Supraglacial lakes are an important component of the Greenland Ice Sheet's mass balance and hydrology, with their drainage affecting ice dynamics. This study uses imagery from the recently launched Sentinel-1A Synthetic Aperture Radar (SAR) satellite to investigate supraglacial lakes in West Greenland. A semi-automated algorithm is developed to detect surface lakes from Sentinel-1 images during the 2015 summer. A combined Landsat-8 and Sentinel-1 dataset, which has a comparable temporal resolution to MODIS (3 days vs. daily) but a higher spatial resolution (25-40 vs. 250-500 m), is then used together with a fully automated lake drainage detection algorithm. Rapid (<4 days) and slow (>4 days) drainages are investigated for both small (<0.125 km 2 , the minimum size detectable by MODIS) and large (≥0.125 km 2 ) lakes through the summer. Drainage events of small lakes occur at lower elevations (mean 159 m), and slightly earlier (mean 4.5 days) in the melt season than those of large lakes. The analysis is extended manually into the early winter to calculate the dates and elevations of lake freeze-through more precisely than is possible with optical imagery (mean 30 August; 1,270 m mean elevation). Finally, the Sentinel-1 imagery is used to detect subsurface lakes and, for the first time, their dates of appearance and freeze-through (mean 9 August and 7 October, respectively). These subsurface lakes occur at higher elevations than the surface lakes detected in this study (mean 1,593 and 1,185 m, respectively). Sentinel-1 imagery therefore provides great potential for tracking melting, water movement and freezing within both the firn zone and ablation area of the Greenland Ice Sheet.

Abstract. A set of supraglacial ponds filled rapidly between April and July 2017 on Changri Shar Glacier in the Everest region of Nepal, coalescing into a ∼180 000 m2 lake before sudden and complete drainage through Changri Shar and Khumbu glaciers (15–17 July). We use PlanetScope and Pléiades satellite orthoimagery to document the system's evolution over its very short filling period and to assess the glacial and proglacial effects of the outburst flood. We also use high-resolution stereo digital elevation models (DEMs) to complete a detailed analysis of the event's glacial and geomorphic effects. Finally, we use discharge records at a stream gauge 4 km downstream to refine our interpretation of the chronology and magnitude of the outburst. We infer largely subsurface drainage through both of the glaciers located on its flow path, and efficient drainage through the lower portion of Khumbu Glacier. The drainage and subsequent outburst of 1.36±0.19×106 m3 of impounded water had a clear geomorphic impact on glacial and proglacial topography, including deep incision and landsliding along the Changri Nup proglacial stream, the collapse of shallow englacial conduits near the Khumbu terminus and extensive, enhanced bank erosion at least as far as 11 km downstream below Khumbu Glacier. These sudden changes destroyed major trails in three locations, demonstrating the potential hazard that short-lived, relatively small glacial lakes pose.

Runoff from high-elevation debris-covered glaciers represents a crucial water supply for millions of people in the Hindu Kush-Himalaya region, where peak water has already passed in places. Knowledge of glacier thermal regime is essential for predicting dynamic and geometric responses to mass balance change and determining subsurface drainage pathways, which ultimately influence proglacial discharge and hence downstream water availability. Yet, deep internal ice temperatures of these glaciers are unknown, making projections of their future response to climate change highly uncertain. Here, we show that the lower part of the ablation area of Khumbu Glacier, a high-elevation debris-covered glacier in Nepal, may contain ~56% temperate ice, with much of the colder shallow ice near to the melting-point temperature (within 0.8 °C). From boreholes drilled in the glacier’s ablation area, we measured a minimum ice temperature of −3.3 °C, and even the coldest ice we measured was 2 °C warmer than the mean annual air temperature. Our results indicate that high-elevation Himalayan glaciers are vulnerable to even minor atmospheric warming.

Surface melting of High Mountain Asian debris-covered glaciers shapes the seasonal water supply to millions of people. This melt is strongly influenced by the spatially variable thickness of the supraglacial debris layer, which is itself partially controlled by englacial debris concentration and melt-out. Here, we present measurements of deep englacial debris concentrations from debris-covered Khumbu Glacier, Nepal, based on four borehole optical televiewer logs, each up to 150 m long. The mean borehole englacial debris content is ≤ 0.7% by volume in the glacier’s mid-to-upper ablation area, and increases to 6.4% by volume near the terminus. These concentrations are higher than those reported for other valley glaciers, although those measurements relate to discrete samples while our approach yields a continuous depth profile. The vertical distribution of englacial debris increases with depth, but is also highly variable, which will complicate predictions of future rates of surface melt and debris exhumation at such glaciers.

The hydrological characteristics of debris-covered glaciers are known to be fundamentally different from those of clean-ice glaciers, even within the same climatological, geological, and geomorphological setting. Understanding how these characteristics influence the timing and magnitude of meltwater discharge is particularly important for regions where downstream communities rely on this resource for sanitation, irrigation, and hydropower, as in High Mountain Asia. The hydrology of debris-covered glaciers is complex: rugged surface topographies typically route meltwater through compound supraglacial-englacial systems involving both channels and ponds, as well as pathways that remain unknown. Low-gradient tongues that extend several kilometres retard water conveyance and promote englacial storage. Englacial conduits are frequently abandoned and reactivated as water supply changes, new lines of permeability are exploited, and drainage is captured due to high rates of surface and subsurface change. Seasonal influences, such as the monsoon, are superimposed on these distinctive characteristics, reorganising surface and subsurface drainage rapidly from one season to the next. Recent advances in understanding have mostly come from studies aimed at quantifying and describing supraglacial processes; little is known about the subsurface hydrology, particularly the nature (or even existence) of subglacial drainage. In this review, we consider in turn the supraglacial, englacial, subglacial, and proglacial hydrological domains of debris-covered glaciers in High Mountain Asia. We summarise different lines of evidence to establish the current state of knowledge and, in doing so, identify major knowledge gaps. Finally, we use this information to suggest six themes for future hydrological research at High Mountain Asian debris-covered glaciers in order to make timely long-term predictions of changes in the water they supply. water supply is a fundamental part of building sustainable development (United Nations, 2015), which in High Mountain Asia can only be realised by improving understanding of future changes in the timing and magnitude of meltwater production from hitherto poorly studied debris-covered glaciers.

