|Review of “Rapidly-changing subglacial hydrology pathways at a tidewater glacier revealed through simultaneous observations of water pressure, supraglacial lakes, meltwater plumes and surface velocities” by How et al.|
This is my second review of this manuscript, and although it has certainly improved since I first reviewed it, some of the changes and additions are superficial and do not deal sufficiently thoroughly with my initial suggestions (and those of the other reviewers). Also, there remains a tendency for unjustified speculation. On several occasions (e.g. regarding the potential effect of tides on periodic drainage of subglacially stored water) the speculation from the first submission remains, but an extra sentence along the lines of ‘Although we can’t tell with our data, it would be an interesting topic for future work’ has been added. I don’t think this is good enough. Surely for the specific case of tidal influence, a quick check of the relative frequencies of plume visibility and tides would be informative and is fairly simple to achieve?
Title: ‘hydrology pathways’ should be ‘hydrological pathways’
P1, L3-4: ‘water pressure’ should be ‘borehole water pressure’
P2, L3: ‘provides’ should be ‘provide’ (data are plural)
P2, L5: ‘across’ should be ‘beneath’
P2, L11: ‘marine’ plume seems an odd term to use here as these could be caused by wind-driven upwelling for example. How about ‘glacial’ or ‘meltwater’ plume instead?
P2, L22: Is it worth including some references for the observation of high pressures for most of the melt season?
P2, L24: ‘and’ should be ‘where’
P2, L26-27: This has also been observed at land-terminating margins of the Greenland Ice Sheet
P2, L27: ‘are’ should be ‘is’
P3, L4-5: I’m not quite sure what is meant here by the ‘two component structure’. I think this requires clarification.
P3, L6-7: The wording here is a little confusing. What if the implementation of the approach were perfect but the model didn't include key features of the real system? If model outputs no not match real-world velocities, is the representation of the subglacial environment really 'adequate'?
P3, L10-11: But lakes are also formed from surface melt, so this is not really 'additional'. This needs to be clarified.
P3, L11: Once drained (and therefore once a surface-bed connection has been made), the perched lakes become subglacially connected? I think this characterization is a bit confusing. How about surface melt-fed vs. subglacially-fed or something similar?
P3, L14: be clear that you mean supraglacial lakes here.
P3, L15: ‘contains entrained’ instead of ‘entrains’
P3, L22: ‘Detailed’ is vague. Do you mean higher temporal and/or spatial resolution?
P3, L25: ‘meltwater in the subglacial zone’ could be replaced with ‘subglacial meltwater’
P3, L26: The sentence beginning ‘This has…’ does not add anything useful – what other settings are there anyway?
P3, L27-28: ‘to make a channelized system become more efficient’ could be ‘to increase the efficiency of a channelized system’
P3, L29: ‘significant periods of time (i.e. decadal)’ should be ‘decadal timescales’
P3, L33: How do these long residence times this relate to surface uplift which typically recedes within 48 hrs? Is there any evidence (apart from modelling) which supports this idea of long residence times of subglacial water 'blisters'? Also, the excessive use of parentheses to characterise timescales is unnecessary (e.g. see my suggested simplification above).
P4, L10: Both satellite and time-lapse imagery are always temporally intermittent (unless you use video!). It is the relative rate of change in plume behaviour vs the temporal frequency of image acquisition which is important.
P4, L12-13: I don’t think this adds anything worthwhile.
P4, L27: I think the calving rate should be positive without the context of its net impact on glacier mass balance (which I suppose would be negative).
P4, L29: Consider replacing the second use of retreat with ‘receded’ to avoid repetition.
Figure 1 caption: ‘8 km LONG tongue’ otherwise not obvious which dimension you are referring to.
P7, L2: ‘underwent refreezing’ should be ‘refroze’
P7, L29: Both models should be cited here.
P8, L14: ‘up-glacier’ rather than ‘upper’
P8, L20: ‘is’ should be ‘are’
P8, L23-24: The fact that the GPS did not provide any extra useful information seems surprising - were the data simply too noisy?
P9, L18: ‘on’ should be ‘in’
P11, L22: How was the runoff ‘detected’? Surface lakes become visible or plume first reaches the fjord surface?
Figure 3 caption: You cannot know that the lakes are entirely drained from the camera angle… just that you cannot see any water in them.
P14, L29: ‘The relative timing of these components…’ could be ‘The relative timing of variations in these components…’
Figure 5: It is difficult to tell the difference between >2.4 m/d and no data. Could you use a more effective colour scheme?
Figure 5 caption: use ‘up-glacier’ instead of ‘upwards’
P16, L20: ‘enable ENHANCED glacier flow’ (after all, the glacier is already flowing…)
P16, L21: Some missing words? ‘but the influence of submarine ice melt on glacier dynamics could be…’?
P16, L23: ‘supported because of the coinciding observations’ could be ‘supported by coincident observations’
P16, L26: Other surface catchments perhaps? But not other catchments as determined by the combined bed and surface topography surely?
P18, L6: ‘this’ is a bit ambiguous. How about 'the sequence of events described above'
P18, L11: The hydraulic connection (rather than the distance per se) is key here: is the borehole connected to the channelised system and the adjacent regions which experience temporal variations in water pressure? From the data it looks like it is not (although you say that the water level in the borehole dropped substantially when the bed was reached).
P18, L16-17: No. The speedup propagates all the way up-glacier to the extent of the data shown. It would be useful (as I mentioned in my previous review) to have plots of the relative change in ice flow (i.e. percentage change for each pixel) using data from the earliest image pair as the baseline. This would show the speed-up much more clearly.
P18, L17: ‘appear to be’ should be ‘are’
P18, L27-28: But you have already admitted that the borehole is unlikely to be connected to the main hydrological system. Therefore you cannot make inferences about the extent of variations in meltwater storage at the bed from the borehole data.
P18, L25: ‘influenceS’
P18, L26: ‘aN’
P20, L13: The speed-up IS a change in glacier dynamics...!
P20, L21: ‘tongue is a heavily crevassed surface’ could be ‘tongue is heavily crevassed’
P20, L23: Why a possibility? Why don't you check? Timelapse? Satellite data?
P20, L27-28: Even with a distributed subglacial drainage system? Wouldn't you expect the plume to be more spread out along the calving front if this is the case? Do you see this?
P21, L13: ‘coincident timing’ could be ‘coincidence’
P21, L16: ‘thr’ should be ‘the’
P22, L9: Sufficiently melted the cavity wall for what?
P22, L10-11: This is really not worth including without some further analysis: for a start, does the pulsing show any variation on tidal frequencies?
P22, L32-33: But you have already admitted that the borehole is unlikely to be connected to the main hydrological system. Therefore you cannot make inferences about the diurnal subglacial water pressure variations from the borehole data.
P23, L8: But not every inch of the bed will be connected to the channels even in an ‘efficient catchment’ (although I’m not sure what that is)
P24, L24: Really unprecedented? How do you know? Within the last few years only?
P25, L1-2: ‘the basal pressure environment’ should be ‘basal water pressure’