| 17 Aug 2026
The use of multi-source cryospheric datasets to improve the simulation of future icemelt contribution to streamflow in large glaciarized basins
Abstract. The glacierized mountains are impacted by dramatic changes in the context of global warming with large implications for hydrology. However, the projection of icemelt contribution to streamflow remains a complicated task and is subject of large uncertainties, especially when the calibration-validation process was performed with little or no cryospheric data. In this study, we present the integration of a glacier module in the distributed conceptual hydrological model J2000, taking into consideration the evolution of glacier surface in the simulations. A calibration and a validation was done using a large panel of hydro-cryospheric datasets, including glacier mass balance observations and simulations, snow reanalyses, snow-cover by satellites and stream gauges. The calibration-validation was performed by applying J2000-Rhône-glaciers to the entire Rhône basin in the historical period (1976–2022) using the SAFRAN climate reanalysis at 8 km horizontal resolution. The set of parameters identified in the past and meteorological data from 5 “GCM-RCM” chains of climate projections under RCP 8.5 scenario were used to simulate the hydrological response of the Rhône basin under climate change projections (1976–2095). The results show that snowmelt, icemelt and streamflow are satisfactorily simulated by J2000-Rhône-glaciers in the Rhône basin. The projections show a decrease of annual streamflow expected by the end of the 21st century due to a decrease of precipitation and glacier shrinkage in highly glaciated subbasins. The results of the simulations also show an increase of streamflow in winter but a dramatic decrease in summer mainly due to a change in snow seasonality. The icemelt will become extremely low in Isère, Drac and Durance subbasins, while in the Arve, upper Rhône, and along the Rhône River, the future evolution of icemelt contribution to summer streamflow is very uncertain. Despite these uncertainties, our results suggest that the remaining glaciers will still be crucial to sustain the summer streamflow at the end of the century. However, the peak water may be passed by 2040s in the Arve subbasin, and 2050s in the upper Rhône, and the change in snow seasonality will still threaten the availability of water resources in most of the Rhône subbasins. The use of multi-source cryospheric datasets allowed to gain confidence in the projections of icemelt contribution to streamflow in a large basin in the Alps.
This study models streamflow in the large Rhone basin, from the glacierized headwaters in Switzerland and France, to almost all the way to the outlet at the Mediterranean sea. The aim of the study is to quantify the ice- and snowmelt contributions throughout the 21st century, after evaluating how well the model can simulate snow cover, snowmelt, glacier mass balance and streamflow during a historical period.
Large-basin glacio-hydrological assessments are still scarce, so this could be a very valuable contribution, even if the framework itself is not novel. In its current form, however, I do not think the manuscript meets the standards of HESS. My main concerns are:
1. The glacier module is not built on or compared against existing literature of coupled glacio-hydrological models
Section 2.1 describes the glacier module, but it mostly describes the overall hydrological model and refers to other studies for the glacier module. It doesn’t become clear how glacier evolution over time was implemented? How is it given as input? Later in the manuscript I found some descriptions of linearly interpolating between inventory dates and getting the glacier area evolution from two different (global scale) glacier models, ALPGM and OGGM. Since coupled glacio-hydrological models exist including glacier retreat, an argumentation is needed for why taking this model and approach is best to analyze changing glacier melt contributions considering what has already been done before.
Generally, the modelling approach seems too inconsistent; 1) using different glacier evolution models, 2) no coupling between calculated melt and retreat, 3) “merging” observed and modelled glacier retreat (past and future period) without stating how and 4) getting precipitation correction factors from one global glacier model (OGGM), while taking glacier area evolution from another global glacier model (GloGEMflow).
2. Several model assumptions and approaches are not justified or unclear, or appear even physically inconsistent
If parameters from the “classic” model were calibrated with “trial and error”, and for this study only “new” parameters related to snow and ice were “manually adjusted in a stepwise approach”, how were the other parameters transferred from the classic model version to the glacier version? The streamflow data includes snow- and glacier melt, so parameters were likely compensating for these missing processes in the classic version and seem therefore not transferable to the “glacier” version of the model. Please explain in more detail how all model parameters were obtained, including objective functions, extrapolating or taking over from earlier model versions.
The description of the precipitation factors is unclear. How is precipitation generally changing with elevation? Are there lapse rates? How large are HRU’s? The use of French meteorological data to estimate precipitation in the whole Swiss upper part of the Rhone basin, to then correct this differently for elevations above and below 2800m seems arbitrary but probably quite influencing the results. Why is a Swiss snow product used for correcting the precipitation for the lower elevated gridcells, but why could not a Swiss precipitation product be used as input? The argumentation that glaciers at high elevations due to their topographical location may receive more precipitation input is plausible, but why is this correction factor also used for non-glacierized HRU’s? The snow of avalanches must come from somewhere? Moreover, why would OGGM snowfall be a valid target? Global scale glacier models calibrate their models to long-term annual mass balances and for that reason accumulation and ablation processes are highly uncertain.
How are parameters calibrated? Per glacier, per subbasin, per HRU? How are soil, geology and landuse used in the model framework?
The size of HRU’s would also be crucial to understand if processes like snow towers (snow accumulating in a model HRU and never melting because of temperatures always below the melt threshold) or absence of snow redistribution processes play a role. Currently this information is not available.
Why would non-glacier fed rivers not require a validation? These river sections contribute to the main river further downstream and influence relative glacier melt contributions. The strength of this study could be in analyzing the Rhone river basin as a whole, and how contributions change from up- to downstream. For that, the model, processes and results also need to be discussed for the non-glacierized parts of the basin.
The study states that natural streamflow is simulated and dams and diversions are not included. Which observations have been used then for calibrating the “classic” model? Did the classic model include dams (Line 604 suggests so)? Then how can these model versions (“Glacier” and “Classic”) be compared? Does Figure 2 include the 48 or 19 stations? It seems that only the 19 natural ones are relevant in this case? More broadly, I wonder what the results of this study will tell us if they represent a situation that is not the actual situation. What can we learn from the findings? A detailed discussion on the use of this “natural” streamflow situation simulation is currently missing, describing how much it matters or when and where results would change.
How are glacier and snowmelt routed through the storages? Do they mix? What happens in the lakes?
How is ET calculated in the model?
3. Direction of the study
As it stands, the study could go in two directions, and neither works at the moment. The first is quantifying glacier and snowmelt contributions to streamflow and how they change over the 21st century. This requires a careful model set-up and a convincing evaluation, but the inconsistencies mentioned in the points above and the weak streamflow performance (section 3.1.3) do not allow for reliable conclusions on this. The improvements the authors identify in the conclusions, such as adding a glacier model and using consistent meteorological input should have been implemented here rather than left as an outlook.
The second option would be a more technical one, showing how to set up and calibrate a large-basin glacio-hydrological model, and what improvement the glacier module brings over the "classic" model version, including for example which processes can and cannot be analyzed where or how the glacio-hydrological model compares to glacier models. This would be a useful contribution, but only with a systematic comparison, which is currently missing.
4. Figures, tables and text need considerable editing for clarity, some examples: