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.
<p>High Alpine catchments are precious water-resources since they act as natural storage reservoirs, storing water in the snow cover and in the subsurface and thereby providing water during the dry seasons. Thus, a deeper knowledge of the hydrological functioning of these systems is necessary, in particular to make climate change projections. The role of seasonality is crucial in these catchments that generally exhibit a snow-dominated hydro-climatic regime.</p><p>Here we use high-frequency observations of stable isotopes of water to identify the seasonal origin of streamwater in a high-elevation Alpine catchment located in the Valle d&#8217;Aosta Region, Italy. We quantify the relative contribution of winter and summer precipitation reaching the stream through the Seasonal Origin Index (SOI<sub>Q</sub>), calculated using the &#948;<sup>18</sup>O values and the volumes of precipitation and streamflow. Highly negative SOI<sub>Q </sub>values are obtained suggesting that streamwater is mainly composed of winter precipitation. Conversely, the Seasonal Origin Index for evapotranspiration (SOI<sub>ET</sub>), which can be directly inferred from SOI<sub>Q</sub>, returns a positive value reflecting that plants preferentially take up water deriving from summer precipitation.</p><p>These findings allow us to develop a conceptual model of this Alpine system. This conceptual model suggests:</p><ul><li>a deep infiltration component, mainly composed by snowmelt water, reaching the stream through a preferential flow.</li> <li>a shallow infiltration component, predominantly represented by summer rainfall, that dominates the shallow soils and that is used by plants.</li> </ul><p>Therefore, we presume a seasonal compartmentalisation of water in this high-elevation catchment.</p><p>Nevertheless, a previous study in Switzerland revealed SOI<sub>Q </sub>&#8776; 0 for the Allenbach and Dischmabach snow-dominated catchments, indicating that similar fractions of summer and winter precipitation become streamflow. This different result achieved in systems with an apparently similar functioning highlights the need for a deep insight into the flow paths governing high-elevation catchments and it opens the way for new challenges to understand the hydrological processes hidden behind this difference.</p>
<p>The concept of young water fraction, introduced by Kirchner (2016) and defined as the fraction of streamflow that was stored less than about 2-3 months in the catchment, is increasingly used in catchment intercomparisons studies to understand and conceptualize the hydrological processes governing the catchment's functioning. However, the development of perceptual models is not always as straightforward as it may seem. Past works have shown that high mountainous catchments worldwide reveal small young water fractions. These low young water fractions at high elevations have been explained by different hydrological processes, including deeper vertical infiltration promoted by the presence of both fractured bedrock and freely draining soils (e.g., luvisols and cambisols) and long groundwater flow paths driven by the topographic roughness. But, a harmonious explanation of how the relevant mechanisms in mountainous catchments lead to low young water fractions at high elevations is missing.</p> <p>Using a data set composed of 27 study catchments, located both in Switzerland and in Italy (of which 22 are from the previous work of von Freyberg et al., 2018), we explore both the drivers and the conceptualization of the processes that potentially clarify this surprising result. We assume that this lowering can be explained by groundwater storage potential and the interplay of the seasonal dominance of hydrological processes. For groundwater storage potential we use the proportion of catchment area covered by Quaternary deposits (a parameter that is readily available for the studied catchments). For the interplay of seasonal processes, we use the length of the low-flow period as a measure for the duration of the groundwater (in terms of age, old water) dominated recession period.</p> <p>Our results suggest that the length of the low-flow period is clearly the main driver of low young water fractions at high elevation. Here, the long winter period, characterised by absence of liquid water input and hence by a low-flow regime, promotes a progressive emptying of the groundwater storage. Even during summer, recent snowmelt and rainfall that transit through the subsurface push out old groundwater into the stream, as reflected by high proportions of baseflow also during high-flow periods. However, during summer, the relative share of old water remains lower than during winter and accordingly, the longer the winter period (with very low young water fractions), the lower the annual young water fraction. Quaternary deposits could play a role in reducing young water fractions via their capacity to store groundwater, but further detailed geological information would be necessary for a complete picture about the role of geology.</p>
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.