (𝑡 𝑖 ) is that we generally cannot accurately determine the endmembers ECyw and ECow from measurements as they correspond to the (rare) scenarios in which 𝐹 𝑦𝑤 (𝑡 𝑖 ) is either 0 or 1. The first scenario (𝐹 𝑦𝑤 (𝑡 𝑖 ) = 0) might occur only after prolonged periods without rainfall or snowmelt while the second scenario (𝐹 𝑦𝑤 (𝑡 𝑖 ) = 1) is unlikely to occur in most natural catchments where baseflow is usually older than 3 months (Gentile et al, 2023), and thus we cannot directly measure ECyw (Kirchner, 2016b).…”

“…Accordingly, 𝐹 𝑦𝑤 𝑜𝑝𝑡 (𝑡 𝑖 ) rapidly increases after an event (peak 𝐹 𝑦𝑤 𝑜𝑝𝑡 (𝑡 𝑖 ) is reached on average after 1.98±1.25 days), while it recedes slower during no-input days (the next minimum 𝐹 𝑦𝑤 𝑜𝑝𝑡 (𝑡 𝑖 ) is reached on average after 3.36±3.10 days). The largest daily young water fractions in the ERL catchment occurred during spring snow melt (March-May), suggesting that the melt water of the ephemeral snowpack is an important source of young water (since no relevant water aging is observed in such snowpack) that flows off quickly in the stream (Gentile et al, 2023). Rapid surface runoff of snow melt can occur due to soil freezing (temperatures < 0°C) or high soil moisture contents (temperatures > 0°) both of which can limit infiltration (Harrison et al, 2021;Keller et al, 2017;Fig.…”