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:
- To improve the knowledge on the time in which snowmelt is converted
into runoff.
- To determine the possible influence of the cumulative winter snowpack
on the observed hydrological behavior during spring and early summer.
- To assess the extent to which the annual hydrologic balance and
hydrograph might change in a likely future with less snow.