What drives Fyw variations with elevation in Alpine catchments?

Abstract. The young water fraction (Fyw), defined as the fraction of catchment outflow with transit times of less than about 2–3 months, is increasingly used in hydrological studies, replacing the widely used Mean Transit Time (MTT), 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’s hydrological function. Past work has shown the remarkable and counterintuitive evidence that steep (and generally high elevation) catchments worldwide reveal small Fyw values. However, the topographic slope only partially explains the observed Fyw variance, and the mechanisms hidden behind this lowering with slope remain basically unclear. The main aim of this paper is to investigate what drives Fyw variations with elevation in Alpine catchments clarifying why Fyw 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 Fyw elevation gradients: the fractional snow cover area (FSCA), the fraction of quaternary deposits (Fqd), and the fraction of baseflow (Fbf). We also consider a fourth variable, namely the Winter Flow Index (WFI), 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 Fyw 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 FSCA, 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 Fyw. Finally, our work highlights that Fbf, considered as a proxy for groundwater flow, is roughly the one’s complement of Fyw. In harmony with the model, we find high Fbf 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 Fyw. In conclusion, our data set suggests that Fbf is the best explanatory variable of Fyw 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.