the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Oct 2021
Quantifying the glacial meltwater contribution to streams in mountainous regions using highly resolved stable water isotope measurements
Abstract. This study aims to determine the contribution of glacial meltwater to streams in mountainous regions based on stable water isotope measurements (δ18O and δ2H). For this purpose, three partially glaciated catchments were selected as the study area in the central Swiss Alps being representative of catchments that are used for hydropower energy production in Alpine regions. The glacial meltwater contribution to the catchments’ stream discharges was evaluated based on high-resolution δ18O and δ2H measurements of the end-members that contribute to the stream discharge (ice, rain, snow) and of the discharging streams. The glacial meltwater contribution to the stream discharges could be unequivocally quantified after the snowmelt in August and September when most of the annual glacial meltwater discharge occurs. In August and September, the glacial meltwater contribution to the stream discharges corresponds to up to 95 ± 2 % and to 28.7 % ± 5 % of the total annual discharge in the evaluated catchments. The high glacial meltwater contribution demonstrates that the mountainous stream discharges in August and September will probably strongly decrease in the future due to global warming-induced deglaciation, which will be, however, likely compensated by higher discharge rates in winter and spring. Nevertheless, the changing mountainous streamflow regimes in the future will pose a challenge for hydropower energy production in the mountainous areas. Overall, this study provides a successful example of an Alpine catchment monitoring strategy to quantify the glacial meltwater contribution to stream discharges based on stable isotope water data, which leads to a better validation of existing modelling studies and which can be adapted to other mountainous regions.
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Interactive discussion
Status: closed
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RC1: 'Review of hess-2021-512', Bettina Schaefli, 18 Nov 2021
This paper proposes to quantify the contribution of ice melt to total streamflow in three highly glaciated catchments in the central Swiss Alps with the help of astable isotopes of water. The aim is to come up with results that are more reliable than previous modelling-based results and with recommendations for future sampling campaigns.
I cannot recommend the paper for publication because some fundamental hydrological process knowledge is ignored. The obtained results are not plausible (glacier melt contribution of between 80% and 95% to total streamflow during August in catchments with only between 6 and 28% glacier cover). One key result is summarized in Figure 8, which shows glacier melt in Mio m3 against glacier area. For the smallest glacier investigated, this result indicates meltwater production of 4*106 m3 on an area of 0.3 km2, which corresponds to a melt water production of 4*106 m3/0.3*106 m2 = 13.3 m of melt water production over the glacier area. For the largest glacier, it is 18*106 m3/6.8*106 m2 = 2.7 m of meltwater production. The first value is impossible, the last value is in the order of observed summer mass balances in Switzerland in 2019 (see Figure 1 one in the attached complete review).
The reasons for the erroneous estimates are certainly related to the wrong assumption that streamflow during summer is only composed of glacier melt and of rainfall. In reality, an important part of streamflow is groundwater (baseflow) released by the hillslopes; the isotopic values of groundwater are strongly influenced by snow melt and thus close to the values of glacier melt (see below). Accordingly, the separation into glacier melt and not-glacier melt is impossible with the help of isotopes alone. EC values could help separating ground water from non-groundwater input but this would require values for groundwater and values for ice melt at the glacier snout (which was already in contact with the ground).
See my full review in the attached pdf.
-
AC1: 'Reply on RC1', Philipp Wanner, 11 Mar 2022
General author response to the reviewer’s comments for Manuscript hess-2021-512
We thank the three reviewers for their insightful comments regarding our manuscript. We agree with the major comment of the reviewers that our approach of neglecting groundwater as significant interim storage for glacial melt, rainwater, and snowmelt was somewhat simplified and that our dataset does not allow making strong quantitative statements regarding the glacial meltwater contribution to mountainous streams for all hydrological set-ups. Therefore, we plan to slightly shift the scope of the manuscript and we intend to focus more on the opportunities, challenges, and limitations of using stable water isotopes to quantify the contribution of glacial meltwater to mountainous streams. We think that such a scope is still of high and novel scientific value since our stable isotope dataset covering 13 months of continuous sampling in three catchments shows that the quantification of the glacial meltwater contribution works well if two conditions are met: a) The snow must be absent for instance in late summer due to its highly variable stable isotope signature impairing the quantification of the glacial meltwater contribution to mountainous streams and b) the groundwater contribution during this snow-free period must be low in relation to those of the other end-members (ice and rain) or the corresponding groundwater subsurface residence time must be short such that water flow through the groundwater system into the stream does not strongly delay the end-member signal arriving in the streams.
