The climate projections (CPs) used to force the AquiFR platform primarily originates from the DRIAS-2020 database (DRIAS 2021; http://www.drias-climat.fr/, last access: 25 October, 2024) which provides downscaled and unbiased climate projections on the French region. This database is based on a subset of projections from the Euro-CORDEX project (CooRdinated Downscaling EXperiment – European Domain; ), which is included in CMIP5 . As part of the EXPLORE2 database implementation, the original DRIAS-2020 database has been extended to include the latest EURO-CORDEX simulations with evolving aerosol forcing resulting in the EXPLORE2-Climat 2022 database.

To obtain future climate variables, General Circulations Models (GCMs; ; ) are forced by Radiative Concentration Pathways (RCPs), greenhouse gas emission scenarios that follow specific socio-economic trajectories through the 21st century. However, GCM outputs have a coarse spatial resolution of around 2.5°, i.e., between 100 and 200 km, which is insufficient to adequately represent local meteorological phenomena and extreme events. This prevents them from being directly used as climate forcing in hydrological models for regional or local scale studies. Thus prior to use a statistical downscaling method, a dynamic downscaling is made by using RCMs (Regional Climate Models) to provide detailed estimates of meteorological parameters (e.g., temperature, precipitation, humidity, wind, solar radiation, …) at an hourly time step for regional applications using a dynamic downscaling approach , with a spatial resolution from 10 to 20 km.

Then, the ADjustment to MOuNTain regions (ADAMONT; ) method is applied to the RCM outputs to both improve spatial resolution and correct bias according to an historical meteorological reference by a two-steps method:

Weather regime computation : each day from both the RCMs and the meteorological reference is clustered into one of four weather regimes, based on the clustering of the geopotential height at 500 hPa from the ERA-40 analysis, per each one of the four seasons: (1) December, January, February (DJF); (2) March, April, May (MAM); (3) June, July, August (JJA); (4) September, October, November (SON). Then, quantile distributions of each climate variables from both the RCMs and the meteorological reference are computed for the 16 weather regime/season combinations ;

The historical meteorological reference used is the SAFRAN reanalysis (“Système d'Analyse Fournissant des Renseignements Adaptés à la Nivologie” in French; ), which provides “pseudo-observed” data useful for bias correction across France.

At the end of the procedure, the CPs simulated by the GCM/RCM couples provide downscaled climate variables at hourly time step and on a regular grid of 8 km × 8 km. The total set comprises 6 GCMs coupled with 9 RCMs, and forced by 3 RCPs : (1) RCP2.6, a stringent mitigation scenario which assumes an efficient international environmental policy ; (2) RCP4.5, an intermediate stabilisation scenario ; (3) RCP8.5, a very high green house gas emission scenario without efficient environmental international policy . The EXPLORE2 project uses 17 GCM/RCM pairs forced by one to three RCPs, for a total of 36 simulations (Table  in Appendix A), all downscaled and bias-corrected by ADAMONT.

The large number of GCM/RCM couples in the EXPLORE2 ensemble is one of the project's main strengths, which allows to better account for uncertainty when extracting future trends using a probabilistic approach. identified climate components as being the primary source of uncertainty in particular for precipitation, which is the main factor influencing recharge and, consequently, groundwater. Indeed, the climate models can produce different results for the same future scenario, depending on the formulation and parametrisation of physical and thermodynamic processes, which can vary considerably from one model to another, as well as on the spatial resolution. Further details on the biases induced by each specific GCM and RCM can be found in . In light of this divergence, which limits the extraction of future trends using a probabilistic approach, a second approach is adopted. This approach involves selecting four contrasting storylines (i.e., CPs based on contrasted trends) from the available GCM/RCM couples using the RCP8.5. This allows us to focus on four contrasting possible futures . Unlike the probabilistic approach, the aim here is to consider each narrative individually to maintain its physical consistency without mixing it with the other narratives in the set. The selected storylines are :

Orange storyline (EC-EARTH/HadREM3-GA7-05 couple): climate affected by a strong warming and very dry in the summer;

It should be noticed that the latter CMIP6 projections are not used here, since they have not yet been downscaled or bias-corrected at the regional level, which prevents their use for regional studies. However, the EXPLORE2 dataset was compared with projections from CMIP6, and the projections used here are included within the CMIP6 confidence interval . This lends credibility to the use of these climate projections, particularly given that they have been used in recently published papers .

Following the recommendations of the EXPLORE2 project, the analyses of CPs were conducted over four time periods comprising 1 historical period and 3 future horizons for a total time frame ranging from 1975 to 2099. This considers that an hydrological year is set from 1 August to 31 July (Table ).

