Beyond equilibrium: Multi-method mass-balance monitoring at the vanishing Alpine glacier Stubacher Sonnblickkees, Austria

Anna Siebenbrunner , Bernhard Zagel, Andreas Gschwentner, Robert Delleske, Amelie Sonnleitner, Hans Wiesenegger, Lea Hartl

Glaciers in the Eastern Alps are approaching extinction, raising the question of how reliably established mass-balance monitoring methods perform on glaciers in severe disequilibrium. We present a multi-year (2018–2025) intercomparison of three independent methods – direct glaciological, semi-direct (accumulation-area-ratio based) and UAV-geodetic – at Stubacher Sonnblickkees (SSK), Austria, complemented by geodetic–(semi-)direct comparisons over two historical periods (1969–1998, 2008–2023). After homogenisation onto a common area time series, no method pair differs significantly at the 95 % level during the intercomparison period. The mean annual deviation of 0.1 m w.e./a between the direct and geodetic methods is among the lowest reported in comparable studies. Cumulative annual UAV-geodetic mass balances agree with 2018–2025 DEM differencing to within 0.25 m w.e., demonstrating that annual high-resolution surveys capture multi-year change without systematic drift. While these multi-year comparisons indicate good overall alignment, the annual comparison reveals substantial year-to-year deviations between methods that warrant further investigation. Since 2022, accelerating disintegration exposes method-specific limits, most critically the breakdown of the semi-direct method as the accumulation area vanishes. Sustaining adaptive, multi-method monitoring is essential to preserve long-term mass-balance records as these glaciers vanish, and to extend them onto larger successor glaciers.