Preprints
https://doi.org/10.5194/esurf-2022-58
https://doi.org/10.5194/esurf-2022-58
 
16 Nov 2022
16 Nov 2022
Status: this preprint is currently under review for the journal ESurf.

Estimating surface water availability in high mountain rock slopes using a numerical energy balance model

Matan Ben-Asher1, Florence Magnin1, Sebastian Westermann2, Emmanuel Malet1, Johan Berthet3, Josué Bock1, Ludovic Ravanel1, and Philip Deline1 Matan Ben-Asher et al.
  • 1EDYTEM laboratory, Université Savoie Mont Blanc, CNRS, Le Bourget-du-Lac, 73376, France
  • 2Department of Geosciences, University of Oslo, Oslo, Norway
  • 3Styx 4D, Le Bourget du Lac, France

Abstract. Water takes part in most physical processes that shape the mountainous periglacial landscapes and initiation of mass wasting. An observed increase in rockfall activity in several mountainous regions was previously linked to permafrost degradation in high mountains, and water that infiltrates into rock fractures is one of the likely drivers of these processes. However, there is very little knowledge on the quantity and timing of water availability for infiltration in steep rock slopes. This knowledge gap originates from the complex meteorological, hydrological and thermal processes that control snowmelt, and also the challenging access and data acquisition in the extreme alpine environments. Here we use field measurement and numerical modeling to simulate the energy balance and hydrological fluxes in a steep high elevation permafrost affected rock slope at Aiguille du Midi (3842 m a.s.l), in the Mont-Blanc massif. Our results provide new information about water balance at the surface of steep rock slopes. Model results suggest that only ~25 % of the snowfall accumulates in our study site, the remaining ~75 % are redistributed by wind and gravity. Snow accumulation depth is inversely correlated with surface slopes between 40° to 70°. Snowmelt occurs between spring and late summer and most of it does not reach the rock surface due to the formation of an impermeable ice layer at the base of the snowpack. The annual effective snowmelt, that is available for infiltration, is highly variable and ranges over a factor of six with values between 0.05–0.28 m in the years 1959–2021. The onset of the effective snowmelt occurs between May and August, and ends before October. It precedes the first rainfall by one month on average. Sublimation is the main process of snowpack mass loss in our study site. Model simulations at varying elevations show that effective snowmelt is the main source of water for infiltration above 3600 m a.s.l.; below, direct rainfall is the dominant source. The change from snowmelt-dominated to rainfall-dominated water availability is nonlinear and characterized by a rapid increase in water availability for infiltration. We suggest that this elevation of water availability transition is highly sensitive to climate change, if snowmelt-dominated permafrost-affected slopes experience an abrupt increase in water input that can initiate rock slope failure.

Matan Ben-Asher et al.

Status: open (until 08 Jan 2023)

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Matan Ben-Asher et al.

Matan Ben-Asher et al.

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Short summary
Water in fractures drive many processes that destabilize steep permafrost-affected rock walls. However, quantitative knowledge on water availability for infiltration is limited. Here we use a numerical model and field measurements to estimate the water balance in a steep rock walls site. We show that snowmelt is the main source of water at elevations >3600 m and that snowpack hydrology and sublimation are key factors. Below 3600 m, surface water availability is increased rapidly due to rainfall.