Isotopic variation of snow cover and streamflow in response to changes in canopy structure in a snow‐dominated mountain catchment

Abstract

Isotopic composition of snow cover and streamflow was determined in a snowdominated, forested watershed to quantify the spatial variability and processes that alter stable isotope (oxygen-18, 18 O and deuterium, 2 H) composition under different forest canopy conditions (clear-cut, partial-cut (thinned), and unimpacted forest). Snow sampling was carried out on 4 days in late winter and early spring 2006. Meteorological data, precipitation, and streamflow were continuously monitored during the study. Isotope analyses of precipitation samples were conducted weekly through the 2005-2006 snow season. Values of d 18 O varied between −22·0 and −9·5‰ , and d 2 H varied between −170 and −76‰ . Isotope concentrations from snowpack samples varied between −17·5 and −13·8‰ for d 18 O, and between −129 and −102‰ for d 2 H. These ranges reflect differences in precipitation, accumulation, sublimation, and melting of the snow cover. Streamflow samples were collected during the snowmelt season from two locations every alternate day from the beginning of April until the end of May. Streamflow and snow from a partial-cut and an uncut forest were enriched in the heavy isotopes ( 18 O and 2 H) relative to streamflow and snow from a clear-cut forest.Based on the low water contents of the snowpack under dense canopies, we infer that the isotope enrichment resulted primarily from sublimation of snow intercepted by the canopy, with more enrichment in denser canopies. There was no significant correlation between snowpack isotope concentration and altitude. Results indicate that variations in canopy structure can alter snow isotope composition. This finding will provide a useful index of snowpack sublimation, and thus, improved parameterization of distributed hydrological models.

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, 2008). 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, 2012;Dietermann & Weiler, 2013;Koeniger, Hubbart, Link, & Marshall, 2008;Moran, Marshall, Evans, & Sinclair, 2007;Wen, Tian, Weng, Liu, & Zhao, 2012;Winograd et al, 1998). Moran et al (2007) 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 4 in Moran et al (2007)), which may occur only if the warmer temperatures and hence smaller vapor-liquid or vapor-ice isotopic fractionation factors offset the "rain-out" effect.…”

Using isotopes to constrain water flux and age estimates in snow-influenced catchments using the STARR (Spatially distributed Tracer-Aided Rainfall–Runoff) model

“…The animation shows for each model cell its water age (top row) and runoff (middle row), and the simulated stream hydrograph at the outlet (bottom row). Simulations span 1 calendar year and a typical hydrological year is shown for each catchment (2008for Krycklan, 2014for Bruntland and 2011-2012. A threshold of 0.1 mm for runoff is selected to highlight the spatial variability in runoff generation.…”

“…Furthermore, snow sublimation and melt fractionation primarily take place in the top snow layers, and are probably not instantaneously mixed in the snowpack (Claassen and Downey, 1995;Evans et al, 2016), whereas we assume fractionation with respect to the bulk snowpack. However, the error caused by the fullmixing assumption is reduced by the fact that snowpack is typically homogenized during snowmelt when diurnal meltrefreeze processes take place in the snowpack (Taylor et al, 2001;Unnikrishna et al, 2002;Koeniger et al, 2008). The majority of snowpack outflow is generated during the overall snowmelt when isotopes in the snowpack are subjected to mixing, which gives empirical ground to our simplification.…”

“…Internal mixing processes do not considerably influence the bulk isotopic composition of the snowpack, but fractionation due to snow sublimation has the potential to isotopically enrich the snowpack in relation to snowfall (Moser and Stichler, 1974;Earman et al, 2006). Furthermore, canopy snow interception can provide an additional transient storage subjected to sublimation, and thereby fractionation processes, further amplifying the snow isotopic enrichment (Claassen and Downey, 1995;Koeniger et al, 2008). Finally, several field, laboratory, and modelling studies (Shanley et al, 1995;Taylor et al, 2001;Feng et al, 2002) demonstrate how the onset of snowmelt tends to be depleted in heavy isotopes in comparison to average snowpack, and the snowpack isotopically enriches over the course of snowmelt.…”

“…Both processes are well documented in laboratory, field, and modelling studies (Cooper et al, 1993;Claassen and Downey, 1995;Taylor et al, 2001;Laudon et al, 2002, see e.g. Carey and Quinton, 2004;Koeniger et al, 2008;Schmieder et al, 2016), but have not before been incorporated to tracer-aided modelling in a spatially explicit manner.…”