Philipp Wanner et al.
Pleased find attached the author's responses to the comments of reviewer 1.
-
AC1: 'Reply on RC1', Philipp Wanner, 11 Mar 2022
-
RC2: 'Comment on hess-2021-512', Anonymous Referee #2, 29 Nov 2021
General comments
The authors of this manuscript analyzed the temporal variability in the isotopic composition of rain water and snow samples, and quantified the contribution of glacial melt water to stream runoff, by means of stable water isotopes, in three study catchments in the Swiss Alps.
The topic of this manuscript is potentially interesting for the readers of Hydrology and Earth System Sciences. In general, I think that more studies investigating the contribution of snowmelt and glacier melt to stream runoff in high elevation catchments are needed to improve our understanding of hydrological processes in such complex areas. Overall, the paper is well structured and well written, but I have several (major) concerns about the methodological approach.
Firstly, the authors have not considered the contribution of groundwater to runoff both in the accumulation and the ablation period. Groundwater is expected to be the dominant end-member during the accumulation period, but a large contribution of groundwater to runoff may be possible from the glacier-free areas of the catchments during the ablation period.
Secondly, more details are needed in the section 2.4 about the hydrograph separation. The authors should explain the choice of the end members, provide the assumptions at the base of the hydrograph separation technique (please see Klaus and McDonnell, 2013), and describe how uncertainty was estimated (it is mentioned only at lines 420-423).
Thirdly, the authors should consider more and discuss the temporal and spatial variability in the isotopic composition of the end members. Previous studies conducted in Alpine catchments (e.g., Schmieder et al., 2016; Schmieder et al., 2018; Zuecco et al., 2019) have already shown that a high spatial and temporal variability in the tracer composition of the end members can greatly affect the results of the hydrograph separation and/or hamper its application. In this study, the authors used only three samples of glacier ice (and from only one of the glaciers) to characterize the glacier-melt end member. This sample size is too small for making any consideration on hydrograph separation.
Finally, the authors have not described which approach was used to assess the end of the snowmelt period in the three catchments (using snow cover data collected at only one station at 2063 m a.s.l. is not sufficient).
Specific comments
- The introduction is mainly focussed on the role of hydropower in Alpine catchments, whereas there is too little attention towards the application of tracers in high-elevation catchments to quantify the contribution of glacier-melt water to stream runoff.
- Lines 47-48: This concept repeats the text at lines 32-35.
- Line 54: I would not describe the tracer-based methods as low cost compared to other methods, such as hydrological modelling.
- In the legend of Figure 1, I suggest indicating the glacierized area.
- Line 123: 19 snow samples is not a high sample size.
- Line 131: I suggest indicating the number of ice samples that were collected.
- Lines 132-133: Three samples collected at the glacier fronts cannot be representative of the whole ablation zone. Additional samples are needed to support the main findings of this manuscript.
- Lines 274-276: These two sentences are not supported by rain samples collected during the accumulation period.
- Lines 363-371 and Figure 6: I suggest comparing discharge values after normalization by catchment areas.
- Lines 410-411: The author should provide evidence about the presence/absence of snowmelt in all three catchments during the ablation period.
- Lines 420-423: These sentences belong to section 2.4.
- Figure 8: This figure could be interesting if more catchments were considered; is it possible to gather data from other Alpine catchments? If not, I suggest deleting the figure.
Technical corrections
- Line 172: It is unclear what the authors mean with “binary mixing approach”. I suggest using another term, such as “two-component hydrograph separation”.
- Line 223: Please indicate the water source for “heavy isotopes”.
- Lines 225-226: Please mention the water source considered in the sentence.