AquiFR is a hydro-meteorological modelling platform regrouping existing models, one model being defined as a pair of catchments and the hydrogeological modelling software used, to simulate French rivers and groundwater systems . Each model is developed and configured for a specific region to meet the needs of stakeholders. The platform was assessed over a sixty-year period , and has demonstrated good performance in simulating groundwater levels, compared to observed piezometric heads from the ADES (“Accès aux Données sur les Eaux Souterraines”) database (http://www.ades.eaufrance.fr/, last access: 29 November 2024, ). Nowadays, the AquiFR platform is used in an operational context for real-time monitoring and seasonal forecasting on the main sedimentary basins and aquifers for groundwater decision-makers .

The SPLI maps for each RCP8.5 projection of the median value over the long-term horizon (Fig. ) show heterogeneity between the different projections, from dry to wet values. Some GCM/RCM couples, such as the orange storyline (EC-EARTH/HadREM3-GA7-05 couple), lead to a increase in the area corresponding to dry events, specifically 10-year events which span almost the entire AquiFR domain on the long-term horizon. On the contrary, other couples like the green storyline (HadGEM2-ES/ALADIN63) and the IPSL-CM5A-MR/RCA4 couple lead to predominantly stable or increasing groundwater levels for the three horizons, with a large area characterised by median groundwater level above the 10-year wet level throughout the entire period.

Overall, most of GCM/RCM couples lead to an increase in groundwater level across the domain (11 out of 17, ≈ 65 %), while 4 induce a decrease (≈ 23 %) and 2 induce relatively stable values (≈ 12 %).

Focusing on the purple storyline, the results indicate stable groundwater levels in the northern part of the domain for the short-term horizon while in the Beauce and in the Loire basin, groundwater levels increase (median above 2.5-year wet). This pattern changes over time, with conditions close to normal for the intermediate and long-term horizon over the major part of the domain, except in the Basse-Normandie region with decreasing trend. The yellow storyline shows the least changes. Groundwater levels in southern Beauce, the Loiret department and the northern part of the Loire basin decrease slightly (median SPLI below the 2.5-year dry, and around normal elsewhere). Similar results are observed for the long-term horizon, except in the northern France (Nord-Pas-de-Calais and Basse-Normandie), where groundwater levels slightly increase (median SPLI between 2.5- and 5-year wet). Let us note that the MARTHE Tarn-et-Garonne and MARTHE Alsace models show an opposite trend with decreasing groundwater levels for all the storylines, corresponding to extreme dry events, from the short-term horizon for Alsace and from the long-term horizon for Tarn-et-Garonne.

The same analyses for the short-term (Fig. ) and the intermediate horizons (Fig. ) under the RCP8.5 are available in Appendix B. Despite the divergent results, two main patterns are distinguishable:

From the northern part of the AquiFR domain to the southern part of the Loire basin, the median of the groundwater levels increases up to what is today considered as 5-year wet events on the short-term and intermediate horizons, and to extreme wet events on the long-term horizon;

These findings suggest a North-South divide in the future evolution of groundwater level over the AquiFR domain under the RCP8.5, as highlighted by 3 out of the 4 storylines (green, purple and yellow).

Under the RCP2.6 (Figs., and in Appendix B respectively for the three horizons), projections show the largest heterogeneities. On the long-term horizon, 4 out of 10 GCM/RCM couples (40 %) project an increase of the groundwater level to a relatively high value, 3 couples (30 %) project almost no change and 3 couples (30 %) project a decrease of the groundwater level, especially the ones related to the EC-EARTH GCM. These results are consistent across the three horizons. Similarly, projections with RCP4.5 for the short-term and intermediate horizons (see Figs. and ) are divergent, preventing any statement as to the future trend in groundwater levels for this scenario. On the long-term horizon (see Fig. ), the results are more homogeneous with 6 out of 9 GCM/RCM couples inducing increasing groundwater level corresponding to high values over more than half of the AquiFR domain, while 3 couples induce either no change or slight decrease of the groundwater level.

Due to these large heterogeneities in the results from all the CPs, we focus the further analyses on the storylines, while the results corresponding to all the CPs are available in Appendices B and C.

