Comment on egusphere-2022-921

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> The young water fraction (<em>F_yw</em>), defined as the fraction of catchment outflow with transit times of less than about 2&ndash;3 months, is increasingly used in hydrological studies, replacing the widely used Mean Transit Time (<em>MTT</em>), which is subject to aggregation error. The use of this new metric in catchment intercomparison studies is helpful to understand and conceptualize the relevant processes controlling catchment&rsquo;s hydrological function. Past work has shown the remarkable and counterintuitive evidence that steep (and generally high elevation) catchments worldwide reveal small <em>F_yw</em> values. However, the topographic slope only partially explains the observed <em>F_yw</em> variance, and the mechanisms hidden behind this lowering with slope remain basically unclear. The main aim of this paper is to investigate what drives <em>F_yw</em> variations with elevation in Alpine catchments clarifying why <em>F_yw</em> is low at high altitudes. In this regard, we use a dataset composed of 27 study catchments, located both in Switzerland and in Italy, that we categorize as rainfall-dominated, hybrid or snow-dominated according to a proposed formal classification scheme that considers both a common-used monthly streamflow ratio and the snow cover regime. We analyze three not previously investigated variables that could potentially explain the <em>F_yw</em> elevation gradients: the fractional snow cover area (<em>F_SCA</em>), the fraction of quaternary deposits (<em>F_qd</em>), and the fraction of baseflow (<em>F_bf</em>). We also consider a fourth variable, namely the Winter Flow Index (<em>WFI</em>), for comparing our results about the groundwater contribution to streamflow with those of previous scientific publications. Our results suggest that unconsolidated sediments could play a role in modulating <em>F_yw</em> elevation gradients via their capacity to store groundwater, but further geological information, such as the portion of fractured bedrocks, would be desirable for a complete picture of the role of geology. Based on our analysis concerning the <em>F_SCA</em>, we develop a perceptual model that explains how the increasing duration of the snowpack promotes a progressive emptying of the groundwater storage during winter, thereby increasing the streamwater age, while ephemeral snowpack generally favors rapid flow paths that increase <em>F_yw</em>. Finally, our work highlights that <em>F_bf</em>, considered as a proxy for groundwater flow, is roughly the one&rsquo;s complement of <em>F_yw</em>. In harmony with the model, we find high <em>F_bf</em> during all low-flow periods, which underlines that streamflow is mainly sustained by groundwater in such flow conditions. For catchments where the winter low-flow period is long compared to the summer high-flow period, this results in low <em>F_yw</em>. In conclusion, our data set suggests that <em>F_bf</em> is the best explanatory variable of <em>F_yw</em> elevation gradients in Alpine catchments, implying the key-role of major groundwater storages that, with the increasing snowpack duration, are actively involved in streamflow generation processes.