|Second round of comments on ‘Estimating surface mass balance patterns from UAV measurements…’ by Lander van Tricht and others, submitted to The Cryosphere after revision|
I’d like to congratulate the authors on a nicely structured and well-presented study, which was a pleasure to read. In their manuscript revision, the authors have carefully addressed all my comments (and those of the other reviewers). They have consequently made careful adjustments to their methods and throughout the manuscript text. I have a few minor comments below that have no impact on the calculations, discussion, or conclusions. I am very happy to recommend the manuscript for publication following these technical corrections.
L52. Suggest ‘do therefore not’ -> ‘therefore do not’
L96. Suggest ‘both glaciers disconnected’ to ‘the glaciers disconnected’ as the glaciers are disconnecting from one another rather than from a third body
L112-113. A bit awkward – ‘…has been measured twice , and again in 2020…’ perhaps simplify to ‘… has previously been measured twice .’ as the new measurements are described in L208-214. I’d recommend some basic depiction of the new measurements in an Appendix (Table?) or Supplementary Material , or you could indicate transects or spot measurement locations in Figure 3c. Perhaps mention these data in the Code and Data Availability statement -> I guess they have been or will be submitted to GlaThiDa?
L209. Suggest ‘considerable effect for’ -> ‘considerable effect on’
L290. A small detail, but Brun et al (2018) tested different sizes of Gaussian filter for this calculation. What size Gaussian do you use?
L408. The ratio correction works since the surveyed area is almost entirely within the ablation area. However, if your survey area was the entire glacier, the net flux divergence would be zero by definition, and the ratio approach would set flux divergence to zero everywhere – not ideal! I don’t think you need to recalculate with a different method, though; instead, perhaps just remind the reader of the survey domain ‘To provide for conservation of mass _within our survey area_’
Figure 6 is referenced before figure 5.
Table 2. Could you additionally indicate the number of GVPs?
L473. It looks like the coregistration of the 2017-2018 DEM pair is slightly inferior to the other pairs (looks like the 2017 DEM might be shifted). I don’t think this is a problem but it’s worth keeping in mind in the discussion and your uncertainty testing – this was your first UAV survey in the area and had the least constraint from control points, so this is not surprising. My main question from this is – what level of uncertainty to ascribe to the DEM-differencing? To me it looks like 20cm is ample for 2018-2019 and 2019-2020, while a higher value is needed for 2017-2018. It is not surprising that this is slightly higher than you might derive from Table 2, but these domains are to some degree the worst-case for repeat surveys (steep slopes), entirely independent from the GCPs, and this metric includes the uncertainty from each of 2 surveys. The only change to make is that in the uncertainty section you should make reference to these results to justify the chosen perturbation for dh/dt
Figure 6. Nice depiction of the results before and after flow-correction.
L515 and 516. Possibly broken internal cross-reference links to Figure 7?
L521-525 could go into methods after current L320
L534. The contours are for velocity (m yr-1 rather than m), right?
L618, 630, 631. Broken cross-reference to a Figure
Figure 12. Nice! It would be very interesting to quantify the mean or median absolute deviation of differences, and possibly the mean error ablation error from a simple linear fit to stakes; these would be relatively simple numbers to put in the text ~L668
L712. The qualitative description here is fine, but it would be nice to have a basic numerical justification to support the findings. Perhaps you could indicate a representative MAE for the THIZ/THIL setups here as for the discussion in 5.2.
L739. I’d recommend to give the actual value here (PM average).
L786-787. Can you provide the values here?
L789. The method can certainly be applied, but may not be as _robust_