- Figure 4: Please indicate in the caption what the error bars represent.
- Figure 5: Please indicate in the caption what the error bars represent.
References
Klaus, J., & McDonnell, J.J. (2013). Hydrograph separation using stable isotopes: Review and evaluation. Journal of Hydrology, 505, 47-64. DOI: 10.1016/j.jhydrol.2013.09.006
Schmieder, J., Garvelmann, J., Marke, T., & Strasser, U. (2018). Spatio-temporal tracer variability in the glacier melt end-member – How does it affect hydrograph separation results? Hydrological Processes, 32, 1828–1843. DOI: 10.1002/hyp.11628
Schmieder, J., Hanzer, F., Marke, T., Garvelmann, J., Warscher, M., Kunstmann, H., & Strasser, U. (2016). The importance of snowmelt spatiotemporal variability for isotope-based hydrograph separation in a high-elevation catchment. Hydrology and Earth System Sciences, 20, 5015-5033. DOI: 10.5194/hess-20-5015-2016
Zuecco, G., Carturan, L., De Blasi, F., Seppi, R., Zanoner, T., Penna, D., Borga, M., Carton, A., & Dalla Fontana, G. (2019). Understanding hydrological processes in glacierized catchments: Evidence and implications of highly variable isotopic and electrical conductivity data. Hydrological Processes, 33, 816-832. DOI: 10.1002/hyp.13366
Citation: https://doi.org/10.5194/hess-2021-512-RC2 - AC2: 'Reply on RC2', Philipp Wanner, 11 Mar 2022
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RC3: 'Comment on hess-2021-512', Anonymous Referee #3, 21 Dec 2021
Summary of the paper:
This article estimates the role of glacial meltwater in generating stream discharge in three Alpine catchments located in the Central Swiss Alps. Stable water isotopes (2H, 18O) are used to quantify the proportion of streamflow generated from ice melt vs rainfall while electrical conductivity measurements are qualitatively used to understand the dominant hydrologic processes. The article concludes that ice melt is the dominant driver of streamflow generation in August and September and propose that due to climate change, glacial coverage will reduce which might adversely impact streamflow generation during this period of the year. The article then estimates annual glacial melt discharge in these three catchments and propose a power law relationship between minimum annual glacial meltwater discharge and the glaciated area, which can potentially be extrapolated to catchments with known glaciated areas.
The paper is well written but lacks significantly in terms of robustness of the methods used and the inferences made thereafter. The key problem that I see is one missing end-member which is “groundwater” that has not been considered in this article. In Alpine environments, groundwater has a significant role is sustaining streamflow during low flow periods in August-October period. In this particular case study, I think groundwater is significantly contributing to streamwater generation during August-September period as can be inferred from the high EC values during that part of the year (Figure 6C). If this period was completely dominated by ice melt originating from glaciers, EC values would be much lower and similar to that observed in the June-July period in Steinwasser catchment when snowmelt was dominating streamwater recharge (Figure 6C). As Steinwasser is the only catchment which has a longer timeseries of EC values, we can see that snowmelt was probably dominating stream recharge in June, July (low EC values) and then groundwater kicked-in in late August which is why EC values increased significantly. As the article has only relied on stable isotope measurements, this distinction is missing. I want to see if the results would be similar if the end member mixing exercise was undertaken with EC values and not stable water isotopic ratios. This also makes sense because electrical conductivity is largely a conservative tracer.
In terms of mechanism, I think there might be significant subsurface storage that is getting recharged by snowmelt and ice melt (hence very depleted) and this storage is then recharging the stream during August September period. If this mechanism is indeed true, then the underlying hypothesis that rapidly retreating glaciers will lead to very low flows in August September period will not be true as groundwater can be recharged via rainfall, snowmelt and ice melt. I would like to hear the authors’ perspective on this and if this was considered as a possible hypothesis.
Variability in the isotopic ratio of ice melt (originating from the glacier) is very low and might not be very realistic. This is probably due to very limited ice sampling (only sampled two times in August and September, L418). Hence, the distinction in isotopic ratio of ice melt and snowmelt might be more of a function of sampling bias rather than any underlying hydrologic process.
