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).