|This paper addresses the problem of snow redistribution in conceptual hydrological models. The authors present a methodology for the redistribution of snow in the conceptual distributed rainfall-runoff (RR) model COSERO at the scale of 1 km2. The snow is redistributed from the summit of the mountains to the valley using the slope angle to determine the route of the snow redistribution, and the snow density and vegetation cover to determine the amount of snow available for transport. The implementation of the snow redistribution module in the RR-model improved the model efficiency regarding the estimation of the snow cover and the discharge in the Ötztaler Ache catchment in Austria. The main improvement of this method is the elimination of “snow towers” at high elevations, which is a well-known problem for hydrological modeling in mountainous regions. |
The study presented in this paper represents an important contribution to hydrological modelling in mountainous regions, since the inter-annual accumulation of snow at high elevations is a well-known and unsolved problem. Furthermore, snow redistribution and therefore the variability of the snow cover may influence the discharge and the glacier mass balance. The method is clearly described in this paper and shows one interesting and simple approach to deal with the problem of snow redistribution in conceptual models. Although the method is certainly connected with uncertainties, the idea is worth to be published. However, the presented paper needs to be improved on four substantial aspects (described below). The issue of the manuscript is in the scope of HESS and I would support its publication after major revisions.
(1) Aims of the study: Throughout the paper, the authors focus on saying that the main goal of the study is to better model the discharge. In fact, the improvement of model efficiency achieved by the simulation with snow distribution compared to the usual model is very little for discharge (KGE 0.92 vs 0.9). In my opinion, the main improvement achieved by the model is the elimination of “snow towers” and the better representation of the snow cover according to the efficiency coefficient r2 (0.66 vs 0.74), which can indicate, as suggested in the conclusion, that the model works right for the right reasons. Therefore I suggest that the authors focus their discussion more on the improved modelling of the snow cover, snow melt, and glacier melt and less on the improvement of the simulated discharge. This would also allow being more optimistic regarding the potential of such a conceptual model, since it is stated in the discussion that snow redistribution is not important for discharge modelling at the large scale (p.13, l.20-21).
(2) Transferability of the method: The presented method showed very interesting results in the Ötztaler Ache catchment in Austria, but was it also tested in other catchments? Can you find similar results in other alpine catchments with different topographical properties, land cover, etc.? How transferable is this model? Since the model is conceptual and therefore should not need long calculation time, I suggest that the authors support their findings and test the model in 2-3 other catchments or at least discuss the transferability of their model in the discussion. This would add validity to the presented study.
(3) Reliability of the model: The authors should discuss how their results are situated compared to other studies. Are their results reliable? How did other studies model snow redistribution? Do they come to similar results (physically-based as well as conceptual model)? If not, why not? The model results for example show enhanced glacier melt (100 mm/yr) because of the glacier area being snow free earlier during the year (p. 12, lines 16-17; p.14, line 10-12, fig.8). Are 100 mm/year in agreement with mass balances studies made on glaciers in the catchment or in catchment with similar properties? A comparison of the results with other studies related on snow redistribution would improve the reliability of the results and therefore also of the model.
(4) Structure of the paper: The structure of the paper presents some weaknesses that need to be improved:
a. Overall, the introduction makes a good review of the snow transport processes and the existing models. However this section needs to be restructured more logically. For example in section 1.1 processes responsible for the variability of the snow cover are described, that are not all snow transport processes. I suggest that the authors rename this section accordingly. As other processes are also involved in the variability of the snow cover, they should better explain why their model focuses on the snow redistribution. The objectives of the study need to be reformulated (see above, point 1).
b. The authors should pay attention not to put results or part of the discussion or the methods in the figure captions. The figure captions should describe the figure but not analyze it. For example fig. 7: “Specific runoff at the outlet at Huben modelled with (model A) and without (model B) using the snow redistribution routine.” This is enough for the caption of the figure, the rest “In the early snow melt period, more runoff is generated by model A because snow accumulates rather in lower than in higher levels. In summer, enhanced glacier melt leads to more runoff by model A.” is part of the results and should be moved to the results section. The caption of fig. 2 to fig. 11 should be revised.
c. The conclusion should be reformulated. It should summarize the main findings of the study and relate to the importance of the work. Part of the conclusion should be moved to the discussion. The second paragraph for example (p. 14, lines 13-20) is rather a discussion of the results than a conclusion. This is also the case for the last paragraph (p.14, lines 26-29). In comparison, paragraph 3 belongs perfectly to the conclusion since it points out the contribution of the paper and the further work that needs to be done to have better hydrological models.
