1. INTRODUCTION
Snowmelt plays a critical role on streamflow generation in cold-regions mountain headwaters (Barnhart et al., 2016; Li et al., 2017) and provides large amounts of water for ecosystems and human uses in their surrounding lowlands (Viviroli et al. , 2020). During the last years, intense research has been conducted in order to improve observational and modelling capabilities, and to better understand the physical mechanisms that connects the snow dynamics and the streamflow generation (Gordon et al., 2022). One of the most challenging aspects of this research topic is to determine the timing and routing from the snowmelt onset into the river flow (Ceperley et al., 2020). Routing may involve processes such as water percolation through the snowpack, the portion of snowmelt that quickly reach the streams as surface runoff, and water that infiltrates to aquifers or circulates as subsurface flow (Carroll et al., 2019). The difficulty to analyze such routing dynamics relies partly on the complexity of maintaining hydrological and hydrometrics measurements in snow dominated areas (Ala-aho et al., 2017).
Depending on the dominant hydrological processes, the transit time of melted snow to reach the stream at each catchment will vary and consequently will strongly determine its vulnerability to drought periods and climate change scenarios (Jeelani et al., 2017; Taylor et al., 2013). Some studies suggest that snowmelt dominated catchments show higher runoff coefficients than ephemeral snowpack and rain dominated catchments (Barnhart et al., 2016; Berghuijs et al., 2014; Li et al., 2017; Lone et al., 2023). However, other studies have not found any strong relationship between changes in the snowpack duration and magnitude of the annual runoff (López-Moreno et al., 2020). The transit time of snowmelt water in a catchment determines to which extent the accumulated snowpack during the precedent winter(s) and spring season(s) will affect the streamflow during summer time. Some studies have identified a clear role of the antecedent snowpack to explain anomalies in summer streamflow (Carroll et al., 2019; Godsey et al., 2014; Rebetez & Reinhard, 2008). For example, summer low flows in Czechia are driven by seasonal precipitation and evapotranspiration but also by previous winter snowpack dynamics (Jenicek and Ledvinka, 2020). On the opposite, the analysis of 380 Swiss catchments revealed that snow water equivalent and winter precipitation plays a minor role in the magnitude and timing of the warm season low flows (Floriancic et al., 2020).
The comparison between streamflow diel cycles and snow depletion time series also provides useful information about the snowmelt contribution to the total streamflow and their transit time (Holko et al., 2021; Jin et al., 2012; Kirchner et al., 2020; López-Moreno et al., 2023; Miller et al., 2020). During the melting season, rain provides a large streamflow contribution, and the meltwater contribution is often difficult to infer. Stable water isotopes (generally δ2H and δ18O) have resulted extremely useful to better understand the contribution of snowmelt to streamflow and the residence time of melting water in the catchments (Leuthold et al., 2021; McGill et al., 2021; Penna et al., 2017), thanks to the more depleted values of snow isotopy compared to streamflow (McGill et al., 2021; Vystavna et al., 2021). However, a full separation of the contribution of each component is difficult to obtain, since it requires a very intense spatially and temporally isotopic sampling of each component. Further, at the catchment scale there is still a high spatial, as well as temporal (inter- and intra-annual) variability of the isotopic signal of the snowpack, precipitation (liquid and solid) and streamflow water (Wenninger et al., 2011). For this reason, the available literature often uses the water isotopy evolution to perform qualitative rather than quantitative analyses, in combination with other source of data such as water characteristics (i.e., water temperature or electrical conductivity, geochemistry) and piezometric levels (Woelber et al., 2018).
In line with this, we analyzed the streamflow response of a snow dominated basin in the central Spanish Pyrenees, in combination with water table data, streamflow and precipitation isotopy, and additional information of water temperature and electrical conductivity. The general objective was to better understand the hydrological dynamics induced by snowmelt in this experimental catchment (Izas catchment), which is representative of large subalpine sectors in the Pyrenees. The results of this study are important to better predict the future hydrological response of similar catchments in the Pyrenees when snow duration and accumulation will decrease as a consequence of temperature scenarios for the next decades (López-Moreno et al., 2013, 2017). The specific objectives of this work were:
  1. To improve the knowledge on the time in which snowmelt is converted into runoff.
  2. To determine the possible influence of the cumulative winter snowpack on the observed hydrological behavior during spring and early summer.
  3. To assess the extent to which the annual hydrologic balance and hydrograph might change in a likely future with less snow.