Other major comments
L521-523: I find it very surprising that the ice melt contributes to ~25% of total discharge in Giglibach when the extent of glacial coverage is only 8%. On the other hand, the extent of glacial coverage is as high as 28% in Steinwasser but the contribution of glacial melt to total discharge is only slightly higher at ~29%. Are these estimates reasonable or to put it differently, have these kinds of number been reported at any other place where despite very high glacial coverage (>3x for Steinwasser compared to Giglibach), contribution to annual stream discharge only increases slightly.
L377: Groundwater might also be a significant contributor to stream recharge. I propose the authors to explore this hypothesis.
L381-385: If snow and glacial meltwater show lower EC compared, then August and September discharge cannot be explained by glacial meltwater as EC values are high across catchments.
L418: Two samples is very few to make any meaningful statistical judgement
L420-423: Details about Gaussian error propagation has not been explained anywhere in the article. Additionally, ±2% uncertainty bound seems to be very small. This might be due to small sample size.
L483-486: Has this been reported for the first time? I am not familiar with this literature, are there other studies which have reported similar results? In that case, it might be good to include relevant references.
L544-545: Using temporally high resolution isotope measurements leading to superior quantification of glacial meltwater hasn’t been shown in this article.
Minor comments:
L284: It should read as “… in the ablation compared to the accumulation period …”
L285: It might be clearer if its written as “… which has a heavier isotopic signature compared to the snow that fell during the accumulation period…”
L538: Should be “. This is of major importance ..”
Figures:
Figure 1: Incorrect figure caption, Wendenwasser is shown in grey and not pink.
Figure 5: Should also include snowmelt isotopic ratios here to make the comparison between snowmelt and ice melt easier. Is this any reason to believe that both will have different isotopic signature?
Figure 6: In subplots B, C and D there is a lot of whitespace due to very large y-axis bounds. For e.g. there are no discharge measurements below 0.1 m3/s, so showing y-axis values up to 0.01 m3/s is not necessary. Similar is the case for EC values < 10. I will suggest the authors to consider using tighter y-axis bounds so that the underlying data variability is more clearly visible.
Figure 6A: Is the unit mm or mm/hr?
Figure 7: I will suggest adding uncertainty bounds in this figure. Also, is 90%+ glacial melt contribution (Figure 7A) a plausible estimate at the end of July in a catchment which is only 6% glaciated?
Figure 7 caption: Should be “.. glacial melt water contribution to the three ..”
Citation: https://doi.org/10.5194/hess-2021-512-RC3 - AC3: 'Reply on RC3', Philipp Wanner, 11 Mar 2022
Interactive discussion
Status: closed
-
RC1: 'Review of hess-2021-512', Bettina Schaefli, 18 Nov 2021
This paper proposes to quantify the contribution of ice melt to total streamflow in three highly glaciated catchments in the central Swiss Alps with the help of astable isotopes of water. The aim is to come up with results that are more reliable than previous modelling-based results and with recommendations for future sampling campaigns.
I cannot recommend the paper for publication because some fundamental hydrological process knowledge is ignored. The obtained results are not plausible (glacier melt contribution of between 80% and 95% to total streamflow during August in catchments with only between 6 and 28% glacier cover). One key result is summarized in Figure 8, which shows glacier melt in Mio m3 against glacier area. For the smallest glacier investigated, this result indicates meltwater production of 4*106 m3 on an area of 0.3 km2, which corresponds to a melt water production of 4*106 m3/0.3*106 m2 = 13.3 m of melt water production over the glacier area. For the largest glacier, it is 18*106 m3/6.8*106 m2 = 2.7 m of meltwater production. The first value is impossible, the last value is in the order of observed summer mass balances in Switzerland in 2019 (see Figure 1 one in the attached complete review).
The reasons for the erroneous estimates are certainly related to the wrong assumption that streamflow during summer is only composed of glacier melt and of rainfall. In reality, an important part of streamflow is groundwater (baseflow) released by the hillslopes; the isotopic values of groundwater are strongly influenced by snow melt and thus close to the values of glacier melt (see below). Accordingly, the separation into glacier melt and not-glacier melt is impossible with the help of isotopes alone. EC values could help separating ground water from non-groundwater input but this would require values for groundwater and values for ice melt at the glacier snout (which was already in contact with the ground).
