5. DISCUSSION
This work combines several data sources, including snow measurements,
meteorological information as well as hydrological records in order to
better understand the hydrological response of the Izas catchment during
the snow melting period. Results show that meltwater is the driver of
the main soil water fluctuations and streamflow during the melting
period, which is in accordance with results found in other cold mountain
sectors (Barnhart et al., 2016; M. Feng et al., 2022; Schreiner-McGraw
& Ajami, 2022). Liquid precipitation enhances the streamflow peaks
controlled by melt, and keep the peak flows high once snow cover is
almost depleted over the catchment (Gordon et al., 2022). This suggests
that monitoring winter and early spring snow conditions is useful to
better anticipate the spring water availability. Also, results provide
evidence that liquid precipitation in spring can highly counterbalance
snow poor years, since streamflow levels during rainy springs are
comparable to snow abundant years. The importance of liquid
precipitation for the spring hydrological response of snow-dominated
catchments was also highlighted for alpine sites in the Dolomites (Penna
et al., 2016).
The measurement of piezometric levels during spring in 2019 and 2020
revealed that the storage of infiltrated water from snowmelt in the
catchment is very variable among different points of the basin.
Meltwater infiltration is probably controlled by the soil types and the
terrain slope (Woelber et al., 2018). In most cases, the water levels
fluctuations are very fast. Water level increases when melting starts,
even if the entire catchment is still snow covered. Saturation
conditions (when water table reaches the surface) only happens during
short periods. Saturation is often associated to the snow depletion
period at each specific point. Afterwards, water levels decline
considerably, and saturation conditions are not reached even in periods
of heavy rain. Therefore, results suggest that under rainy conditions,
the overland flow controls the hydrological response in the catchment.
The snow depletion triggers an increase in the water temperature,
electrical conductivity and δ18O values. Further, the
piezometric levels and the streamflow show very low values two weeks
after the snow depletion, independently of the snowpack magnitude and
the duration of the snow season. Our results suggest that there is no
clear relationship between winter snowpack and summer runoff flows,
which is probably explained by the very fast hydrological response of
the catchment to meteorological fluctuations. Similarly, the analysis of
380 Swiss catchments revealed that summer streamflow is controlled by
the seasonal rainfall and evapotranspiration interannual variability
(Floriancic et al. , 2020). The lack of relation between
antecedent snowpack and summer streamflow contrast to other mountain
snow-dominated sectors, where snowmelt drives the streamflow anomalies
several months after the snow depletion (Godsey et al., 2014; Staudinger
et al., 2017). Thus, streamflow reacts immediately to the onset of melt
events, but also declines quickly when new snowfalls or cold periods
occur. After these interruptions, streamflow rises quickly when
conditions that favor melting returns, or rain events occur (Figure 2).
The fast hydrological response of the catchment during the melting
period is also suggested by the rather sudden change in water
temperature, electrical conductivity and streamflow water isotopy after
the snow cover depletes over the catchment. Such fast hydrological and
water properties response to dominant climatic conditions is generally
characteristic of many small alpine catchments with relatively shallow
soils (Ceperley et al., 2020; Segura, 2021). Such behavior contrasts
with other alpine and subalpine catchments, where thick soils or
sedimentary deposits favor the existence of alpine aquifers (Cochand et
al., 2019; Hayashi, 2020) and intense subsurface flow (Ceperley et al.,
2020; Jin et al., 2012; Tague & Grant, 2009) that favor longer transit
times and a slow hydrological response independently regardless the
short term climatic fluctuations.
Streamwater isotopy showed marked diel cycles of water isotopy during
the melting periods, with almost systematic low values during daily
maximum snow melting rates, and high values when baseflow controls the
runoff generation. This is a clear indication of the low transit time of
water in the Izas catchment during melting period. However, groundwater
storage and interflow processes are not discarded in the catchment.
Direct in-situ observations suggest that that most of the tributary
ravines to the main stream are completely dry during the driest period
of the summer, but there is always some runoff at the gauge station
thanks to lateral flow and few small perennial springs in deep soils in
convergent by topographic zones.
This work confirms the relevance of the isotopy monitoring for the
catchment streamflow evolution. The δ18O magnitude and
spatial variability across the catchment increases while the snow
disappears, which is consistent with previous results (Dietermann &
Weiler, 2013; X. Feng et al., 2002; Holko et al., 2013). The mean
isotopic values of the recorded series show strong interannual
differences, exhibiting higher (lower) values during the snow poor
(rich) seasons. However, the application of hydrograph separation based
on water isotopes is complicated by the lack of detailed control of
isotopic variations in individual precipitation events, water stored in
soils and groundwater, and distributed samples of snowpack isotopy
(Kamensky, 1998; Lee et al., 2010; Leuthold et al., 2021; Schmieder et
al., 2016). Such monitoring should be considered in further research.
Overall results point out that snow plays a strong control in the
hydrology during the melt period the expected future with reduced and
shorter snowpack and a major influence of rainfall (López-Moreno et al.,
2013, 2017) may lead to shifts in the occurrence of the maximum peak
flow, and an earlier rise of stream water temperature that may impact
with river ecology (Kamarianakis et al., 2016). However, the fast
hydrological response of the catchment, the limited water storage
capacity of the ground, and the importance of spring rainfall suggest
that the main characteristics of the annual water balance and its
hydrograph would not change in a drastic way. These results must be
considered as local and explained by the main lithological, edaphic and
climatological characteristics of the studied catchment. Mountain
regions where most of the precipitation only fall during the coldest
months of the year and where melt plays a major role in groundwater
recharge will show a major dependence with the amount of timing of snow
dynamics (Fayad et al., 2017).