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

Abstract

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.

From Rockfall Observation to Operational Solutions: Nearly 20 years of Cryo-gravitational Hazard Studies in Mont-Blanc Massif

“…32 Another source of improvement lies in the accuracy of the predicted surface temperature that could be better achieved by using energy balance modelling taking into account snow processes (Ben-Asher et al, 2022). It would also be interesting to consider other variables than the daily air temperature to determine the parameters associated with failure occurrences, such as the date of snow melt which partly controls the surface temperature but can also be used as a proxy for water infiltration, or the maxima and minima of air temperature, for example.…”

“…The peak of rockfall occurrence in MBM in July 2019 (Fig. 4b), that was reached far before the maximum ALT developed, could for example be related to the rather high amount of available water from snowmelt and rainfall in the early summer (Ben-Asher et al, 2022).…”

“…In MBM, characterisation of snow accumulation was performed on the AdM S face using time-lapse photogrammetry, high-resolution photogrammetric models, and numerical modelling. About 25% of the fallen snow accumulates on the 55° steep and rugged rock face(Ben-Asher et al, 2022), and that average snow depth steadily decreases between 40° slope angle, where it is approx. 1 m (up to > 2.5 m at point scale), and 70°, a threshold above which snow no longer accumulates.…”