Debris covers glaciers worldwide and controls sub-debris melt rates by modifying energy transfer from the atmosphere to the ice. Physical properties like thermal conductivity (k) and surface roughness (z0) have been derived from limited local measurements, with models often relying on literature values from a few sites and studies. Accurate representation of these properties in energy-balance models is crucial for understanding climate-glacier interactions and predicting future behaviour of debris-covered glaciers. We studied these properties using established and modified approaches to derive k and z0 from field data at three locations on Pirámide Glacier in the central Chilean Andes. We compared existing methods and evaluated the modelled melt using these values. Our study reveals substantial inconsistencies between methods, leading to discrepancies between ice melt from energy-balance simulations and observed data, highlighting the impact of method choice on calculated ice melt. For energy-balance modelling, optimising k against measured ice melt appears a viable method to constrain melt simulations. Determining z0 is less critical due to its smaller impact on total ice melt, and profile aerodynamic method measurements, despite higher economic costs, are independent of ice melt calculations. The large, unexpected differences between existing methods indicate a substantial knowledge gap that the community should address. Values of k and z0 from field measurements do not work well when used in energy-balance models, suggesting that model values are bulk properties that do not necessarily correspond to field-derived values from theoretical approaches.

Ice cliff distribution plays a major role in determining the melt of debris-covered glaciers but its controls are largely unknown. We assembled a dataset of 37537 ice cliffs and determined their characteristics across 86 debris-covered glaciers within High Mountain Asia (HMA). We complemented this dataset with the analysis of 202 cliff formation events from multi-temporal UAV observations for a subset of glaciers. We find that 38.9% of the cliffs are stream-influenced, 19.5% pond-influenced and 19.7% are crevasses. Surface velocity is the main predictor of cliff distribution at both local and glacier scale, indicating its dependence on the dynamic state and hence evolution stage of debris-covered glacier tongues. Supraglacial ponds contribute to maintaining cliffs in areas of thicker debris, but this is only possible if water accumulates at the surface. Overall, total cliff density decreases exponentially with debris thickness as soon as debris gets thicker than 10 cm.

River systems originating from the Upper Indus Basin (UIB) are dominated by runoff from snow and glacier melt and summer monsoonal rainfall. These water resources are highly stressed as huge populations of people living in this region depend on them, including for agriculture, domestic use, and energy production. Projections suggest that the UIB region will be affected by considerable (yet poorly quantified) changes to the seasonality and composition of runoff in the future, which are likely to have considerable impacts on these supplies. Given how directly and indirectly communities and ecosystems are dependent on these resources and the growing pressure on them due to ever-increasing demands, the impacts of climate change pose considerable adaptation challenges. The strong linkages between hydroclimate, cryosphere, water resources, and human activities within the UIB suggest that a multi- and inter-disciplinary research approach integrating the social and natural/environmental sciences is critical for successful adaptation to ongoing and future hydrological and climate change. Here we use a horizon scanning technique to identify the Top 100 questions related to the most pressing knowledge gaps and research priorities in social and natural sciences on climate change and water in the UIB. These questions are on the margins of current thinking and investigation and are clustered into 14 themes, covering three overarching topics of ‘governance, policy, and sustainable solutions’, ‘socioeconomic processes and livelihoods’, and ‘integrated Earth System processes’. Raising awareness of these cutting-edge knowledge gaps and opportunities will hopefully encourage researchers, funding bodies, practitioners, and policy makers to address them.