3.2 Hydrological data and water sampling for isotopic analyses
Water level was measured every 5 minutes at the gauging station (V-shape weir), using a CT2X Seametrics probe (Seametrics, USA). Water level data were corrected from barometric pressure fluctuations using the software Aqua4plus 2.2 and converted into runoff (l s-1). The water level information for 2018 was missing due to a sensor malfunctioning. Water temperature and temperature-corrected electrical conductivity were also measured with the same probe. Electrical conductivity data were only available in 2019 and 2020 years when a proper calibration provided reliable values.
Nine piezometers were drilled at different locations within the catchment (see Figure 1) to monitor the fluctuations of the water table levels with LevelScout sensors (https://www.seametrics.com/product/levelscout/) that were also corrected from barometric pressure fluctuations. In this study, we focused on water table data from 1st April to 1st July to assess water table dynamics from before the onset of the main melting period until some weeks after snow has completely melted in the catchment.
An ISCO 3700 automatic water sampler (https://www.teledyneisco.com/en-us/water-and-wastewater/3700-sampler) was used to sample streamwater at the gauging station twice daily in 2017, 2018 and 2019. The sampler malfunctioned in 2020 and also caused data gaps in 2019. The sampler was programmed at 6 AM and at 6 PM in order to capture the diel cycle from inexistent or very low snowmelt conditions (6 AM), and to very high input from snowmelt (6PM). After early July, when melting did not drive daily runoff cycles, the sampler only collected one sample per day (12 AM). Water sampler had storage capacity of 24 bottles, making necessary to collect water samples every 12 days. During the days we collected ISCO samples, 11 water samples from different streams and springs across the basin were also collected following a fixed itinerary that started at 12 AM, aiming to reduce the impact of daily cycles in water isotopy. At the same time, we also collected bulk precipitation fallen between two sampling days using a water collector designed to prevent evaporation (Gröning et al., 2012). For all samples, 15 ml of water were conserved in narrow neck propylene tubes and stored in an isothermal bag with cold ice packs to avoid evaporation during the 4 hours of transport to the laboratory facilities. In the laboratory, the water tubes were kept in a fridge at +6ºC. A Picarro L2130-i isotope analyzer was used to measure δ18O isotopes in streamwater and precipitation samples. The isotopic values were determined from the eight replicates of the same sample to minimize sample carryover effects (Penna et al., 2012). A total of 553 samples of streamflow and 19 for precipitation were analyzed for this study.