the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 20 May 2025
A large transient multi-scenario multi-model ensemble of future streamflow and groundwater projections in France
Abstract. A large transient multi-scenario and multi-model ensemble of future streamflow and groundwater projections in France developed in a national project named Explore2 was recently made available. The main objective of Explore2 is to provide rich and spatially-consistent information for the future evolution of hydrological (surface and groundwater) resources and extremes in France to support adaptation strategies. The Explore2 dataset was obtained using a nested multi-scenario multi-model approach to estimate future uncertainty and to assess local climate at the catchment scale: three greenhouse gas (GHG) emission scenarios, a set of 17 combinations of Global Climate Models and Regional Climate Models (GCM-RCM), and two bias correction methods provide the meteorological forcing for nine surface hydrology models and four groundwater hydrology models (one to simulate groundwater recharge and three to simulate groundwater level). In this paper, we present the methodology underlying the dataset, the evaluation of the hydrological models against daily streamflow and groundwater level observations, the assessment of the future streamflow and recharge projections, the data availability and the ways of accessing the data and understanding the results (mainly through visualisation tools).
This large set of hydrological projections shows a high model agreement on the decrease in seasonal flows in the South of France under the RCP8.5 high-emission scenario, confirming its hotspot status. The surface HMs agree on the decrease in summer flows across France under the RCP8.5 scenario, with the exception of northern part France. This area may indeed benefit from more active winter recharge that may counterbalance decrease in summer precipitation and increase in evapotranspiration. In the mountainous areas, winter flows will increase as a result of higher air temperature and the high degree of agreement between the models holds regardless of the RCP considered. Unsurprisingly, the higher the GHG emission scenario, the higher the median changes. Most of these changes are organised in France along a north-south gradient, regardless of the RCP considered.
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- RC1: 'Comment on egusphere-2025-1788', Anonymous Referee #1, 17 Jun 2025 reply
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General comments
Sauquet et al. presented in their manuscript the results of French national project called Explore2 to provide hydrological climate projections in France for streamflow and groundwater changes until the end of this century. It is a successor project of Explore2070 which took place in in the early 2010ies. The motivation to update projections were to extend projections to the end of the century with newer climate model data and to increase the locations for projections. Many institutions were involved over several years from science, administration and stakeholders in water management. The objective of the project also to provide a better accessibility and understanding of the data for stakeholders compared the predecessor project. The main objectives of the manuscript were to describe the ensemble of the simulations, to evaluate the ensemble in past climate and present main results of the climate projections.
They used a subset from the EURO-CORDEX CMIP5 climate chain ensemble, two bias correcting methods, nine surface hydrology models and four groundwater models. Models were evaluated in the past both using observed forcing and climate model data at large number of observation location, i.e. > 600 gauging stations for surface hydrology models > 200 piezometers for groundwater models over many years. Some of the models were calibrated using similar data as for evaluation. Not all models cover whole France. The authors present evaluation under multiple aspects and mention thresholds for acceptable performance.
The projections cover up to approx. 4000 simulation points for streamflow projections covering whole France and parts of France for groundwater recharge projections. Results for evaluation and projections focused on the surface hydrology models as results of the groundwater models are (or will be) presented elsewhere. However, there is a sufficient overview of the groundwater part presented in this manuscript.
The substantial content of the manuscript is, however, not matched with the quality presenting the data. Most importantly I miss a literature review on the current knowledge of hydroclimatic projections in France (and Europe). The question must be answered, to my opinion, how the presented results embed in and expand existing knowledge.
The second largest issue is the readability of the manuscript, which suffers under many aspects. Most problematic are the large number of the results, the imprecise language which requires the reader to interpret what could be meant and the manuscript structure. To my opinion the manuscript will largely improve with using a commonly used structure to separate data/methods and results/discussion. I will detail the arguments below. With this impression I recommend to major revisions for this manuscript.
Detailed comments
Literature review
As mentioned above the manuscript is not largely referring to previous work. This should also include regional studies. This aim was mentioned in the overview of the manuscript to be part of section 7, but I could not find a relevant discussion with previous literature. This literature review should also include model decisions (e.g. the impact of bias correcting methods …). Section 7 rather adds new own results not presented before.
Readability
First is the large number of results. The objective was to show only main results, and I think this can be more condensed. The large number of results in the figures and tables is supplemented with additional numbers in the text, which makes it difficult to read (e.g. on page 28). Generally, I advise that these numbers in the text could be presented in a table, however, given the need to condense results for this manuscript, I would vote for a more qualitative text description to put the chosen results in tables and figures into value.
Second is the imprecise language. As it is a complex data set covering many dimensions it is essential to be precise. The reader may be able to follow what is meant but it requires a very thorough reading. A table and a figure, however, should be self-explanatory to a certain extent. This is often not given. As an example for many other parts is the presentation of Table 6. The unit is not provided, so one cannot say if the numbers are percentage changes or absolute changes. It can only be assessed from the context that the “interquartile range of the median change …” in the caption description refers to an interquartile range between simulation points (space) and the median change to a set of simulations varying in climate models, bias corrections and hydrological models.
Additionally, I could often not follow what the authors interpret from figures, either by wrong references to figures or that they were imprecise with mentioning patterns or regions. For example, in lines 482-483 “The most significant and positive changes with an agreement between projections can be found for QDJF in the Alps and Pyrenees (Fig. 8) for the three RCPs.” I could follow this statement with looking at Figure 7. Similarly, I could not follow the impressions of the elasticity values (lines 338-342) shown in Fig. 3: Are the values for temperature good, or for precipitation in the summer?
Manuscript structure
The reading flow is often interrupted by switching between methods and results as the manuscript is more structured by content (evaluation, future projections …) than separating data/methods and results/discussion. For example, the results on projected changes start with a description of a multi-model index of agreement (Eq. 2). Ideally the manuscript would keep the content structure but replicate this structure for the data/methods section and the results/discussion section.
Focus of the manuscript
In the abstract the authors mention prominently that they want to present the data availability and understanding the results through visualisation tools. This part is in the manuscript rather short. If the authors want to present their data availability, the understanding thereof and their visualisation tools as promised in the abstract I think this needs to be more elaborated in the manuscript.
For example, the fact sheet in the supplemented is quite complex and is not put in value in the manuscript text. I would have liked to see more details on the description of these sheets and how these sheets help to make informed choices pre-select hydrological models (mentioned in section 5)
The data availability is only presented with a link (section Data availability). There is no guidance in the manuscript or on the webpage how I can get netCDFs of a certain figure for example. The webpage is available in an English version but 99% of the text is still in French. Besides the language barrier (which can be solved with translation tools nowadays) the data is hidden in multiple links. There is a link for each model for each RCP for each subregion, which leads to a new webpage with new links with filenames, which relate to different climate chains and bias correction methods.
Similarly, the visualisation tool is presented just with a link but not put in value in the manuscript
In a similar note: In the conclusion in line 595 the authors advertise their data set with the ability to step back from the results in case of disagreement and promoting an understanding of the differences between hydrological models. This is indeed possible with such a data set, but it is not presented in the manuscript. The authors show mainly median results between hydrological models (except of a minor subtopic with Figure 5). In Table 3 there are thresholds presented for model performance, but nothing mentioned about the consequences. It would be interesting if the authors would include a detailed description of their fact sheets and how one could potentially use them.
Minor points