the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Dec 2025
New insights from fully-integrated surface-subsurface hydrological modeling in high-elevation glaciated environments
Abstract. Recent modeling efforts go towards distributed physics-based modeling for improved hydrological process understanding. The sparse hydro-climatological observations and complex topography in high-elevation mountainous environments have, however, hampered the application of such data-intensive and sophisticated techniques for detailed process exploration. In particular, characterizing surface-subsurface water exchange processes in these terrains remains understudied. Here we implement a fully-integrated and fully-distributed surface-subsurface hydrological model, WaSiM, to quantify the observed streamflow variations and their interactions with groundwater in the high-elevation glaciated environment (Martell Valley) in the central European Alps since the 2000s. Extensive field observations (meteorology, vegetation, glacier mass balance, soil properties, river discharge), especially in-situ groundwater levels, were collected to investigate hydrological processes and constrain model parameters in the surface and subsurface. We observe that in contrast to the delayed responses typically observed in aquifers, shallow alpine groundwater responds nearly as quickly as streamflow to peak snowmelt and extreme rainfall events, highlighting the need for improved subsurface parametrization in hydrological modeling. Surprisingly, we are able to obtain satisfactory model performance when subsurface lateral flow is constrained to zero, indicating its limited influence in river discharge generation at the site and providing new insights into hydrological processes in such an environment. Moreover, to overcome the challenges of integrating point-scale groundwater observations into a fully-distributed hydrological model in high-elevation catchments, we suggest pre-calculating the Topographic Wetness Index to guide piezometer placement. Despite the inherent uncertainties associated with the necessary assumptions of modeling, this study sheds new light on surface-subsurface hydrological processes in high-alpine landscapes.
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Status: open (until 17 Apr 2026)
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CC1: 'Comment on egusphere-2025-6065', Nima Zafarmomen, 08 Jan 2026
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AC1: 'Reply on CC1', Xinyang Fan, 15 Jan 2026
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Thanks for your interests and appraisal of the value of our work. Please kindly find the attached response to your comments.
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AC1: 'Reply on CC1', Xinyang Fan, 15 Jan 2026
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RC1: 'Comment on egusphere-2025-6065', Anonymous Referee #1, 19 Mar 2026
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Overall, this is robust research presented in a well written manuscript. I was impressed with the thorough analysis you performed when comparing the simulated results to the in situ field data. You also produced clear visuals to report the results. I have some comments below in response to a few places in the paper where I thought you could add additional context.
Introduction
A paragraph is needed introducing, explaining, and justifying the use of the Water Balance Simulation Model (WaSiM) as the hydrologic model for this study. Given that this research is the first to apply WaSiM o in high-elevation environment, this explanation/knowledge gap should also be addressed in the introduction.
Additionally, I think a paragraph is needed explaining how in situ data can be used in these fully-integrated models would be helpful – particularly by highlighting the rarity of in-situ groundwater data.
Study Area
Figure 1b – The basemap is present showing the Germany label cut off. I would recommend fixing your map.
Section 3.2 You mention using the sen slope and Mann-Kendall test – please also add your p-value significance threshold, confidence intervals, etc.
Section 4.4.3 It seems to me that there should be additional statistical significance tests such as RMSE, MAE and p-values.
I am recommending adding the RMSE and MAE analysis. Also, I recommend expanding section 4.4.3 to include the NSE, spearman rank correlation coefficient, and p-value thresholds that you considered “significant” in this analysis.
Line 368: please provide the river gauge and borehole ID 4479 water fluctuation in order to compare it to the 0.8 reported for the borehold ID 4478.
Line 565: I’m not sure that the hypothesis that “subsurface lateral flow may have limited contribution to streamflow generation at the high-elevation headwater landscapes.” Is fully explained in the introduction. Therefore, I would recommend ensuring that this narrative is present throughout the paper including an additional explanation in the introduction. I’m also not sure how this is an “outrageous hypothesis” – perhaps expand on why this hypothesis is unusual for hydrologic research?
Conclusion: I think you can add a paragraph on how the findings of this research can be applied to similar high alpine ecosystems throughout the world in order to give the research a “global application”
Citation: https://doi.org/10.5194/egusphere-2025-6065-RC1
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This study implements a fully-integrated and fully-distributed surface-subsurface hydrological model using the WaSiM framework to examine water exchange processes in the high-elevation, glaciated Martell Valley in the Italian Alps. The authors utilize extensive field data, including rare in-situ high-elevation groundwater level observations—to constrain model parameters across snow, glacier, and subsurface modules
This paper is worthy of publication as it addresses a significant gap in mountain hydrology: the quantification of surface-subsurface interactions in cryosphere-dominated environments. The integration of groundwater level observations into a distributed physics-based model for a high-alpine catchment is a notable technical achievement.
1- While the model achieved satisfactory performance with subsurface lateral flow constrained to zero, this remains a counter-intuitive finding for steep alpine terrains. The authors acknowledge that their piezometers were placed in relatively flat floodplains. The finding might be a localized artifact of piezometer placement rather than a catchment-wide process. More discussion is needed on how this parameterization might affect the model’s ability to simulate hillslope-to-stream connectivity in steeper parts of the catchment.
2- The study notes a specific numerical requirement in WaSiM where the lower boundary of the soil layers must be identical to the lower boundary of the aquifer (calibrated here to 1.30 m). This 1.30 m constraint is quite shallow for an entire catchment. While it may represent the shallow porous aquifers observed, it automatically excludes any simulation of deeper groundwater flow paths. The authors should explicitly clarify if this thickness (1.30 m) is applied uniformly across the entire 77 km2 catchment, including the valley floor where sedimentary deposits are likely much deeper.
3- In Section 6.3, the recommendation to match the TWI spatial resolution to the model resolution (25m×25m) is clear. It might be helpful to include a brief sentence advising how practitioners should handle areas where the 25m grid might smooth over critical local topographic features (like small moraines) mentioned earlier in the text.
4- It would be beneficial to explicitly discuss if this "zero interflow" finding is a physical characteristic of the Martell Valley (e.g., due to high vertical conductivity in porous aquifers) or if it highlights a structural limitation in how current physics-based models distribute lateral flow in steep, shallow-soil alpine terrains. In Section 5.3, consider clarifying if "zero interflow" refers to the model parameter dr being set to 0, and briefly reiterate the physical implication for the reader.
5- As you are working on surface-subsurface hydrological model I do strongly recommend to broad your literature review and cite "Assimilation of sentinel‐based leaf area index for modeling surface‐ground water interactions in irrigation districts"