Spatial distribution of stable water isotopes in alpine snow cover

Abstract

The aim of this study was to analyze and predict the mean stable water isotopic composition of the snow cover at specific geographic locations and altitudes. In addition, the dependence of the isotopic composition of the entire snow cover on altitude was analyzed. Snow in four Swiss catchments was sampled at the end of the accumulation period in April 2010 and a second time in Mai 2010 and analyzed for stable isotope composition of ²H and ¹⁸O. The sampling was conducted at both south-facing and north-facing slopes at elevation differences of 100 m for a total altitude difference of approximately 1000 m. The observed variability of isotopic composition of the snow cover was analyzed with stepwise multiple linear regression models. The analysis indicated that there is only a limited altitude effect on the isotopic composition when considering all samples. This is due to the high variability of the isotopic composition of the precipitation during the winter months and, in particular in the case of south-facing slopes, an enrichment of heavy isotopes due to intermittent melting processes. This enrichment effect could clearly be observed in the samples which were taken later in the year. A small altitudinal gradient of the isotopic composition could only be observed at some north-facing slopes. However, the dependence of snow depth and the day of the year were significant predictor variables in all models. This study indicates the necessity to further study the variability of water isotopes in the snow cover to increase prediction for isotopic composition of snowmelt and hence increase model performance of residence time models in alpine areas and to better understand the accumulation processes and the sources of water in the snow cover of high mountains

Characterizing seasonal groundwater storage in alpine catchments using time‐lapse gravimetry, water stable isotopes and water balance methods

“…In theory, the isotopic composition of snow is significantly more depleted than other waters (Beria et al, 2018). However, melt processes and snow transformation can lead to an enrichment of the snowpack (Dietermann & Weiler, 2013). It has been shown that an enrichment of heavy isotopes in the upper snow layers takes place due to diffusion of water vapour in the pores of the snowpack and also partial melting, which causes evaporation and percolation of meltwater in the remaining snow (Gat, 1996; Stichler et al, 2001) as well as kinetic fractionation occurring during sublimation (Biederman et al, 2014; Gustafson, Brooks, Molotch, & Veatch, 2010).…”

Understanding snow hydrological processes through the lens of stable water isotopes

“…Here, the average isotope composition of precipitation shifts by 1.9‰/100/m for δ 2 H and 0.27‰100/m for δ 18 O, noting that in winter, above around 800 m asl, the precipitation is dominated by snow (Marty, ). Some version of this isotopic lapse rate is seen in almost all mountainous environments except on the leeward or “rain‐shadow” side of mountains, which receive precipitation from clouds that have already passed over the highest elevation of the ridge and are no longer continuing to rise, keeping the cloud condensation temperature relatively stable (Bershaw, Penny, & Garzione, ; Dietermann & Weiler, ; Koeniger, Hubbart, Link, & Marshall, ; Moran, Marshall, Evans, & Sinclair, ; Wen, Tian, Weng, Liu, & Zhao, ; Winograd et al, ). Moran et al () reported positive isotopic lapse rates (enrichment in heavier isotopes with increasing elevation) in snow samples on the leeward side of a glacierized valley in the Canadian Rockies (refer to Figure in Moran et al ()), which may occur only if the warmer temperatures and hence smaller vapor–liquid or vapor–ice isotopic fractionation factors offset the “rain‐out” effect.…”

“…Snowmelt that leaves the snowpack preferentially discharges isotopically light water, thereby enriching the residual snowpack in heavier isotopes (Ala‐aho et al, ; Feng et al, ; Laudon, Hemond, Krouse, & Bishop, ; Shanley, Kendall, Albert, & Hardy, ; Soulsby, Malcolm, Helliwell, Ferrier, & Jenkins, ; Taylor et al, ; Taylor, Feng, Williams, & McNamara, ). It has been widely observed that early meltwater is more depleted in heavier isotopes and that, as the melt season progresses, both the residual snowpack and the generated meltwater become more enriched in heavier isotopes (Dietermann & Weiler, ; Taylor et al, ), which is also referred to as the melt‐out effect (Ala‐aho et al, ). To the best of our knowledge, the physical mechanisms of this melt‐out effect are not well understood but likely involve the partial melting of snowpack which results in preferential loss of lighter isotopes in the early season meltwater.…”

Spatial variability in the isotopic composition of water in small catchments and its effect on hydrograph separation

“…Additional factors are melt‐related fractionation during the accumulation and/or the melting season, which is particularly affected by aspect, microtopography, vegetation, slope and (blowing) snow redistribution, isotopic fractionation between ice crystals and water within a snowpack during melt (Taylor, Feng, Williams, & McNamara, ), as well as differences in the time since the development of the snowpack or onset of melt (Table ). Fractionation due to sublimation leads to a strong enrichment of the topmost layers of the snowpack (Sokratov & Golubev, ; Dietermann & Weiler, ).…”