the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Jan 2026
Retreating glaciers and snow cover are amplifying summer droughts in the Adige River Basin (Italy)
Abstract. Snow and glacier meltwater play a critical role in sustaining summer streamflow in mountains and downstream regions. Yet, understanding glaciers' contributions to buffer river streamflow during droughts remains limited and pose major barriers to improve present and future water management within the context of climate change.
This study evaluates the contribution of glacier melt to summer flows and its mitigation effect of hydrological droughts in the upper Adige River basin, (Italy). We developed and implemented a new dynamic glacier module into the ICHYMOD-TOPMELT hydrological model, annually updating glacier area and improving the quantification of meltwater contributions under progressive glacier retreat (from 111 km2 in 1997 to 79 km2 in 2017).
The hydrological model exhibited robust performances (KGE = 0.89 for 1997–2022; 0.88 in summers) capturing observed glacier area, mass balance, and seasonal melt trends. Results show that glacier melt in the upper Adige River basin contributed to an average of 4.5 % to total summer streamflow during 1997–2022, with significant spatial variability and reaching 30 % in glacierized subbasins. During the severe drought of 2003, 2005 and 2022, glacier melt contributions ranged between 4 and 12 % at the upper Adige closing section. In 2003, high temperatures and limited SWE led to glacier melt accounting for 11.67 % of summer flows. By contrast, colder temperatures in 2005 reduced contributions to 4.85 % compounding with low SWE conditions and leading to a significative runoff deficit. In 2022, the combination of low precipitation, low snow cover and high temperature drove glacier melt. Differently than the 2003 drought, reduced glacier areas led to lower absolute contributions (8.17 %).
Our findings reveal that despite the increased melt rates in recent warm years, retreating glacier areas have reduced their absolute buffering effect. Glacier retreat is weakening their contribution to summer flows, increasing the upper Adige River basin's dependence on precipitation and snowmelt, which is also showing a decreasing trend. For these reasons, accounting for dynamic glacier and snow changes is essential to improve future drought projections and inform adaptive water management in glacier-fed basins within the context of climate and anthropogenic changes.
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Status: open (until 28 Feb 2026)
- RC1: 'Comment on egusphere-2025-6387', Anonymous Referee #1, 20 Feb 2026 reply
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RC2: 'Comment on egusphere-2025-6387', Anonymous Referee #2, 27 Feb 2026
reply
Shrestha, Terzi et al. present a study of the contribution of glacier retreat to recent droughts in the Alps. Using a framework that includes both a glacier model and a hydrological model, the authors produce a glacier-through-streamflow model from 1997 to present, and then focus on three drought-year case studies to identify the different amount of glacial contribution to streamflow, and thereby the role of glacier melt (or lack thereof) in buffering (or not) droughts in the region. This is an important contribution to the literature and addresses important questions about both the past and the future. I find that the most impactful results are those related to the role of glacial retreat in decreasing hydrological buffering of droughts, and specifically the use of counterfactual modeling. My recommendations are for edits that more clearly focus the manuscript on this question, at the expense of some more exploratory material about hydroclimate in general. I feel this will be a nice contribution to the field, but at present is not focused clearly enough on the main message, contains numerous writing errors, contains some important assumptions that are insufficiently justified, and the current analysis does not fully support the final claims. On the last point, I specifically mean that some claims about the comparative role of glaicers in different years’ droughts should be more clearly evidenced in the results section, with statistics and uncertainty, and the results should be clearly referenced in the discussion.
General:
- There are many grammatical and writing errors in the current version of the manuscript. I have highlighted some of them in the specific comments section of this work, but have not listed them comprehensively. I recommend the authors conduct a close reading to improve the clarity of the writing, even in places with no scientific comments.
- I recommend that the authors re-structure the introduction. As it stands, it reads
- the 2022 European/Italian alps drought
- glacier shrinking in Europe
- knowledge/modeling gap related to glacier modeling and glacier inclusion in hydrologic models
- the 2022 drought again
- what this study does
I would like to see a structure that more methodically frames the impact of this study, which I see as an accounting for the presence or lack of glacier buffering of droughts in several important drought years. To me, this would start with an explanation of glacier-driven hydrological buffering, then the argument that this may be lessening due to glacier shrinking, then that modeling is required to understand how much buffering occurs (and any technological gaps that might be related), then an introduction of the domain of this study (the 2022 Alps drought). - For the ICHYMOD-TOPMELT model: this seems to be an interesting and important application of this model. The discussion of the model does not make it clear which components were developed in previous work and which components are novel in this paper. Please make this more clear, which will also highlight the novelty of this work!
- Albedo calibration: There should be, at minimum, a sensitivity analysis performed on the albedo calibration, and more justification of choosing a singular albedo value. The resulting albedo value should be externally validated in some way (i.e., establish that the value is a reasonable value for glacier per the literature, observations, etc.) It seems that the results are moderately dependent on this value, but it is calibrated on only two individual observations and is held to be one singular value for all times of the year in all years. If such a calibration is required, all of these should be carefully justified. The choice to calibrated on albedo and not another parameter should also be justified.
