Seasonal evolution of the supraglacial drainage network at Humboldt Glacier, North Greenland, between 2016 and 2020
Abstract. Supraglacial rivers and lakes are important for the routing and storage of surface meltwater during the summer melt season across the Greenland Ice Sheet (GrIS), yet remain poorly mapped and quantified across the northern part of the ice sheet, which is rapidly losing mass. Here we produce, for the first time, a high-resolution record of the supraglacial drainage network (including both rivers and lakes) and its seasonal behaviour at Humboldt Glacier, a wide-outlet glacier draining a large hydrologic catchment (13,488 km2), spanning the period 2016 to 2020 using 10 m spatial resolution Sentinel-2 imagery. Our results reveal a perennially extensive yet interannually-variable supraglacial network extending from an elevation of 200 m a.s.l to a maximum of ~1440 m a.s.l recorded in 2020, with limited development of the network observed in the low melt years of 2017 and 2018. The supraglacial drainage network is shown to cover an area ranging between 965.7 km2 (2018) and 1566.3 km2 (2019) at its maximum seasonal extent, with spatial coverage of up to 2685 km2 recorded during the early phases of the melt season when a slush zone is most prominent. Up-glacier expansion and the development of an efficient supraglacial drainage network as surface runoff increases and the snowline retreats is clearly visible. Preconditioning of the ice surface following a high melt year is also observed, with the earlier widespread exposure of the supraglacial drainage network in 2020 compared to other years; a finding that may become representative with persistent warmer years into the future. Overall, this study provides evidence of a persistent, yet dynamic, supraglacial drainage network at this prominent northern GrIS outlet glacier and advances our understanding of such hydrologic processes, particularly under ongoing climatic warming and enhanced runoff.
Lauren D. Rawlins et al.
Status: final response (author comments only)
- RC1: 'Comment on tc-2023-23', Anonymous Referee #1, 22 Mar 2023
- RC2: 'Review on tc-2023-23', Anonymous Referee #2, 04 Apr 2023
Lauren D. Rawlins et al.
Lauren D. Rawlins et al.
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Rawlins et al TC 23
This is a super well written, clear, thoroughly referenced (rare!), and interesting paper. I don’t get many of these to read, so this was a delight. I do have some suggestions below for how the paper could be improved, but they are all easily done and I think will make the paper stronger. I am very familiar with the subject matter and literature here, so my eyes glaze over on the methods sections and I took them for granted.
Slush definition- Slush zones are a key part of these results, but the authors have not described how they were defined/mapped/identified. I’d like to see their definition of a slush zone and how they uniquely identified them in images from other supraglacial features.
MAR uncertainty- I do not think the authors need an ensemble of models, but I would expect to see some discussion of the fact that MAR (or any coupled or uncoupled atmos-ice model) is our least bad representation of reality. There are known issues in this sort of modelling, and errors in the model will only strengthen your conclusions by potentially reducing scatter. Figure 5, for instance, should have a MAR uncertainty plotted on it, perhaps as a confidence interval. Discussion should also be added.
MF uncertainty- as above, please discuss uncertainties in your MF. You have spatial errors resulting from pixel size, spatial errors from undetected sub-S2 channels, DEM resolution errors, and errors of classification that might omit/comit water area. This seems a larger omission from the paper- I (and readers) want to know where the method is good and where it is not. Since you aren’t about to map a scene manually to provide true validation (although I wouldn’t object to that!), I think you can propagate the variance from each of those terms based on the literature surrounding your classification methods. I think this information is essential to this paper.
Drainage density and other stats- you have the data to create a vector river network and determine drainage density, stream orders, and other stats for comparison (see last point). Author Yang has published many papers on this topic and therefore I believe this should be a straightforward task that would add needed richness to this paper in terms of comparison.
Split MF into river and lake areas- I am quite interested in this divide. This is figure S3, but for me this belongs in the main text as a very interesting expression of the supraglacial hydrology here. Some discussion should also occur.
Section 5.4- I am ok with this section as it is fairly hedged and well referenced, but other reviewers may not like to see such speculative conjecture.
Missing comparisons- the 2nd major omission I see (beyond uncertainty discussion noted above) is a lack of comparison to the rich literature of the SW GrIS. This is the right paper to use the discussion to first outline this bit of ice sheet (as you have done) and then explicitly compare to the SW to see what is the same and what is different- are the fractions of rivers and lakes the same? Elevations of highest melt features? Density of persistent channels? Channel lengths? Width distributions of these channels? Prevalence of slush zones/bare ice and their interaction with the network? I think you have all the data to answer those questions (and more) and I think this paper really needs it to move this beyond an interesting and well written observational study into a richer contextual understanding of this unique bit of ice. I’d like to see these differences quantified where possible (e.g. from a vector network) or discussed qualitatively and referenced where not possible (as you have nicely done throughout this paper!!)