Figure  illustrates the temporal evolution of the percentage of the surface within the AquiFR surface area that falls into one of the six SPLI categories (in line), as defined in Table , for the four storylines under the RCP8.5. The left column corresponds to wet events and the right column to dry events. Overall, the results depend largely on the hypotheses on which the storylines are based. The results show strong temporal variabilities, mostly on the 10-year events, probably related to climate natural variability. The orange storyline tends to significantly decrease the surface affected by wet events and increase the ones affected by 10-year and 5-year dry events. The surface corresponding to 10-year dry events almost reaches 60 % of the AquiFR domain by the end of the 21st century. The green storyline induces an increase the surface related to wet events, i.e., 10-year wet events covering almost 20 % of the AquiFR domain (from 3.7 % on the historical period), and a decrease of the surface dedicated to dry events, except for the 10-year dry event where the surface increases to 10.2 %. The changes induced by the purple and the yellow storylines are smaller, except for both wet and dry extreme events where the contrasts are particularly pronounced. These two storylines tends to increase the extreme events (mainly the purple storyline) and reduce the surface allocated to 2.5-year events. These results are confirmed by the statistical tests with p-values below 0.05 for almost all the 6 categories of events (see Table  and Table ), meaning that the future is significantly different from the reference period (Kolmogorov-Smirnov test) and changes significantly over the 21st century (Mann-Kendall test), for the four storylines.

The results for all the CPs are available in Appendix C (see Fig.  and Table ). The results show that the envelopes between the minimum and maximum values are wide, mainly for the extreme events, which highlights the heterogeneity between the projections. The surfaces related to 10-year events increase for both wet and dry events, spreading with time and increasing greenhouse gas emission, particularly for wet events. The surface related to extreme dry events is also projected to increase, albeit with a much less pronounced trend, reaching at most 10.0 % under the RCP8.5. The changes of surface proportions corresponding to the 5-year dry or wet events are low, slightly increasing for the wet events, and decreasing for the dry events. Eventually, both dry and wet 2.5-year events are expected to decrease over the 21st century, indicating an intensification of extreme events, which aligns with the findings from the storylines.

Figure  illustrates the maps of the median of the time proportion corresponding to extreme events (i.e., at least 10-year events), for wet (1st line) and dry (2nd line) events for the four storylines on the long-term horizon. The results for the short-term and intermediate horizons are available in Appendix C (see Fig. ). In consistency with the previous analyses, the results vary depending on the storyline. The orange storyline tends to reduce the time proportion corresponding to extreme wet events to less than 5 % over almost the entire AquiFR domain, while increasing the time proportion of extreme dry events to more than 75 % over the whole domain. The green storyline strongly increases the time proportion of extreme wet events while reducing the one corresponding to extreme dry events. The purple storyline induces an increase in both wet and extreme dry events, higher as time goes to 2100, and representing more than 50 % of the time over half of the AquiFR domain by the end of the 21st century. The results for the yellow storyline are more spatially and temporally contrasted and do not highlight a noticeable future tendency over the 21st century.

The results for all the CPs (see Fig. ) show that, in the short-term horizon, the time proportion of extreme wet events corresponds to almost half of the time over half of the AquiFR domain for the three RCPs. This proportion remains close to the historical conditions over the remaining part of the domain. Then, this time proportion increases as time goes to 2100, primarly in the northern part of the AquiFR domain. Results for the time proportion corresponding to extreme dry events are more contrasted. On the long-term horizon, the results suggest a decrease of the median of the time proportion in extreme dry event with values on more than half of the AquiFR domain under the RCP2.6 and RCP4.5. Under the RCP8.5, the results show a decrease of the time proportion in the northern part of the AquiFR domain, and higher values on the southern part of the domain, suggesting a North-South contrast in the evolution of the time proportion corresponding to extreme dry events.

In this study, the evolution of the groundwater is simulated by the AquiFR modelling platform using 36 climatic projections spread over 3 socio-economic scenarios RCPs. As highlighted in the introduction section, the use of such an extensive range of CPs enables a robust assessment of uncertainties associated with internal climate variability. When combined with the exploration of four distinct storylines to illustrate contrasting future scenarios, this methodological approach remains rare in the existing literature, particularly in studies examining groundwater dynamics under climate change. Furthermore, the AquiFR platform provides a noticeable advantage by accounting for the heterogeneity of aquifers across the French aquifers covering by the platform. This capability is further enhanced by the integration of the SPLI, which facilitates spatially explicit analyses of future groundwater level trends. Collectively, these elements – the CP ensemble, the storyline-based approach, and the high-resolution aquifer modelling – represent the primary scientific advancements of this paper, with direct implications for groundwater management and climate adaptation strategies.