See my full review in the attached pdf.
-
AC1: 'Reply on RC1', Philipp Wanner, 11 Mar 2022
General author response to the reviewer’s comments for Manuscript hess-2021-512
We thank the three reviewers for their insightful comments regarding our manuscript. We agree with the major comment of the reviewers that our approach of neglecting groundwater as significant interim storage for glacial melt, rainwater, and snowmelt was somewhat simplified and that our dataset does not allow making strong quantitative statements regarding the glacial meltwater contribution to mountainous streams for all hydrological set-ups. Therefore, we plan to slightly shift the scope of the manuscript and we intend to focus more on the opportunities, challenges, and limitations of using stable water isotopes to quantify the contribution of glacial meltwater to mountainous streams. We think that such a scope is still of high and novel scientific value since our stable isotope dataset covering 13 months of continuous sampling in three catchments shows that the quantification of the glacial meltwater contribution works well if two conditions are met: a) The snow must be absent for instance in late summer due to its highly variable stable isotope signature impairing the quantification of the glacial meltwater contribution to mountainous streams and b) the groundwater contribution during this snow-free period must be low in relation to those of the other end-members (ice and rain) or the corresponding groundwater subsurface residence time must be short such that water flow through the groundwater system into the stream does not strongly delay the end-member signal arriving in the streams.
Philipp Wanner et al.
Pleased find attached the author's responses to the comments of reviewer 1.
-
AC1: 'Reply on RC1', Philipp Wanner, 11 Mar 2022
-
RC2: 'Comment on hess-2021-512', Anonymous Referee #2, 29 Nov 2021
General comments
The authors of this manuscript analyzed the temporal variability in the isotopic composition of rain water and snow samples, and quantified the contribution of glacial melt water to stream runoff, by means of stable water isotopes, in three study catchments in the Swiss Alps.
The topic of this manuscript is potentially interesting for the readers of Hydrology and Earth System Sciences. In general, I think that more studies investigating the contribution of snowmelt and glacier melt to stream runoff in high elevation catchments are needed to improve our understanding of hydrological processes in such complex areas. Overall, the paper is well structured and well written, but I have several (major) concerns about the methodological approach.
Firstly, the authors have not considered the contribution of groundwater to runoff both in the accumulation and the ablation period. Groundwater is expected to be the dominant end-member during the accumulation period, but a large contribution of groundwater to runoff may be possible from the glacier-free areas of the catchments during the ablation period.
Secondly, more details are needed in the section 2.4 about the hydrograph separation. The authors should explain the choice of the end members, provide the assumptions at the base of the hydrograph separation technique (please see Klaus and McDonnell, 2013), and describe how uncertainty was estimated (it is mentioned only at lines 420-423).
Thirdly, the authors should consider more and discuss the temporal and spatial variability in the isotopic composition of the end members. Previous studies conducted in Alpine catchments (e.g., Schmieder et al., 2016; Schmieder et al., 2018; Zuecco et al., 2019) have already shown that a high spatial and temporal variability in the tracer composition of the end members can greatly affect the results of the hydrograph separation and/or hamper its application. In this study, the authors used only three samples of glacier ice (and from only one of the glaciers) to characterize the glacier-melt end member. This sample size is too small for making any consideration on hydrograph separation.
Finally, the authors have not described which approach was used to assess the end of the snowmelt period in the three catchments (using snow cover data collected at only one station at 2063 m a.s.l. is not sufficient).
Specific comments
- The introduction is mainly focussed on the role of hydropower in Alpine catchments, whereas there is too little attention towards the application of tracers in high-elevation catchments to quantify the contribution of glacier-melt water to stream runoff.
- Lines 47-48: This concept repeats the text at lines 32-35.
- Line 54: I would not describe the tracer-based methods as low cost compared to other methods, such as hydrological modelling.
