French groundwater model ensemble projects wetter north, drier south

Climate-change simulations indicate that groundwater conditions in France could diverge geographically during the 21st century, with most projections showing wetter aquifers in the north and drier conditions in the south. The finding comes from a modelling study that forced the AquiFR hydrogeological platform with 36 regional climate projections under three greenhouse-gas scenarios. Under RCP8.5, the scenario with the highest emissions among those examined, three of four contrasting climate storylines supported higher groundwater levels in northern France, while all four indicated a southward concentration of more severe low-water conditions. The researchers describe a north–south contrast rather than a uniform national trend and emphasize that these are model projections, not observations of future groundwater levels.

The study assessed 13 aquifer models within the AquiFR platform, covering Nord Pas-de-Calais, Basse-Normandie, the Paris Basin, the Loire and Poitou-Charentes basins, Alsace, and Tarn-et-Garonne. AquiFR combines physically based hydrogeological models, including EauDyssée and MARTHE, with the SURFEX land-surface model, which calculates runoff and drainage used as groundwater-recharge terms. Its climate forcing came from the EXPLORE2 ensemble, a downscaled and bias-corrected subset of Euro-CORDEX projections derived from the CMIP5 climate experiment. The simulations covered 1975–2099, with a 1975–2004 reference period and future horizons of 2021–2050, 2041–2070, and 2071–2099.

Groundwater conditions were evaluated with the Standardized Piezometric Level Index, or SPLI, a normalized measure that expresses each monthly water level relative to the historical distribution for the same calendar month. Values above zero represent levels above the historical median, while negative values represent lower, drier conditions. The index also expresses conditions as historical return periods, allowing wet and dry events to be compared across heterogeneous aquifers. Kolmogorov–Smirnov tests assessed whether future and historical distributions differed, while Mann–Kendall tests evaluated whether conditions changed monotonically through time. The significance threshold was p ≤ 0.05. In the full ensemble analysis, areas associated with more moderate 2.5-year events declined, areas associated with 5-year events changed little, and the footprint of 10-year extremes expanded.

Disagreement among the climate projections was substantial, particularly over the short- and intermediate-term horizons, preventing a single trend from emerging for every scenario. Among 17 available RCP8.5 global and regional climate-model combinations, 11, or about 65%, indicated generally increasing groundwater levels, four produced decreases, and two indicated relatively stable conditions. The study linked much of this spread to the climate component of the modelling chain, including internal climate variability and differences among global and regional climate models. Historical groundwater cumulative-distribution functions derived from the climate projections generally followed those produced using the SAFRAN reanalysis, suggesting that much of the future divergence originated in projected rainfall, drainage, and runoff rather than in the construction of the groundwater index.

Four RCP8.5 storylines were selected to represent contrasting but internally consistent climate futures without combining their trajectories into an averaged projection. The orange storyline paired strong warming with very dry summers and produced the most adverse groundwater outcome. By the end of the century, nearly 60% of the modelled domain was affected by 10-year dry events in the area-based analysis, while more than 75% of the domain experienced extreme dry conditions in the separate time-proportion analysis. The green storyline, characterized by serious warming and increased rainfall, instead generated widespread wetter conditions. Under it, the area affected by 10-year wet events increased from 3.7% in the historical period to almost 20%.

The purple storyline, defined by a strong seasonal contrast in precipitation with wet winters and dry summers, increased both wet and dry extremes. Under this projection, extreme dry conditions occupied more than half of the time across more than half of the modelled domain late in the century. The yellow storyline, designed around relatively modest future changes, produced the smallest overall shifts and a more spatially varied pattern without a dominant tendency. Statistical tests identified significant differences between historical and future distributions and significant temporal changes for almost all event categories in the four storylines. Those results apply separately to each physically consistent projection, however, and do not establish one deterministic future.

Across the full RCP8.5 ensemble, the area affected by extreme wet groundwater conditions increased by a factor of 4.5 over the long-term horizon, a result supported by the green, purple, and yellow storylines. Extreme dry events also increased overall, although generally by less than wet extremes; the driest orange storyline produced a much larger increase, reaching a factor of 25. The ensemble analysis found that the surface assigned to 2.5-year dry and wet conditions decreased, while 5-year dry and wet areas changed only slightly and 10-year events expanded. This pattern points toward greater separation between ordinary conditions and the most extreme states, rather than a broad increase in the frequency of all wet or dry categories.

Maps of the proportion of time spent in extreme events revealed clear differences among the storylines. The orange projection reduced the time associated with extreme wet events to less than 5% over almost the entire AquiFR domain while extending extreme dry conditions across more than 75% of it. The green projection had the opposite tendency, strongly increasing extreme wet events while reducing extreme dry ones. The purple projection increased both, whereas the yellow projection remained more heterogeneous. In the combined RCP8.5 analysis, the time spent in extreme wet events was close to historical conditions over part of the domain in the short term but increased toward 2100, especially in northern France. Extreme dry conditions showed a contrasting long-term pattern, with lower values in the north and higher values in the south.

The simulations were designed to isolate climate effects by holding land use and groundwater abstraction constant at historical climatological levels. This separation avoids confounding climatic change with human water use, but it also means the projections do not represent how abstraction, demand, or policy might change as groundwater availability evolves. The assumption is particularly consequential in southern areas where some simulations produce lower levels. AquiFR also does not apply multiple interchangeable groundwater models throughout its full domain, so the contribution of hydrogeological-model uncertainty cannot be fully quantified. Southern coverage is sparse: the Alsace and Tarn-et-Garonne models each cover about 1,100 square kilometres, compared with roughly 7,000–66,000 square kilometres for most other modelled aquifer systems. These limitations prevent the findings from establishing a nationwide north–south divide with the same confidence across all of metropolitan France.

The study’s broad conclusion is that climate projections imply contrasting regional futures and a growing separation between exceptionally wet and exceptionally dry groundwater conditions, rather than a uniform rise or fall across France. Under RCP2.6 and RCP4.5, the 2021–2050 and 2041–2070 results were too divergent for reliable trend statements. Over 2071–2099, they showed greater convergence toward wetter conditions in the north, while the remote Tarn-et-Garonne and Alsace models continued to disagree. The authors identify expansion of southern model coverage, fuller quantification of uncertainty across groundwater software, and integration of socio-economic water demand as priorities for future assessment. Until then, the four storylines demonstrate how a single national projection can conceal materially different outcomes among aquifers, including futures in which extreme groundwater drought expands across nearly the entire modelled region.

Subject of Research: Modelled climate-change impacts on groundwater levels and extremes across French aquifers

Article Title: Climate change impacts on groundwater simulated using the AquiFR modelling platform

Article References: Jeantet, A., Vergnes, J.-P., Munier, S., & Habets, F. (2026). Climate change impacts on groundwater simulated using the AquiFR modelling platform. Hydrology and Earth System Sciences, 30(20), 6381-6420. https://doi.org/10.5194/hess-30-6381-2026

Image Credits: AI Generated

DOI: 10.5194/hess-30-6381-2026

Keywords: groundwater, climate change, France, AquiFR, hydrogeological modelling, climate projections, extreme events, aquifer recharge, RCP8.5, water resources, Climate, change