Monitoring snowpack outflow volumes and their isotopic composition to better understand streamflow generation during rain-on-snow events

Abstract

Rain-on-snow (ROS) events in mountainous catchments can cause enhanced snowmelt, leading to an increased risk of destructive winter floods. However, due to differences in topography and forest cover, the generation of snowpack outflow volumes and their contribution to streamflow are spatially and temporally variable during ROS events. In order to adequately predict such flood events with hydrological models, an enhanced process understanding of the contribution of rainwater and snowmelt to stream water is needed.In this study, we monitored and sampled snowpack outflow with fully automated snowmelt lysimeter systems installed at three different elevations in a pre-Alpine catchment in central Switzerland. We measured snowpack outflow volumes during the winters of 2017 and 2018, as well as snowpack outflow isotopic compositions in winter 2017. Snowpack outflow volumes were highly variable in time and space, reflecting differences in snow accumulation and melt. In winter 2017, around 815 mm of snowpack outflow occurred at our reference site (grassland 1220 m a.s.l. -metres above sea level), whereas snowpack outflow was 16 % less at the nearby forest site (1185 m a.s.l.), and 62 % greater at another grassland site located 200 m higher (1420 m a.s.l.). A detailed analysis of 10 ROS events showed that the differences in snowpack outflow volumes could be explained mainly by rainfall volumes and initial snow depths.The isotope signals of snowpack outflow were more damped than those of incoming rainwater at all three sites, with the most damped signal at the highest elevation site because its snowpack was the thickest and the residence times of liquid water in its snowpack were the longest, thus enhancing isotopic mixing in the snowpack. The contribution of snowpack outflow to streamflow, estimated with an isotopebased two-component end-member mixing model, differed substantially among the three lysimeter sites (i.e. between 7 ± 4 and 91 ± 21 %). Because the vegetation in our study catchment is a mixture of grassland and forest, with elevations ranging from 1000 to 1500 m a.s.l., our site-specific hydrograph separation estimates can only provide a range of snowpack outflow contributions to discharge from different parts of the study area. Thus, the catchment-average contribution of snowpack outflow to stream discharge is likely to lie between the end-member mixing estimates derived from the three site-specific data sets. This information may be useful for improving hydrological models in snow-dominated catchments.

Effects of Spatial Variability in the Groundwater Isotopic Composition on Hydrograph Separation Results for a Pre‐Alpine Headwater Catchment

“…However, two of the snapshot sampling campaigns took place shortly (2–9 days) after the sampled rainfall events (Table 1). Daily precipitation measurements and samples from the neighboring Erlenbach catchment (precipitation gauge and sampler ~500 m from the Studibach outlet; Rücker et al, 2019) suggest that there was no precipitation between the sampled events and the snapshot sampling campaigns (Event A) or there were only small, low‐intensity events with an isotopic composition similar to the rainfall before the sampled event (Events B and C; see supporting information S3). We assume that these small‐ to medium‐sized events did not significantly change the isotopic composition of the shallow groundwater because their isotopic composition was typical for the season.…”

The CH-IRP data set: a decade of fortnightly data on <i>δ</i><sup>2</sup>H and <i>δ</i><sup>18</sup>O in streamflow and precipitation in Switzerland

“…A general observation for isotopes in the Alptal is the large spatial variation which was found for both rainfall (Fischer et al, 2017) and groundwater (Kiewiet et al, 2020). Rücker et al (2019) measured the isotopic composition of snowpack outflow to study runoff generation during rain-on-snow events. Furthermore, in the Alptal a field lab was installed that provides isotope data at high temporal resolution at the outlet of the Erlenbach catchment (von Freyberg et al, 2018a).…”

“…For isotopes in precipitation, data from the National Network for the Observation of Isotopes in the Water Cycle (ISOT; see Schotterer, 2010) were used to predict precipitation isotopes for the selected catchments. For the catchments close to the Swiss border, data from the Austrian network (ANIP) as well as the global network (GNIP) were also used to allow for a better interpolation.…”

The CH-IRP data set: a decade of fortnightly data of δ<sup>2</sup>H and δ<sup>18</sup>O in streamflow and precipitation in Switzerland

“…A general observation for isotopes in the Alptal is the large spatial variation which was found for both, rainfall (Fischer et al, 2017) and groundwater (Kiewiet et al, in press). Rücker et al (2019) measured the isotopic composition of snowpack outflow to study runoff generation during rain-on-snow events.…”