The Role of Debris Cover in Catchment Runoff: A Case Study of the Hailuogou Catchment, South-Eastern Tibetan Plateau

Round 1

Reviewer 1 Report

Review of manuscript water-614240:

“The Role of Debris Cover in Catchment Runoff: A Case Study of the Hailuogou Catchment, South Eastern Tibetan Plateau” by Yong Zhang et al.

This study examines the impact of spatially-varying debris cover depth on glacier meltwater runoff on three debris-covered glaciers in the southeastern Tibetan Plateau. The manuscript is clear and the results are interesting. I don’t have any major concerns with the manuscript but I do have some thoughts/points outlined below that I’d like the authors to address. Below those are a list of minor editorial/clarification comments to be addressed in the revised version.

1) The debris-cover map in figure 1 indicates high debris cover along the northern edge of all three glaciers, and the meltwater results do indeed indicate that these northern edges are where the insulating effect is strongest (Figure 5). This suggests that the slope of the surrounding topography, which provides the debris, is important and looking at the elevation contours it appears that the slopes of the topography north of all three glaciers is indeed steeper than on the southern sides. Is that the case? And could it then be argued, or at least mentioned in the discussion, that the slope angles of the topography surrounding such glaciers is an important factor in providing the debris that then controls the meltwater?

2) I realize debris cover is difficult to determine (even in situ on the larger Himalayan glaciers) but the thermal resistance map used here appears to work quite well on the scale of this study. Also important is the threshold between the debris acceleration/insulating effects and what debris thickness to use as that threshold. I know it depends on the lithology, but my question is: in this study is it possible to say that there is a particular thermal resistance number (to my eye it looks to be about 15-17, maybe?) that could be used as that threshold? Might that be useful for other glaciers if, given a map of thermal resistance, one could say that above this value (17?) the glacier is fairly well insulated? I’d be interested to hear the authors’ thoughts on this.

3) line 162: Why is the value of 2 degrees used as the threshold for the snow/rain transition? i.e., why is it not 0 degrees? I agree with the use of a temperature range over which it can be a snow/ rain mix, and here a +/- 1 degree window is used, but shouldn’t that window be centred around the freezing point of 0 degrees and not around 2 degrees? Using 2 degrees as the threshold means that the lower temperature bound for the snow/rain mix, which should be all snow, is still at 1 degree above freezing. Some justification is required here since it doesn’t appear to make sense physically.

4) In the sensitivity tests (which I liked, by the way) the more sophisticated snow/rain scheme produces a 6% decrease in glacier mass loss and a 1.3% decrease in total catchment runoff. But if glacier runoff is about 45%, how does this 6% decrease in glacier contribution translate to a 1.3% total decrease? Shouldn’t the total decrease be more like 3%? i.e, 45% of 6%=2.7%? Unless there are other factors at play that have not been mentioned that also decreased? Some clarification is needed here.

5) Did the authors do any tests to determine the statistical significance of the temperature increase and the precipitation decrease in the recent decades?

6) Is it possible to present a time series plot of glacier runoff (from data) to show that it has been increasing in recent decades? Or even just the total runoff from the gauging station would suffice since it appears that glacier runoff is the dominant contributor (~45%). The reader sees the model’s runoff results but not the actual data. It would be nice to see them both visually, in addition to the % scores of the Model Performance section.

Minor comments:

line 19: “…observations combined with a…”

line 34: “In the context of…”

line 43: “…Tibetan Plateau and surroundings host a large number…”

line 49: “…supraglacial debris cover is likely to increase with glacier shrinkage…”

line 58: either “…for a debris-covered surface” or “…for debris-covered surfaces”

line 84: “…on the three larger glaciers”

line 85: “Of the three debris-covered glaciers…”

line 89: “…indicates a highly heterogeneous distribution…”

line 101: either “Precipitation increases with altitude in the catchment…” or “Precipitation increases with increasing altitude in the catchment…”

line 108: “…from the GAEORS near the terminus…”

lines 110-111: “…are required as input variables for our model and are available…”

lines 124-125: I’m not sure what the end of this sentence means. Presumably the DEM is used to compute the area of each elevation band for use in the semi-distributed model (fully distributed models do not use elevation bands). But the meteorological dataset is not distributed at all…it’s derived from the one station’s data then extrapolated as described with the lapse rate, etc. So please rephrase this sentence for clarification.

line 134: “…which consists of all melt-…”

line 147: “…based on the DEM…”

line 152: either “…decreases with altitude, while…” or “…decreases with increasing altitude…”

lines 156-158: Please state the values used for the temperature lapse rate and the precipitation gradient.

lines 163-164: “…below the temperature threshold.”

lines 170-171: This is a confusing sentence because on the first reading it seems to say that there are no energy fluxes between the atmosphere and the underlying surface (debris or ice), and this is not the case from the energy balance algorithms detailed later. I would rephrase: “With the exception of considering the energy exchange between debris-covered/debris-free surfaces and the atmosphere” to be “In addition to the energy fluxes between the atmosphere and the underlying surface (debris or ice), the model accounts for…”

line 177: I’m confused by “…no heat conduction into ice…” because there is heat conduction into the ice as described subsequently (specifically line 185). I think this needs rephrasing for clarification.

line 177: I don’t see how a heat flux can be stored. This needs to be rephrased. Heat can be stored, but not a heat flux.

line 191: “follows” not “following”

line 212: re. Nash-Sutcliffe. This is a personal comment just for the authors. John Sutcliffe was my uncle (Nash was his student) and coincidentally while I was reviewing this manuscript he died peacefully at age 94 in England last week. I’m glad to see his efficiency index has been an enduring contribution for him!

lines 335-336: I don’t quite understand the following sentence: “Especially these debris covered glaciers contain a large ice volume”. Does it simply mean that these debris-covered glaciers are of large volume? Which doesn’t really mean much, only that they are large glaciers that happen to have debris on them. Not sure this sentence is needed.

line 367: either "slightly increased trend” or “slight increasing trend”

line 390: “…leads to a slight decrease in…”

line 391: replace “less contributing thereby to an increase” with “thereby contributing less than other components to the increase…” or something like that.

line 409: delete “Especially”

line 492: replace “…in a glacierized catchment. Noting that our calculations are influenced by model limitations discussed above.” with “…in a glacierized catchment, although our calculations are influenced by model limitations as discussed above.”

line 493: “understanding of”

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

See pdf

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

See pdf

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf