High-resolution monitoring of debris-covered glacier mass budget and flow velocity using repeated UAV photogrammetry in Iran

Abstract

Iran's glaciers are commonly debris-covered. Due to climate changes, the frequency of glacial hazards such as glacial lake outbursts floods is increasing, and consequently, the mass of Iran's glaciers is decreasing. Fieldwork in high-altitude mountains is difficult, thus, the dynamics of debris–covered glaciers and their associated driving factors are poorly studied. In this study, changes in the elevation and the surface flow speed of the largest and most dynamic debris-covered glacier in Iran (Alamkouh glacier) are evaluated during 2018 to 2020 for the first time. For this purpose, images from a high-resolution unmanned aerial vehicle (UAV) are used. Additionally, the spatial distribution of glacier ice thickness is estimated based on the principles of ice-flow and glacier surface velocity data. The distribution of ice thickness is evaluated with in-situ Ground Penetration Radar (GPR) data. The acquired high-resolution images from UAV are processed into a digital elevation model (with a spatial resolution of about 15 cm) and orthoimages (with a spatial resolution of about 8 cm), and glacier ice thickness change and surface flow velocity are derived. In order to estimate glacier surface velocity, a frequency cross-correlation model was applied using the COSI-Corr module. Our results show that the mean glacier ice thinning during the study period 2018–2020 is about −0.23 ± 0.03 m yr−1 and it reaches about −5 ± 0.65 m yr−1 at the terminus and accumulation areas. The observed variations in ice thinning are best explained by the existence of debris cover (and their corresponding insulating effects), the existence of ice cliffs, and their associated supraglacial lakes. We found that ice cliffs and supraglacial lakes show mass losses that can be an order of magnitude higher than average. Moreover, the mean surface velocity of Alamkouh glacier is about 1.1 ± 0.06 m yr−1 from 2018 to 2020 with considerable spatial variation in surface velocities. The maximum surface velocity (about 4.5 ± 0.27 m yr−1) belongs to its upper regions, and lower and marginal areas are nearly stagnant. The slow movement of the glacier, along with a continuous increase in debris and ice thinning, provides a suitable condition for supraglacial pond development, which decreases glacier mass. Total volume of glacier was estimated to be ~102 × 106 m3 in 2020, corresponding to an average ice thickness of ~31.3 ± 7.5 m. Moreover, the average ice thinning rate is 0.66% yr−1 in 2018 to 2020. According to this reduction rate, it is expected that more than 51.7% of the glacier will be lost by the end of the 21st century.