Factors influencing spring and summer areal snow ablation and snowcover depletion in alpine terrain: detailed measurements from the Canadian Rockies

• Main Authors: Schirmer, Michael, Pomeroy, John W.
• Format: Text
• Language: English
• Published: 2018
• Subjects: The Fortress; Tundra
• Online Access: https://doi.org/10.5194/hess-2018-254; https://www.hydrol-earth-syst-sci-discuss.net/hess-2018-254/

The spatial distribution of snow water equivalent (SWE) and melt are important to estimating areal melt rates and snowcover depletion dynamics but are rarely measured in detail during the late ablation period. This study contributes the result of high resolution observations made using large numbers of sequential aerial photographs taken from an Unmanned Aerial Vehicle on an alpine ridge in the Fortress Mountain Snow Laboratory in the Canadian Rocky Mountains from May to July. With Structure-from-Motion and thresholding techniques, spatial maps of snow depth, snowcover and differences in snow depth during ablation were generated in very high resolution as proxies for spatial SWE, spatial ablation rates, and snowcover depletion (SCD). The results indicate that the initial distribution of SWE was highly variable due to overwinter snow redistribution and the subsequent distribution of ablation rates was also variable due to albedo, slope/aspect and other unaccountable differences. However, the initial distribution of SWE was five times more variable than that of subsequent ablation rates, even though ablation differences were substantial, with variability by a factor of two between north and south aspects, and somewhat persistent over time. Ablation rate patterns had an insubstantial impact on SCD curves, which were overwhelmingly governed by the initial distribution of SWE. The reasons for this are that variations in irradiance to slopes on north and south aspects in the near-summer solstice period are relatively small and only a weak spatial correlation developed between initial SWE and ablation rates. Previous research has shown that spatial correlations between SWE and ablation rates can strongly influence SCD curves. The results presented here are in contrast to alpine, shrub tundra and forest observations taken during ablation at higher latitudes and/or earlier in spring but in agreement with other near-summer observations in alpine environments. Whilst variations in net solar irradiance to snow were due to small scale variations in localized dust deposition from eroded ridgetop soil and larger scale differences in slope and aspect, variations in SWE were due to intense over-winter blowing snow storms with deposition from multiple directions of snow transport to incised gullies and slope breaks. This condition differs considerably from situations where wind transport from primarily one direction leads to preferential SWE loading on slopes of particular aspects, which can lead to a spatial correlation between SWE and ablation rate. These findings suggest that in near-summer solstice conditions and environments where snow redistribution is substantial, then mountain SCD curves can be calculated using the spatial distribution of SWE alone, and that hydrological and atmospheric models need to implement a realistic distribution of SWE in order to do this.