- In the legend of Figure 1, I suggest indicating the glacierized area.
- Line 123: 19 snow samples is not a high sample size.
- Line 131: I suggest indicating the number of ice samples that were collected.
- Lines 132-133: Three samples collected at the glacier fronts cannot be representative of the whole ablation zone. Additional samples are needed to support the main findings of this manuscript.
- Lines 274-276: These two sentences are not supported by rain samples collected during the accumulation period.
- Lines 363-371 and Figure 6: I suggest comparing discharge values after normalization by catchment areas.
- Lines 410-411: The author should provide evidence about the presence/absence of snowmelt in all three catchments during the ablation period.
- Lines 420-423: These sentences belong to section 2.4.
- Figure 8: This figure could be interesting if more catchments were considered; is it possible to gather data from other Alpine catchments? If not, I suggest deleting the figure.
Technical corrections
- Line 172: It is unclear what the authors mean with “binary mixing approach”. I suggest using another term, such as “two-component hydrograph separation”.
- Line 223: Please indicate the water source for “heavy isotopes”.
- Lines 225-226: Please mention the water source considered in the sentence.
- Figure 4: Please indicate in the caption what the error bars represent.
- Figure 5: Please indicate in the caption what the error bars represent.
References
Klaus, J., & McDonnell, J.J. (2013). Hydrograph separation using stable isotopes: Review and evaluation. Journal of Hydrology, 505, 47-64. DOI: 10.1016/j.jhydrol.2013.09.006
Schmieder, J., Garvelmann, J., Marke, T., & Strasser, U. (2018). Spatio-temporal tracer variability in the glacier melt end-member – How does it affect hydrograph separation results? Hydrological Processes, 32, 1828–1843. DOI: 10.1002/hyp.11628
Schmieder, J., Hanzer, F., Marke, T., Garvelmann, J., Warscher, M., Kunstmann, H., & Strasser, U. (2016). The importance of snowmelt spatiotemporal variability for isotope-based hydrograph separation in a high-elevation catchment. Hydrology and Earth System Sciences, 20, 5015-5033. DOI: 10.5194/hess-20-5015-2016
Zuecco, G., Carturan, L., De Blasi, F., Seppi, R., Zanoner, T., Penna, D., Borga, M., Carton, A., & Dalla Fontana, G. (2019). Understanding hydrological processes in glacierized catchments: Evidence and implications of highly variable isotopic and electrical conductivity data. Hydrological Processes, 33, 816-832. DOI: 10.1002/hyp.13366
Citation: https://doi.org/10.5194/hess-2021-512-RC2 - AC2: 'Reply on RC2', Philipp Wanner, 11 Mar 2022
-
RC3: 'Comment on hess-2021-512', Anonymous Referee #3, 21 Dec 2021
Summary of the paper:
This article estimates the role of glacial meltwater in generating stream discharge in three Alpine catchments located in the Central Swiss Alps. Stable water isotopes (2H, 18O) are used to quantify the proportion of streamflow generated from ice melt vs rainfall while electrical conductivity measurements are qualitatively used to understand the dominant hydrologic processes. The article concludes that ice melt is the dominant driver of streamflow generation in August and September and propose that due to climate change, glacial coverage will reduce which might adversely impact streamflow generation during this period of the year. The article then estimates annual glacial melt discharge in these three catchments and propose a power law relationship between minimum annual glacial meltwater discharge and the glaciated area, which can potentially be extrapolated to catchments with known glaciated areas.
The paper is well written but lacks significantly in terms of robustness of the methods used and the inferences made thereafter. The key problem that I see is one missing end-member which is “groundwater” that has not been considered in this article. In Alpine environments, groundwater has a significant role is sustaining streamflow during low flow periods in August-October period. In this particular case study, I think groundwater is significantly contributing to streamwater generation during August-September period as can be inferred from the high EC values during that part of the year (Figure 6C). If this period was completely dominated by ice melt originating from glaciers, EC values would be much lower and similar to that observed in the June-July period in Steinwasser catchment when snowmelt was dominating streamwater recharge (Figure 6C). As Steinwasser is the only catchment which has a longer timeseries of EC values, we can see that snowmelt was probably dominating stream recharge in June, July (low EC values) and then groundwater kicked-in in late August which is why EC values increased significantly. As the article has only relied on stable isotope measurements, this distinction is missing. I want to see if the results would be similar if the end member mixing exercise was undertaken with EC values and not stable water isotopic ratios. This also makes sense because electrical conductivity is largely a conservative tracer.
