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Understanding snow hydrological processes through the lens of stable water isotopes

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Snowfall may have different stable isotopic compositions compared with rainfall, allowing its contribution to potentially be tracked through the hydrological cycle. This review summarizes the state of knowledge of how different hydrometeorological processes affect the isotopic composition of snow in its different forms (snowfall, snowpack, and snowmelt), and, through selected examples, discusses how stable water isotopes can provide a better understanding of snow hydrological processes. A detailed account is given of how the variability in isotopic composition of snow changes from precipitation to final melting. The effect of different snow ablation processes (sublimation, melting, and redistribution by wind or avalanches) on the isotope ratios of the underlying snowpack are also examined. Insights into the role of canopy in snow interception processes, and how the isotopic composition in canopy underlying snowpacks can elucidate the exchanges therein are discussed, as well as case studies demonstrating the usefulness of stable water isotopes to estimate seasonality in the groundwater recharge. Rain‐on‐snow floods illustrate how isotopes can be useful to estimate the role of preferential flow during heavy spring rains. All these examples point to the complexity of snow hydrologic processes and demonstrate that an isotopic approach is useful to quantify snow contributions throughout the water cycle, especially in high‐elevation and high‐latitude catchments, where such processes are most pronounced. This synthesis concludes by tracing a snow particle along its entire hydrologic life cycle, highlights the major practical challenges remaining in snow hydrology and discusses future research directions. This article is categorized under: Science of Water > Hydrological Processes Science of Water > Methods
Amount of annual snowfall and ratio of snowfall to annual precipitation in world's mountain ranges; monthly snowfall computed with the equation proposed by Legates and Willmott () from monthly precipitation and monthly temperature of the WorldClim data base (Hijmans, Cameron, Parra, Jones, & Jarvis, ); mountain ranges extracted with the mountain shape files provided by Körner et al. (), shown are the latitude of mid points and peak elevation; peak elevation obtained from the elevation dataset (called “alt” file) from the WorldClim data set. Dark gray dots correspond to mountain ranges without snowfall, light gray shading indicates all pixels in the WorldClim dataset. Colored dots on the left and colored asterisks on the right figure represent mountains with snow, with the marker size proportional to annual snowfall in the left figure and proportional to ratio of annual snowfall to annual precipitation in the right figure. The readers are referred to the electronic version for an enlarged version of the figure
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Tracking the evolution of snow isotopes from inputs of snowfall (a) to snowpack (b), and throughout the melting process (d–f). A representative temporal evolution of snowpack (represented as snow water equivalent) is provided in (c), highlighting the ranges over which early, major, and late melting phases and isotope changes are likely to occur. Plots b,d,e,f synthesize the isotopic evolution of snowpack and snowmelt from a “control volume” perspective, while (a) is from the perspective of a “pulse” of snowfall that could fall directly to the ground or be intercepted by the canopy and undergo isotopic modification due to sublimation (along the local evaporation line) and subsequent transport to the ground as snow throughfall (TF). The evolution of hypothetical samples (circles) and their ranges in dual isotope space (colored boxes) are shown in (a), with subsequent plots only showing the range as colored boxes. In subplot (e), σ represents the standard deviation of the meltwater or the snowpack sample
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Dominant drivers of spatial isotopic heterogeneity according to the different snowpack types as defined by Sturm et al. (). Highlighted here is the relative importance of vapor exchange, snowmelt, wind redistribution, and snow sublimation in enhancing the isotopic heterogeneity of the final snowmelt beyond what was initially introduced by snowfall
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Life cycle of snow seen through the eyes of a hydrologist highlighting fluxes that lead to an enrichment or depletion in stable water isotopes of the snowpack (on the ground or intercepted by canopy); fluxes for which there is no systematic effect or no significant effect are also identified. (Graphic based on original work from www.freepik.com)
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Seasonal variation of δ2H and δ18O in precipitation samples collected by the GNIP (Global Network of Isotopes in Precipitation) network of gauging stations in Switzerland (data from 1966 to 2014). The red line in the middle of each box shows median value, the box corresponds to the difference between third and first quartile values, whisker length is 1.5 times of the interquartile range and the points beyond the whiskers represent outliers
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Variation of δ2H and δ18O in precipitation samples as a function of elevation, as collected by the GNIP (Global Network of Isotopes in Precipitation) network of gauging stations in Switzerland (data from 1966 to 2014). Snowfall is widespread during winter at elevations >800 m asl. (Marty, )
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Conceptual representation of possible sample positions in the dual isotope space (formed by δ2H and δ18O) for snow and rainfall samples from an entire hydrological year
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