Stable Water Isotopes in Fresh Snow Along 2 Slopes in the canton of Grisons/Graubünden, Switzerland: the "Chruez", Praettigau and Eastward ascent to the Vilan Peak

This bachelor thesis concerned itself with the composition of \(\delta\)2H (or deuterium), \(\delta\)18O and \(\delta\)17O in fresh snow. Stable water isotopes have been proven to be useful tracers for hydrological processes, such as air mass sources of regional precipitation or the residence time of snowmelt in a catchment (Beria et al., 2018). Whereas different isotopic compositions of precipitation in the form of rain is well documented for different environments in networks such as the NISOT (The Swiss National Network for the Observation of Isotopes in the Water Cycle), the same knowledge about snow is more limited due to the influences of different isotope fractionation processes and lack of onsite snow accumulation and melt observation (Cooper, 1998; Michelon et al., 2018). The aim of this thesis was therefore to analyse and characterize the spatial variability of \(\delta\)2H, \(\delta\)18O and \(\delta\)17O in freshly fallen snow. This was done by sampling fresh snow on six different snowfall events in the month of March over an elevation gradient of 1000m along the southside slope of the mountain Chruez. 2H throughout four sampling days had \(\delta\)-values ranging from −50‰ to −70‰, with two days being more depleted in 2H, showing \(\delta\)-values from −80‰ down to −160‰. \(\delta\)18O showed the same pattern where of six, four days showed similar -values ranging from −8‰ to −12‰ and two days with a stronger depletion of the isotope with \(\delta\)-values ranging from −14‰ to −20‰. It is known that the enrichment of \(\delta\)17O in a water molecule is about half of \(\delta\)18O, which was also seen in the results of this thesis (Nyamgerel et.al, 2021).

In this thesis, two out of six sampling days were examined with statistically significant positive altitude gradients of 0.19‰/100m and 0.64‰/100m for \(\delta\)18O. Additionally, on one of the six sampling days, a statistically significant negative altitude gradient of −0.15‰/100m was observed. The altitude gradients for \(\delta\)17O were about half of \(\delta\)18O. Two statistically significant positive and one negative altitude gradients for \(\delta\)2H were observed during the same days with lapse rates of 1.7‰/100m, 5.5‰/100m and −1.25‰/100m. Having both positive and negative altitude gradients was attributed to the varying amounts of solar radiation and wind direction during different sampling days. However, further research must be done in order to make a valid statement, since additional factors such as water vapor pressure, turbulent fluxes in the air and relative humidity strongly affect fractionation of stable water isotopes.

The aim of this work is to characterize and analyze the elevation effect of water isotopologues in fresh snow at Vilan. By analyzing fresh snow, the influence of fractionation processes, after the precipitation event can be minimized and helps the understanding of snow-melt water transformation. For this purpose, a total of five fresh snow events in March and April 2021 were analyzed. Samples were taken from the eastern slope of Vilan between 1100 m a.s.l. and 2300 m a.s.l. and were collected at 100-meter elevation intervals. The altitude effect was determined with a linear regression. The analysis shows that the altitude has only a small effect on the d-values of the new snow, because for the \(\delta\)18O only one snow event shows a statistically significant altitude effect. For the \(\delta\)2H, it is their two. The statistically significant new snow events show an elevation gradient of + 0.155 ‰ \(\delta\)18O/100 m and + 1.211 ‰ \(\delta\)2H/100 m and + 0.677 ‰ \(\delta\)2H/100 m, respectively. The observed positive gradients can be explained by four effects. First, a lee effect is likely due to the prevailing weather conditions. Second, the top snow layer accumulates heavy isotopologues with increasing time (time effect). Third, an inverse height effect can be observed when water vapor of different origins mixes. Fourth, the amount of precipitation (quantity effect) can have an impact on isotopic composition. The work shows that further research in the analysis of water isotopes in fresh snow is necessary to better identify and quantify possible influencing parameters.