the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Aug 2026
Brief communication: Repeated supraglacial lake drainage from the Purepu Glacier system, central Himalaya
Abstract. We report repeated drainage from the Purepu supraglacial lake system in the central Himalaya using high-resolution satellite imagery. A satellite-inferred 2023 drainage event is constrained to 12–15 July, whereas a reported GLOF occurred early on 8 July 2025. Both followed lake expansion to ~0.70–0.72 km², but DEM-constrained lake-volume loss increased from 0.87 × 10⁶ m³ in 2023 to 3.55 × 10⁶ m³ in 2025. The 2025 event remained incomplete, and flood-volume reconstruction suggests additional englacial or subglacial contributions cannot be excluded. Residual water persisted into 2026, supporting high-frequency monitoring in early July 2026.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-4065', Anonymous Referee #1, 12 Sep 2026
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RC2: 'Comment on egusphere-2026-4065', Anonymous Referee #2, 19 Oct 2026
In this study, the authors investigated repeated drainage events of Purepu supraglacial lake in the central Himalaya. This study is interesting and important, particularly given the destructive ice–rock avalanche that occurred in the same valley this year. The findings provide valuable insights into supraglacial lake drainage processes and the potential volume of water released during outburst events. Overall, I consider this study a useful contribution to understanding supraglacial lake dynamics and associated hazards. I recommend minor revision before publication.
Specific comments:
Title: The title could be further refined, as the current phrasing, particularly “Repeated supraglacial lake drainage,” is somewhat difficult to read and does not clearly convey the main focus of the study. Based on the abstract, a major contribution appears to be the estimation of supraglacial lake water volume. I suggest revising the title to better reflect this key contribution.
Figure 1: The China–Nepal border line is difficult to distinguish on the map. I suggest removing the line and placing greater emphasis on the characteristics of the GLOF events.
PlanetScope imagery: For “High-resolution PlanetScope surface reflectance imagery,” please specify the spatial resolution of the imagery.
Line 105: Given that this manuscript is a short communication, the text around Line 105 could be omitted or substantially condensed to improve conciseness.
Figure 2: I suggest adding more annotations to highlight relevant characteristics of the lake and glacier, which would help readers better understand the study setting and drainage processes.
Figure 3: The overall presentation of this figure could be improved. In particular, some labels and text are too small and should be enlarged to enhance readability.
Citation: https://doi.org/10.5194/egusphere-2026-4065-RC2
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This manuscript documents two drainage events in 2023 and 2025 from the Purepu supraglacial lake system on a debris-covered glacier in the central Himalaya, upstream of the Rasuwagadhi Friendship Bridge and hydropower project. This channel is where the 2026 giant glacier collapse happened. Using primarily PlanetScope high-resolution optical imagery, supplemented by Sentinel-1/2, Landsat 8/9, and an 8 m HMA DEM, the authors reconstruct pre- and post-drainage lake situations, calculate an area-based drainage completeness metric, and derive first-order DEM-constrained drainage water volume. Both events were preceded by similar peak lake extents, yet the 2023 event drained nearly completely while the 2025 event left substantial residual water. The authors analyzed a possible evidence of englacial/subglacial water contribution beyond what is captured by surface lake area change in the drainage event. The paper argues for continued high-frequency, multi-sensor monitoring through the monsoon season given the risk to downstream infrastructure.
Major issues:
The study makes a useful empirical contribution by documenting rare repeated drainage behavior and highlighting monsoon-season monitoring gaps, but its quantitative conclusions about volume loss and hidden water sources would benefit substantially from uncertainty analysis and independent validation before being treated as firm evidence rather than a first-order approximation.
1. The authors used 8m HMA DEM to estimate the drained water volume. However the data does not correspond in time to either drainage event, and no field bathymetry, discharge, or channel-geometry data validate the derived volumes. The authors note their volume estimate for 2025 is roughly half of an independent bathymetric estimate (Zhang et al., 2026). It should be discussed more in detail how the difference of DEM data could influence the results.
2. Some key numbers in the manuscript lack uncertainty quantification. The 16 m shoreline buffer and water-level proxy method were applied uniformly without a sensitivity analysis, so the reported ΔV values lack error bounds. The authors should consider about the uncertainty.
3. The authors attributed the water-budget discrepancies of two events to englacial/subglacial reservoirs. This is reasonable. However, without additional constraints, this remains a plausible hypothesis rather than a demonstrated mechanism, and alternative explanations (e.g., differing DEM resolution or bathymetric assumptions) are not ruled out. It need more discussions in the manuscript.
4. Transboundary Himalayan flows routinely transform into hyperconcentrated or debris flows through channel-bed entrainment. This manuscript lacks of downstream hazard modeling. At least the authors can add discussion on this issue.