An unusually large isotopic step-function change within the current 2011-2012 snowpack offers the means to test a new methodology for quantifying sublimation processes in snowmelt. The team will measure oxygen and hydrogen isotopes in snow profiles over time at 2 cm resolution using a laser spectroscopic instrument (LGR liquid water isotope analyzer). This evolution of the step function with time will enable them to calculate isotopically distinct sublimation losses by fitting the isotopic profiles to the diffusion-advection equation. These data are crucial for understanding snowpack mass balance and thus the timing and magnitude of the delivery of snowmelt to riverflow. This project builds on the heavily instrumented and monitored current capability at the Dry Creek Experimental watershed, Idaho.
Isotopes are defined as atoms whose nuclei contain the same number of protons, but different numbers of neutrons, thus stable isotopes of oxygen (16O, 18O) and hydrogen (1H, 2H) differ from each other in atomic mass. These mass differences lead to differences in the sorting or "fractionation" of water molecules of varied isotopic composition undergoing physical and chemical processes, such as evaporation and precipitation. In general, molecules that are more enriched in 18O or 2H as opposed to 16O or 1H will evaporate more slowly and precipitate more quickly, and these fractionation processes are temperature dependent. The oxygen and hydrogen isotopic composition of snow is influenced by many factors, including the original isotopic composition of the source water, the length of the vapor trajectory, air temperatures under which the water vapor condenses and freezes, and the air temperature near ground deposition during snowfall. All of these factors contribute to large variability in snow isotopic compositions between and even within individual snowfall events. Snowpack isotopic composition is further complicated by post depositional processes, such as melting/refreezing, melting/evaporation, sublimation, rain-on-snow water additions, and snow crystal metamorphosis. Even with so many interacting variables, snowpack isotopic "fingerprints" are often utilized as tracer tools in hydrologic studies and hydrograph models. Since approximately 60 million citizens in the American West, and over 1 billion people worldwide, depend on snowpack-derived water every year for drinking and irrigation, understanding snowpack dynamics highly relevant. The main goal of this project was to create a high-resolution isotopic record in a seasonal snowpack to increase understanding of post depositional isotopic evolution of snow with applications ranging from basic understanding of the natural drivers of snowpack water budgets to hydrologic modeling of snowmelt transport and watershed residence time. Through the winter season 2012, we studied the development and evolution of the snowpack on a sheltered slope (2100m elevation) in the foothills of the Boise Front Range, Boise, Idaho. The site was instrumented with automated weather/snowpack monitoring equipment and a suite of ground-penetrating radar (GPR) equipment deployed from both above and below the snowpack. Weekly snowpack profile samples were collected for isotopic analysis of d2H and d18O at 2cm increments in conventional snowpits. Isotope data were collected using a laser spectroscopy liquid water isotope analyzer at Boise State University. This work is one of the highest resolution isotopic datasets, both temporally and incrementally within a seasonal snowpack, that has been documented to date. Traditional snowpit observations of snow grains, density, temperature, and water content measurements were also collected. The most striking feature of the 2012 snowpack isotopic profile was the high level of variability observed within the snowpack and the degree to which that variability was preserved throughout the season (Figure 1). It was hypothesized that a striking isotopic boundary between basal snow deposited in early November 2011 and later accumulations (late January to late March 2012) would migrate within the snowpack due to isotopic vapor exchange driven by sublimation at the air/snow interface, however this effect was not observed. In fact, the basal snow/upper snowpack boundary represented a snow density boundary that effectively isolated the November snow from interacting with either vapor or melt water transport until the final stages of spring melt when the impermeable layer boundary was compromised. Through most of the winter, isotopic changes that occurred within the snowpack were limited to the upper snowpack and were responses to liquid water penetration. GPR profiles illustrate clear vertical reflectors of liquid water penetration by both rain-on-snow and melting events leading to the formation of persistent ice layers (Figure 2). Isotopic exchange between liquid water and snow/ice crystals produced a marked change in the relationship between 2H and 18O isotopic composition, particularly evident early in the season when liquid water refroze and was subsequently sampled within the snowpack (Figure 3). This work represents the first time isotopic data sets have been used to corroborate snowpack remote sensing data. Further effects of isotopic exchange between ice and liquid water are evident in a pronounced decrease in overall isotopic variance by late March, following several rain-on-snow and melt events (Figure 4). Isotopic homogenization continued through the spring melt, with melt water samples exhibiting near average snowpack isotopic values, although isotopic fractionation during diurnal melt-freeze cycles does generate daily isotopic variations in melt water composition. These findings validate the use of bulk snowpack isotopic compositions in hydrologic melt water studies on time-scales longer than diurnal cycles, however hourly hydrologic measurements of snow melt pulses require more attention to isotopic details of the melting snowpack.