The results demonstrate that the spatial distribution of the SPLI median varies between the projections, resulting in an uncertainty that restrains the identification of a clear trend regarding the future evolution of groundwater. This uncertainty is to be expected as the study area falls within the transition zone of the Atlantic region marked by a large internal variability , where expected trends shift from a wetter climate in the North to a drier climate in the South . Therefore, there is a lack of consensus among the CPs regarding the location of the transition zone, thereby inducing divergence in the future evolution of groundwater levels.

Faced to the divergences observed between all the available CPs in the multi-model ensemble approach, the extraction of reliable future trends is quite challenging and strongly depends on the RCP considered. As regards the RCP8.5, two tendencies have been identified, primarly through to the utilisation of storylines which demonstrate analogous future evolutions despite being based on contrasted future evolutions consistent with their design assumptions . First, groundwater level from the northern part of the AquiFR domain (i.e., Seine, north of Loire catchment, Basse Normandie and Nord Pas de Calais) is expected to increase toward wetter events, mostly corresponding to 10-year wet groundwater level of the reference period. The green storyline induces this increase from the short-term horizon over most of the AquiFR domain, while this increase starts from the long-term horizon for the yellow (Nord Pas de Calais and Basse-Normandie) and the purple (on the Beauce and the Loire catchment) storylines. Second, a decrease in groundwater level from the southern part of the AquiFR domain (i.e., south of Loire catchment and Tarn-et-Garonne) and from the Alsace catchment is expected, at least to levels corresponding to 5-year dry events on the reference period on the South of the AquiFR domain on the long-term horizon. The orange storyline enhances this decrease to the entire AquiFR domain, reaching levels corresponding to 10-year dry events on the reference period. The yellow storyline also induces a slight decrease of groundwater level from south of Loire to the southern part of the AquiFR domain.

This North-South contrast in the evolution of groundwater levels under the RCP8.5 is highlighted by both the majority of the CPs and by 3 out of the 4 storylines, and appears to be the consequence of the North-South contrast observed in future precipitation projections derived from climate models used in the EXPLORE2 project . In this paper, the spatial representation of South of France into the AquiFR platform is limited, which prevents the existence of this North-South divide from being highlighted with certainty. However, these findings are consistent with , who predict an increase in groundwater level in the northern part of the French metropolitan area as time goes to 2100, and a decrease in the southernmost sectors of France, using a subset of the RCP8.5 DRIAS projections. Moreover, in Sect. , we demonstrated that this contrast is already visible in earlier stages of the modelling chain, in particular on the drainage and runoff variables simulated by SURFEX and which constitute the main drivers of the aquifers recharges (see Appendix D). For RCP8.5 (see Fig. ), the North-South divide is already visible for precipitation and real evapotranspiration but the contrast remains moderate. Conversely, the contrast is much clearer for the groundwater recharge term across the three quantiles, particularly for drainage, with northern France becoming wetter and southern regions drier. Note that the Q90 quantile still highlights a decrease in drainage in regions close to the Pyrenean mountains. All these elements are consistent with joint research from the EXPLORE2 project on groundwater recharge and surface hydrology .

Nevertherless, some differences with our findings and the conclusions extracted from the literature are to be noticed. Indeed, the conclusions drawn for the northern part of the AquiFR domain are not consistent with previous studies which, as mentioned in Sect. , suggested a global decrease of both recharge and groundwater level over France . The divergence may come from that CPs from the CMIP5 global project are wetter than the ones from the CMIP3 global project from which these previous studies were based on, particularly over northern France . Hydrological systems are therefore directly affected by this additional amount of water into the water balance between the two versions of the projections. In this way, the consistency between the present results and the ones from was to be expected as they used the hydrogeological MARTHE model, included in the AquiFR platform, and CPs provided by the DRIAS-2020 database, which are included into the CP database established by EXPLORE2. However, the main difference is that the MARTHE hydrological software package was configured on a coarser spatial grid of 2 km × 2 km, and their study is based on 5 selected regionalized CPs, while in this paper 17 different GCM/RCM combinations have been used.

Moreover, most of studies addressing surface hydrology under climate change state that the seasons will be more contrasted in the future, e.g., winter flows will be higher while summer will be lower , shift that can be attributed to a seasonal regime change of rainfall and evapotranspiration balances . However, by assessing the spatial distribution of SPLI for each one of the 12 months of the year (not shown here), we found similar results for each month, suggesting no modification in the seasonal regime of groundwater level. This phenomenon may be attributed to the slow temporal dynamics characterising the aquifers in the study area, particularly those close to the Parisian Basin which constitute a large part of the AquiFR domain. Indeed, these aquifers are subject to variations that often extent beyond the hydrological year and even span multi-decadal . This renders the observation of sub-annual variations a highly arduous undertaking.