In terms of mechanism, I think there might be significant subsurface storage that is getting recharged by snowmelt and ice melt (hence very depleted) and this storage is then recharging the stream during August September period. If this mechanism is indeed true, then the underlying hypothesis that rapidly retreating glaciers will lead to very low flows in August September period will not be true as groundwater can be recharged via rainfall, snowmelt and ice melt. I would like to hear the authors’ perspective on this and if this was considered as a possible hypothesis.
Variability in the isotopic ratio of ice melt (originating from the glacier) is very low and might not be very realistic. This is probably due to very limited ice sampling (only sampled two times in August and September, L418). Hence, the distinction in isotopic ratio of ice melt and snowmelt might be more of a function of sampling bias rather than any underlying hydrologic process.
Other major comments
L521-523: I find it very surprising that the ice melt contributes to ~25% of total discharge in Giglibach when the extent of glacial coverage is only 8%. On the other hand, the extent of glacial coverage is as high as 28% in Steinwasser but the contribution of glacial melt to total discharge is only slightly higher at ~29%. Are these estimates reasonable or to put it differently, have these kinds of number been reported at any other place where despite very high glacial coverage (>3x for Steinwasser compared to Giglibach), contribution to annual stream discharge only increases slightly.
L377: Groundwater might also be a significant contributor to stream recharge. I propose the authors to explore this hypothesis.
L381-385: If snow and glacial meltwater show lower EC compared, then August and September discharge cannot be explained by glacial meltwater as EC values are high across catchments.
L418: Two samples is very few to make any meaningful statistical judgement
L420-423: Details about Gaussian error propagation has not been explained anywhere in the article. Additionally, ±2% uncertainty bound seems to be very small. This might be due to small sample size.
L483-486: Has this been reported for the first time? I am not familiar with this literature, are there other studies which have reported similar results? In that case, it might be good to include relevant references.
L544-545: Using temporally high resolution isotope measurements leading to superior quantification of glacial meltwater hasn’t been shown in this article.
Minor comments:
L284: It should read as “… in the ablation compared to the accumulation period …”
L285: It might be clearer if its written as “… which has a heavier isotopic signature compared to the snow that fell during the accumulation period…”
L538: Should be “. This is of major importance ..”
Figures:
Figure 1: Incorrect figure caption, Wendenwasser is shown in grey and not pink.
Figure 5: Should also include snowmelt isotopic ratios here to make the comparison between snowmelt and ice melt easier. Is this any reason to believe that both will have different isotopic signature?
Figure 6: In subplots B, C and D there is a lot of whitespace due to very large y-axis bounds. For e.g. there are no discharge measurements below 0.1 m3/s, so showing y-axis values up to 0.01 m3/s is not necessary. Similar is the case for EC values < 10. I will suggest the authors to consider using tighter y-axis bounds so that the underlying data variability is more clearly visible.
Figure 6A: Is the unit mm or mm/hr?
Figure 7: I will suggest adding uncertainty bounds in this figure. Also, is 90%+ glacial melt contribution (Figure 7A) a plausible estimate at the end of July in a catchment which is only 6% glaciated?
Figure 7 caption: Should be “.. glacial melt water contribution to the three ..”
Citation: https://doi.org/10.5194/hess-2021-512-RC3 - AC3: 'Reply on RC3', Philipp Wanner, 11 Mar 2022
Data sets
Discharge Data Giglibach, Steinwasser, Wendenwasser catchments Philipp Wanner, Noemi Buri, Kevin Wyss, Andreas Zischg, Rolf Weingartner, Jan Baumgartner, Benjamin Berger, Christoph Wanner https://doi.org/10.5281/zenodo.5571465
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