the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 06 Dec 2024
Large interannual changes in supraglacial drainage basin areas and channels that flow downstream uphill: lessons from field surveys of moulin-connected streams on the Greenland Ice Sheet
Abstract. Internally drained catchments (IDCs) define the ice surface area draining into a moulin. IDCs are thought to be controlled by the influence of basal topography on the ice surface, which should produce IDCs with static, topographically-defined catchment areas. Our observations of lakes overtopping drainage divides, fluvial incision through those drainage divides and connection of previously isolated adjacent lake basins suggests that supraglacial drainage basins are more complicated. Here, we document interannual variability in the size, shape and density of IDCs in a 31.7 km2 area by mapping supraglacial streams within three mid-elevation catchments on the Paakitsoq Region of the Greenland Ice Sheet in 2017 and 2018. In two of the three catchments, snow-infill of the previous year's incised streams diverted meltwater flow away from relic moulins, which rerouted flow over topographic divides and created new incised channels that flowed downstream against the surface topographic gradient and drained to different moulins than in the previous year. Catchment consolidation resulted the growth of our central catchment from 8.2 km2 in 2017, to 27.8 km2 in 2018, and 31.7 km2 in 2019, an area increase of 387 % that was coincident with a decrease in the number of catchments, and moulins, decreasing from 3 to 1 within this area. Our results highlight that wintertime snowplug formation in supraglacial channels can change catchment-scale supraglacial hydrology and potentially impact hydrodynamic coupling across large areas of the ice sheet by turning moulins on and off.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2024-3676', Anonymous Referee #1, 14 Jan 2025
General comments:
The authors present an interesting and engaging manuscript combining remote sensing and field measurements to outline a interesting change to supraglacial hydrology on the Greenland Ice sheet. The quality of the writing is very high, and the paper is clear in its aims and objectives. The results are presented clearly and flow naturally into the interpretation and discussion. I feel the implications of this work could have been taken further as this is very interesting and has implications for further observations and modelling. I feel the greatest changes, albeit minor ones should be made to the study site and results sections to make them easier to follow and compare as outlined in my specific comments. For these reasons I suggest minor edits to be made to the manuscript.
Specific comments
Title: The title is quite long and could be condensed to the key message.
Study Area: In this section I found myself wondering why this site was picked and how the catchments were delineated, I appreciate the method is explained in section 2.2 however a nod towards this would be appreciated.
Figure 1: I find the choice of colour for contours, particularly the 50m contour colour and stream colours very similar and slightly confusing, consider changing symbology. Stream flow direction would also be appreciated.
Tables 1 and 2: I find the arrows in the tables slightly confusing, perhaps this could be explained in the table title.
Line 80: How much elevation change was recorded during the transects? A number here would help the justification
Line 92: “by visual inspection of remote sensing imagery” is very vague, I would like some more specifics here as to how these were determined.
Line 108: How much does stream depth increase?
Section 3.2: I found this section a tad hard to follow. Perhaps this could be augmented with a figure denoting a timeline for key events, perhaps combining some of the field images you have? This would help condense the information and may make it easier to follow. I found the images of varying size with little text hard to follow and this could be better communicated as this information is very valuable.
Citation: https://doi.org/10.5194/egusphere-2024-3676-RC1 -
AC1: 'Reply on RC1', Jessica Mejia, 04 Feb 2025
We would like to thank the reviewer for their positive comments on our work and for the suggestions which we believe will improve the quality and clarity of the final manuscript. We have attached a pdf that includes point-by-point responses to the comments raised by the reviewer.
Sincerely,
Jessica Mejia, on behalf of all coauthors
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RC2: 'Reply on AC1', Anonymous Referee #1, 06 Feb 2025
Thank you, for these, I look forward to reading the revised manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-3676-RC2 -
AC3: 'Reply on RC2', Jessica Mejia, 11 Sep 2025
We would like to thank the reviewer again for their positive comments on our work and for the suggestions which have improved the quality and clarity of our manuscript. We have now incorporated the comments and suggestions raised by the reviewer in the revised manuscript. Below we have responded to each point raised, our response is in blue, and details how the manuscript has been updated to address each concern.
Sincerely, Jessica Mejia on behalf of all coauthors
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AC3: 'Reply on RC2', Jessica Mejia, 11 Sep 2025
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RC2: 'Reply on AC1', Anonymous Referee #1, 06 Feb 2025
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AC1: 'Reply on RC1', Jessica Mejia, 04 Feb 2025
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RC3: 'Comment on egusphere-2024-3676', Anonymous Referee #2, 03 Aug 2025
In this study, Mejia and others present field observations of supraglacial streams in the Paakitsoq region of the Greenland ice sheet that show how the drainage paths of supraglacial lakes can change considerably between different melt seasons. The study provides valuable insights into the processes by which supraglacial streams form and transport melt water from lakes to moulins, which is very relevant information for understanding and modeling the impact of surface melt on ice dynamics. The manuscript is well written but would benefit from a few clarifications especially in the methods section.