Let us note that the storylines inducing contrasted yet possible future evolutions highlight their usefulness to groundwater decision-makers, especially when they do not converge. The northern part of the AquiFR domain, i.e., from Nord Pas-de-Calais to the South of the Loire basin, is the sector where the contrast between the storylines is the most pronounced, particularly in the long-term horizon. In this context, the storylines can encourage the decisions-makers to establish their policy based on the less favourable projection, i.e., the “worst case scenario” (or precautionary principle), or using no-regrets measures based on the consequences of climate change on other surface variables such as the different seasonal flows or the 20-year return period quantile of the annual maximum daily flow to characterise floods .

The extraction of tendencies for the RCP2.6 and the RCP4.5 was an ardous task, because the projections are too divergent to give confidences on the evolution of the groundwater, especially on the short-term and the intermediate horizons. On the long-term horizon under the RCP2.6, the results are still quite divergent, yet they do underscore an increase of groundwater with values that reach the 5-year to 10-year wet return period of the reference. Under the RCP4.5, this result is in line with the statement established by on the climate aspect of the EXPLORE2 project, which projects a 3.5 % increase in precipitation on the long-term horizon under this RCP in North of France, encompassing the majority of the AquiFR domain (see Appendix ). This additional amount of precipitation increase water supply, groundwater recharge and ultimately raise groundwater levels in these areas.

As stated in the introduction section, this study does not address the impacts of anthropogenic factors on water resources, which constitutes a major limitation of this work. As mentioned in Sect. , groundwater abstractions currently applied to the studied aquifers, thus defined according to current water availability and water demand, are supposedly maintained in the future and do not change regardless of groundwater level evolution and groundwater availability to meet future water demand. This assumption may be rendered moot by the fact that, according to several projections, groundwater level is expected to decrease over the 21st century, specifically in the southern AquiFR domain. In such cases, it is imperative that current groundwater abstractions need to be adapted to future conditions. In further assessments, including adapting groundwater abstraction could prove to be a crucial element in enhancing the persistency of our analysis.

In this study, hydrogeological extreme events are defined as decadal events, i.e., at least 10-year events (see Table ), for both wet and dry events. The results show that the surface affected by extreme wet events increases by the end of the 21st century as both time goes to 2100 and greenhouse gases are released by the RCP. For RCP8.5, extreme wet events increase by a factor of 4.5 on the long-term horizon considering all the CPs, a trend supported by 3 out of 4 of the storylines (green, purple and yellow). The increase of wet events could affect surface water, resulting in an increased risk of flooding, as shown by in northern France, due to the strong influence of groundwater dynamics on the amount of water in rivers and wetlands . Let us note that the orange storyline predicts a different future trend characterised by a total disappearance of extreme wet events.

Conversely, the results also show an increase in extreme dry events, as confirmed by the four storylines, which correspond to a decrease of more regular events, i.e., 2.5-years events. It should be noted that even the green storyline induces this increase of extreme dry events, despite its design assumption leading to a larger amount of precipitation. Furthermore, the orange storyline leads to a much larger increase, by a factor of 25 of the extreme dry events, covering almost the entire AquiFR domain. In this case, groundwater recharge may decrease, a phenomenon already observed in some locations nowadays, e.g., the Champagne region . The areas where these increases are larger are Alsace, Poitou-Charentes and Tarn-et-Garonne on the RCP8.5. However, for most of the CPs and the 3 remaining storylines (green, purple and yellow), this increase is lower than the one for wet events. Therefore, the meteorological droughts announced in the EXPLORE2 project do not appear to noticeably involve hydrogeological droughts in the large majority of the AquiFR domain. This is differs notably from recent hydrological drought assessments, which clearly indicate an intensity of drought events, whether in terms of low flows observed or periods of no flow, particularly in South of France . This result contradicts the results from the EXPLORE 2070 project which suggested a strong increase of hydrogeological droughts . As mentioned in Sect. , the CPs used in the EXPLORE2 project are wetter than the ones used in the EXPLORE 2070 project . As such, the ones from EXPLORE2 are likely to induce fewer dry events than those used in the EXPLORE 2070 project. Furthermore, the aquifers corresponding to the study area are defined by annual or multi-annual temporal dynamics and often involve groundwater-river interactions, whereby the groundwater feeds the river . Therefore, droughts may be observed at the surface without having a direct effect on groundwater.