Supraglacial rock debris is present on 7% of the global mountain glacier area, dramatically affecting the sensitivity of these glaciers to climate change (Herreid & Pellicciotti, 2020). Debris-covered ice represents 30% of the glacier mass in ablation areas in High Mountain Asia (Kraaijenbrink et al., 2017). Supraglacial debris in the Everest region is typically sufficiently thick to reduce ablation by insulating the underlying ice surface (Nicholson & Benn, 2013). As a result, these debris-covered glaciers have experienced lower sensitivity to atmospheric warming than would be expected for climatically equivalent clean-ice surfaces (Benn Abstract Sustained mass loss from Himalayan glaciers is causing supraglacial debris to expand and thicken, with the expectation that thicker debris will suppress ablation and extend glacier longevity. However, debris-covered glaciers are losing mass at similar rates to clean-ice glaciers in High Mountain Asia. This rapid mass loss is attributed to the combined effects of; (a) low or reversed mass balance gradients across debris-covered glacier tongues, (b) differential ablation processes that locally enhance ablation within the debris-covered section of the glacier, for example, at ice cliffs and supraglacial ponds, and (c) a decrease in ice flux from the accumulation area in response to climatic warming. Adding meter-scale spatial variations in supraglacial debris thickness to an ice-flow model of Khumbu Glacier, Nepal, increased mass loss by 47% relative to simulations assuming a continuous debris layer over a 31-year period (1984 but overestimated the reduction in ice flux. Therefore, we investigated if simulating the effects of dynamic detachment of the upper active glacier from the debris-covered tongue would give a better representation of glacier behavior, as suggested by observations of change in glacier dynamics and structure indicating that this process occurred during the last 100 years. Observed glacier change was reproduced more reliably in simulations of the active, rather than entire, glacier extent, indicating that Khumbu Glacier has passed a dynamic tipping point by dynamically detaching from the heavily debris-covered tongue that contains 20% of the former ice volume.Plain Language Summary Glaciers in the Himalaya are shrinking rapidly in response to ongoing climate change. Many of these glaciers are covered with thick layers of rock debris that insulate the ice surface from atmospheric warming. Recent observations suggest that, contrary to expectations, debris-covered glaciers are losing mass at similar rates to clean-ice glaciers. We explore the processes driving the rapid loss of ice from a debris-covered glacier using a glacier model with a novel representation of sub-debris melt. Our model shows that the rapid ice loss from Khumbu Glacier, Nepal, since 1984 cannot be explained solely by accounting for variations in debris thickness and the presence of ice cliffs and ponds across the glacier surface. Instead, the glacier has passed a dynamic tipping point i...

12Debris-covered glaciers (DCGs) are characterised by distinct hydrological systems that differ 13 fundamentally from those observed on clean-ice valley glaciers. To date, most studies of DCG 14 hydrology have focused on supraglacial hydrology, given that surface streams are broadly 15 accessible and repeat observations can lead to conceptual models of channel evolution. Few have 16 characterised englacial conduits and their layout, and none have directly investigated potential 17 subglacial drainage networks in any setting. In this review, we summarise the current state of 18 knowledge relating to DCG hydrology with a global focus, and present our own field observations 19 to illustrate the distinct nature of DCG landforms on a receding high-elevation glacier in the 20Himalaya. We draw on recent work that has gone some way towards providing a process-based 21 understanding of the formation and evolution of englacial and subglacial hydrological pathways 22 and consider the role that DCG hydrology plays in regulating water supplies to downstream 23 communities, contrasting this information with clean-ice examples. We conclude by identifying 24 important knowledge gaps that might be considered priorities for future research into DCG 25 hydrology. 26

Abstract. A set of supraglacial ponds rapidly filled between April and July 2017 on Changri Shar Glacier in the Everest region of Nepal, coalescing into a ~ 180,000 m2 lake before sudden and complete drainage through Changri Shar and Khumbu Glaciers 15–17 July. We use a suite of PlanetScope and Pléiades satellite orthoimagery to document the system's evolution over its very short filling period and to assess the glacial and proglacial effects of the outburst flood. We additionally use high resolution stereo digital elevation models (DEMs) to complete a detailed analysis of the event's ablative and geomorphic effects. Finally, measurement of the flood’s passage at a stream gauge 4 km downstream enables a refined interpretation of the chronology and overall magnitude of the outburst. We infer largely subsurface drainage through both glaciers located on its flowpath, and efficent drainage through Khumbu Glacier. The drainage and subsequent outburst of 1.36 &amp;pm; 0.19 x 106 m3 impounded water had a clear geomorphic impact on glacial and proglacial topography at least as far as 11 km downstream, including deep incision and landsliding along the Changri Nup proglacial stream, the collapse of shallow englacial conduits near the Khumbu terminus and extensive, enhanced bank erosion below Khumbu Glacier. These sudden changes led to the rerouting of major trails in three locations, demonstrating the potential hazard that short-lived, relatively small glacial lakes pose.