d. The language of the paper can be improved. Especially, long sentences should be avoided, since it makes them difficult to read and to understand.
p.1, l.18: observed discharge and MODIS derived snow covered area instead of “runoff and MODIS data”.
p.1, abstract: The improvement of the model efficiency for snow cover (r2) is good and should be added to the abstract.
p.2, l.4 and throughout the text: “degree-day” instead of “day-degree”
p.2, l.7-9: Introduce here the term “snow-tower”.
p.2, l.13 & l.18-20: what is the difference between micro/macro scales and plot/catchment scales? If it is the same, there is no need to define it a second time and only one term should be used.
p.3, l.25-28: The meaning of the sentence “Scipión et al. (2013) however…” is not clear to me, please reformulate.
p.5, l18-30: It is not clear to me how the 5 classes are included in the modeling. Is the SWE of the grid cell the mean value of all classes? Please describe how the classes are taken into account and explain in the discussion how the use of the classes influences the results.
p.5, l30: Define the abbreviation “SWE”.
p.6, l.6 & throughout the text: use the same writing for all the units “[°C]”, “[mm° C-1]”.
p.6, l.25: “… a grid cell is partly snow free (due to melting) …” How can a grid cell be partly snow free? Does that mean that at least 1 of the 5 classes is snow free?
p.7, l.5: the unit for water holding capacity is missing.
p.7, l.9: the unit for refreezing factor is missing.
p.8, l.9-10: “Several authors…” this sentence belongs to the introduction or to the discussion
p.8, l.17-18: with “the portion which is available for the redistribution routine” is fp meant here? Clarify.
p.9, l.5-7: “Since other geomorphological properties […] as driving force for the model”. Move to the discussion and rephrase.
p.9, l.18-19: “A similar approach …” How similar? How does the other approach work? What is new in the presented approach? Discuss it and move to the discussion.
p.11, l.8: fig. 7 instead of fig.6. Please check the reference of the figure numbers throughout the text.
p.11, l.10-11: “…the model efficiency (KGE) could be improved for the discharge by 0.05…”
p.11, l.15: replace “than” by “compared with”.
p.11, l.15: Describe the results of fig. 8 (glacier melt and snowmelt) here more in detail.
p.11, section 4.2: equifinality should come after section 4.3 and 4.4
p.11, l.24-28: “So are cold periods in the summer,…” This sentence is very hard to understand. Please rephrase and split in 2-3 sentences.
p.11, l.28: Talk here about the improvement of the model efficiency found for snow cover (see table 2).
p.12, section 4.4: snow accumulation is unclear; rename the section with “inter-annual snow accumulation”.
p.12, l.2-3: “The main reason…” this should be in the introduction or in the discussion.
p.12, l.15-23: Discuss the peaks in runoff difference in September and October. From the snow redistribution, one would expect that snow is redistributed from the steepest slope to the glacier area leading to more snow on the glacier area (e.g. Kuhn 2003). How can you explain that the glacier get snow free earlier in the year? Is the entire glacier area getting snow free or only the lower part? Please discuss this more in detail.
p.12, l.29: snow depth or snow cover area?
p.13, l.3-5: I disagree, some small differences can be observed between the 2 models and what about the better efficiency r2?
p.13, l.20-29: The discussion should not end on a negative thought. It might not have signification for the large scale modelling, but it still allows to better model the snow covered area.
p.13, discussion: A discussion on the meaning and the influence of the chosen scale is missing and discuss the patterns of the net snow deposition (fig.11).
p.13, discussion: Discuss the problem of equifinality more in detail (citing only the supplement is not enough). The main findings of the Monte Carlo simulation (from the supplement) should be summarized in the discussion for better understanding. Why is it expected to have this problem and why is the model anyhow reliable? Cite references to support your point of view.
p.14, l.18-20: “consequently the objective function…” I don’t understand this sentence. Please rephrase. Where is the Rosenbrook’s routine used (refer to the supplement)? It “differ much” from what? Please explain and move it to the discussion.
p.22, table 1: Please give the unit of snow holding capacity (-). And please give the reference for the CORINE land cover data.
p.23, Table2: “Note, that…” does not belong to the table caption, please move to method.
p.29, Figure 6: For clarity, number the different graphs (with a), b))
Throughout the text:
-Be consequent with the expressions used. For example snow redistribution/snow transport, avoid using both for clarity reasons.
-use the terms runoff and discharge adequately, they are no synonyms.
Kuhn, M., 2003. Redistribution of snow and glacier mass balance from a hydrometeorological model. Journal of Hydrology, 282(1-4), pp.95–103.