General comments
1) Section 2.1 describes the stream mapping associated to Mars and ArcSav lakes in 2017 and 2018, but it does not mention Radical lake, the results of which are presented in Figs. 7 and 12. Was this data acquired in the same in-situ way or from e.g. WorlView imagery? The same is true for all the streams in the 2019 melt season. Although not delineated in any map (why not?), the 2019 stream paths are described in the last paragraph of the results section. At this point, the reader has to guess that this information is based on the WorldView image e.g. in the background of Figure 12. It would be good to state this explicitly. If that is how the stream paths were determined for 2019, would it be possible to use such a remote sensing approach to map streams on a larger scale or over more melt seasons?
2) Section 2.2 is slightly short for the reader to fully understand what was done. In particular, I am wondering about two points:
a) Does the `steepest descent algorithm` (L98) refer to the Wang and Liu (2006) method? If so, I suggest citing them again, otherwise it is not clear that their method was not only used for filling the depressions. Furthermore, `the steepest descent algorithm` commonly refers to a search algorithm in optimization that has little to do with how it is used here, so I also suggest avoiding this specific term.
b) How were the DEM-predicted catchments `divided according to the moulins identified in the field` and `corrected for supraglacial streams` (L101-102)? There must be a set of rules that were followed, for instance that streams were not allowed to cross catchment boundaries, etc.? How was this done and how much ambiguity was there in this correction?
Potentially, it could also be helpful and interesting to show the difference between the DEM-predicted catchments and the corrected one. If the corrections were substantial, it would mean that topography alone was not a good predictor of flow paths in this case, which could strengthen the message of this study.3) The manuscript has many figures with partially redundant information, perhaps this could be condensed. For example, the Mars and ArcSav stream paths are depicted in Figures 1, 2, 3, 8, 9 and 12. It takes some effort from the reader to figure out the differences between those. Furthermore, it is not clear why Figure 7 has a different design than Figures 3 and 9. Does it not show the same information, just for Radical stream instead of Mars/ArcSav? Why is there no 2018 profile for Radical stream?
Specific comments
L35: 'through' instead of 'though'?
L117: Technically, topography does not have a direction, perhaps use slope, downward gradient or similar. (Same in L126 and maybe elsewhere.)
L127-128: `... the river flowed downslope...` could be `down the surface slope` because technically the river always flows downslope.
L148: 'stream flowed upslope' sounds like water actually flowed uphill, see L127-128; there might be other such examples that I did not point out. I understand what is meant, and it is a very minor comment, but I still think it could be more precise. Or it could be clarified once in the beginning.
L159-160: `Mars Lake drained into Phobos Moulin...` is a slightly confusing sentence. Should it be Radical Moulin instead of Radical River? And it must have drained the Radical catchment, it could be more clear to add that name again.
L162-163: Do these numbers about all tributary streams come from the DEM-predicted flow path calculations?
L186-188: The whole sentence 'Larger July lake extents before drainage coincided with... ' is unclear. How can a larger lake extent coincide with a location of `upslope streamflow`? Maybe what is meant is that it coincides with `upslope streamflow` in time? The `together indicate` does not have a proper subject in the sentence, unless e.g. `coincided` is changed to `coinciding`, if that is what is meant.
L220-222: Why is snow deposition favored on top of snowplugs? It is not just the albedo that is responsible for snowplugs becoming high points?
L222: Shallow or flat? Shallow topography usually means that the ice thickness is small. This formulation was used in other places, too.
L250: `strong` slopes seems like an unusual formulation; high slopes is more common.
L275: Hoffman and Price (2014) may not be the appropriate reference here. Without knowing the article I would think it is a study that observed the timing of daily peak sliding speeds, which is not at all the case.
L306-307: `We find that ... is magnified at higher elevations ... where surface slopes are shallow and moulin density is low.` seems too strong of a statement here since this study analyzed three catchments in one particular location. I would expect such a formulation from a study that compared the flow of many more streams on a range of elevations and surface slopes. It is something that was discussed here and is expected given the presented data, but it is not a direct finding.
Citation: https://doi.org/10.5194/egusphere-2024-3676-RC3 -
AC2: 'Reply on RC3', Jessica Mejia, 08 Aug 2025
We would like to thank the reviewer for their constructive feedback our work. We believe the implementation of the reviewer’s suggestions will result in the improved clarity of our manuscript. We have attached a pdf that includes point-by-point responses to the comments raised by the reviewer, describing how we will update the manuscript to address reviewer comments.
Sincerely,
Jessica Mejia, on behalf of all coauthors
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AC4: 'Reply on RC3', Jessica Mejia, 11 Sep 2025
We would like to thank the reviewer for taking the time to review our manuscript and provide suggestions for its improvement. We have incorporated the reviewers comments in the revised manuscript which has improved the clarity of the methods section. In the attached document you will find how we have addressed each point raised by the reviewer's initial comments.
Sincerely,
Jessica Mejia on behalf of all coauthors
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AC2: 'Reply on RC3', Jessica Mejia, 08 Aug 2025
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