- Focus on justifying all claims: The major point of this work is to quantify the role of glaciers in buffering/exacerbating hydrological droughts. I see opportunities to strengthen this argument with minimal analysis related to what you currently show and to focus more on this point than on a general accounting of drought conditions (which has been done in some of the literature in this region). Specifically, with your current counterfactual runs, can you present results on how much more buffering would have been provided if the glaciers had not retreated? I think that is the point of the counterfactual, but I do not see this result communicated very clearly. For example, there are currently 2 figures on the hydroclimate of drought, but the counterfactual, which I find more impactful, is a table that receives minimal discussion. What about a timeseries of streamflow showing the observation and then the counterfactual model? The discussion mostly focuses on the difference between dynamic and static, not on the use of the 1997 glacier area counterfactual, which I see as the more impactful result!
Specific:
Use “significant” not “significative” and always include a statistical justification of “significant” when using. Otherwise, use another word, like “meaningful,” “observable,” or “large”
L26: droughts (plural)
L28: using SWE acronym without first defining it
L29-30: Sentence “By contrast…” is written unclearly, please rephrase
L31: maybe “in contrast to” instead of “differently”
L34-35: be clear over what region you make this claim
L35: is the dependence showing a decreasing trend or are precipitation and snowmelt? Rephrase to make this clearer
L51: grammar corrections to “39% of European glacier [is this count? mass? Area?] was lost”
L53: “recession” singular
L56: grammar corrections
L73: This is the second portion of the introduction that introduces the 2022 drought – please restructure the introduction to have this information in one place
L75: clarify if the Adige River Basin is part of the Italian Alps (and therefore an example of the broader context you are describing). Refer to figure 1 here
L111: give the streamflow gauges a density too? #/length of river?
L112: citation or link, not hyperlink
Figure 1: the lake and glacier colors are extremely similar and the lakes and glaciers are quite small so it is difficult to tell them apart
L140: should this be “lower, fixed albedo” ? -- how would a high and fixed albedo enhance absorption and accelerate melt?
L157: why is n=3?
L184: is the glacier albedo calibration a standard practice in this modeling? To me, this seems like a huge range of potential albedos that could have a massive impact on the results – what measures were taken to ensure that the calibrated albedo matches real albedo of the glaciers at this time (especially given the fact that we have spaceborne albedo estimates for many of these regions during this timeline).
Figure 2: What is the time interval for plotting the area of the glacier? If it is relatively coarse, use markers to show when the data points appear. If it is very fine, I am surprised to not see seasonality in glacier area – can you explain why this does not appear? Would there not also be a moderate amount of seasonality in overall albedo?
L206: why SWE at the end of June? This seems to be an unusual choice, given that significant melt occurs earlier in the spring. (April and May)
L230-233: the mean includes the years with positive change (unless you excluded them from the mean?) I think you mean here that qualitatively, there is minimal deviance from the long-term linear trend in any individual year (overall the signal is monotonic and linear), but that there are a few exceptions.
Fig 3 caption: clarify in the caption that the increase is specifically the increase when making the glacier model dynamic.
Fig 3: If there are 65 subbasins in this study, why are most of them not included in this figure?
L266-269: cite for “previously identified” Also, this is discussion/conclusions, not results
L285-286, 292: 0.47 and 0.55 are not “strong” correlations – they are moderate
Section 3.3: this is an interesting section on the hydroclimate drivers of melt, but it is not clear how this is related to the overall message of the paper, which is on modeling the contribution of glaciers to buffering (or not) droughts. For example, some of these topics are explored for the same years and regions in Brunner et al., 2023 (see fig. 4). It is also somewhat unclear from this section (and the corresponding methods section 2.5) which of the values used are from the model and which are from observations/reanalysis.
The following section on counterfactuals is much more related, and, in my opinion, more novel.
Figure 5: please change the color scale to something more intuitive on all graphs. As currently presented, the same color can have a different meaning (and different unit!) about the same variable in different columns. Further, some rows have diverging colormaps for both the value and anomaly while others do not, regardless of whether or not the quantity is diverging. I recommend picking non-diverging colormaps for each quantity, and then a different, maybe even standard between all variables, colormap for the anomalies. Also, improve the coherence of meaning between variables. For example, white currently means a high amount of runoff, a low amount of glaciermelt, and a medium temperature.
Discussion: avoid repeating the results directly, but rather reference them (by figure, table, or section) and then explain the meaning. There should be more citations throughout this section as well, whenever comparing to a prior publication.
L384: reference where this is shown in the results
L435: Suggest completing the public release concurrently with this paper. Available by request does not satisfy open science guidelines, if this is what you are pursuing.
Review References
Brunner, M. I., Götte, J., Schlemper, C., & Van Loon, A. F. (2023). Hydrological drought generation processes and severity are changing in the Alps. Geophysical Research Letters, 50(2), e2022GL101776.
Citation: https://doi.org/10.5194/egusphere-2025-6387-RC2
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Shrestha et al. use a hydrological model (ICHYMOD-TOPMELT) with a newly developed and implemented dynamic glacier module to assess glacial meltwater contribution to streamflow in the upper Adige River Basin, Italy. The authors focus primarily on the contribution of meltwater to streamflow during three drought years (2003, 2005, and 2022), as glacial melt provides a buffering mechanism to hydrological drought caused by lower snowmelt. The authors find that the 2022 drought, despite warmer temperatures, showed a lower contribution of glacial melt to streamflow than the similarly warm 2003 drought due to there being a smaller glacier area in 2022. I think this is a very important topic, and the results are clearly outlined. It is also clear that the addition of the module they developed mostly improved the results in individual sub-basins. However, I have a few recommendations to make the paper potentially more impactful.
General recommendations:
Line-by-line: