Exploring aufeis in the Trans-Himalaya: Remote sensing-based studies of a neglected cryosphere component

Meltwater from the cryosphere plays a vital role for local and downstream communities alike. It provides water for drinking and irrigation purposes as well as hydropower generation. Due to climate change, glacier retreat and mass loss, permafrost degradation, and an increased variability in seasonal snow cover can be observed, leading to increasing uncertainties for water availability and agrarian livelihood security. Cryosphere studies typically focus on glaciers, seasonal snow, and permafrost while studies on other components, such as aufeis, remain limited. Aufeis is a characteristic feature of many cold and permafrost regions of the world, and can also witnessed on remote sensing imagery and historic travel reports from the Trans-Himalaya. Aufeis describes seasonal, laminated ice masses that accumulate during winter by successive freezing of water that seeps from the ground, a spring or emerges from below river or lake ice. It stores the winter base flow, which then becomes available as additional water during the melting period in spring and summer. In Ladakh, aufeis accumulation is actively promoted in ice reservoirs, which have been introduced in several valleys as an adaptive strategy to cope with seasonal water scarcity at the onset of the sowing period in spring. Despite this importance, the regional distribution and hydrological relevance of aufeis in the Trans-Himalaya is largely unknown. The aim of this cumulative dissertation is to provide the first regional assessment of aufeis distribution for the Trans-Himalaya and to explore its hydrological relevance. Furthermore, it aims to evaluate how the applied remote sensing methods can contribute to a better understanding of aufeis regimes in the various environments it occurs. On the example of three basins (Upper Indus Basin, Tso Moriri Basin, and Pangong Tso Basin), the regional occurrence of aufeis was mapped based on freely available satellite imagery from the Landsat and Sentinel-2 missions. In the Upper Indus Basin, aufeis was mapped with a harmonic time-series approach based on all available Landsat data from 2010–2020. In the Tso Moriri Basin, the spatial and temporal pattern of aufeis accumulation and melting were extracted with a machine learning approach using a Random Forest classifier on Landsat and Sentinel-2 data. Based on these results, a threshold method based on the Normalised Difference Snow Index was used to reveal aufeis distribution in the Pangong Tso Basin from 1994–2023. As these optical satellite datasets are not suitable for thickness and volume estimations, digital elevation models were calculated from very high-resolution Pléiades data and dense point clouds from terrestrial photographs obtained during two field trips in September/October 2022 and February/March 2023. Aufeis thickness was estimated in two ice reservoirs and two catchments with natural aufeis occurrence via digital elevation model differencing. Results revealed widespread occurrence that intensifies with increasing continentality and cold-aridity towards the Tibetan Plateau. Aufeis fields predominately occur within an elevation range between 4000 and 5500 m a.s.l.. Individual size is highly variable, but aufeis fields exceeding sizes of 0.1 km² account for the majority of aufeis-covered area. The largest aufeis fields even exceed the size of the largest high altitude glaciers in Ladakh by a factor of three. Temporal patterns indicate two temperature controlled phases: An accumulation phase from November until March, and a melting phase from April until July. The Abstract maximum aufeis area is typically reached in April/May. With the exception of few isolated places in the Pangong Tso Basin, aufeis fields completely disappear until the end of July. This widespread occurrence implies a significant hydrological value that is largest on the local scale. Digital elevation model differencing revealed substantial ice thickness up to 2.50 m in both ice reservoirs, while natural aufeis fields occasionally even reach greater thickness over three meters. Although the computation of dense point clouds from terrestrial photographs was hampered by snow coverage on the ice surface, the reinforcing effect of the walls in the ice reservoirs in terms of ice accumulation could be confirmed. While stereoscopic satellite imagery presents a promising method to investigate aufeis thickness on large spatial scales, terrestrial photographs and derived products, such as digital elevation models, have the potential to allow for analysis on aufeis processes on high spatial and temporal resolutions. This dissertation concludes that aufeis occurrence has to be accounted for in future hydrological studies in the Trans-Himalaya. In this context, the utilisation of integrated approaches that combine the advances in earth observation with in-situ data, are indispensable for enabling more accurate hydrological studies in a mountain environment reshaped by climate change.

PhD Candidate: Dagmar Brombierstäudl
Duration: 2021-2024