Hydrogeological situation on 1 October 2024
In September, groundwater levels continued to fall over a large portion of the country, decreasing at 70% of indicator sites. Levels rose or were stable in central and southern parts of the country, which received some rainfall.
The low-water period in 2024 is looking very promising, with 73% of levels above the monthly norm. The groundwater situation along the Roussillon coast, in Languedoc and in north-eastern Corsica remains a cause for concern, with low to very low groundwater levels.
The trend for October will essentially depend on how effective the rainfall is locally and on the responsiveness of individual groundwater bodies. Where groundwater levels respond quickly to rainfall, recharge is expected to begin in areas receiving some rainfall. How the situation evolves will depend on cumulative rainfall. Aquifers where groundwater levels are slow to respond should remain on a downward trend or stabilise, and the situation is expected to change only moderately.
It should be noted that early and particularly abundant groundwater recharge could have a significant impact on surface environments. The main risk currently concerns groundwater bodies that respond rapidly to rainfall, as they could be a contributing factor to overland flow and fluvial flooding where they limit rainfall infiltration and water discharge, or where groundwater flooding occurs and they discharge back to streams, surface-water systems and springs.
Carte de France hexagonale de la situation des nappes d'eau souterraine au 1er octobre 2024.
© BRGM
Evolution of the trends observed on piezometers from October 2023 to September 2024.
© BRGM
Groundwater trends
During the autumn and winter of 2023–2024, groundwater recharge was substantial over most of France, and low in the Pyrenees, in Roussillon, along the western coastline of Languedoc and in Corsica. Spring rainfall helped support groundwater levels and even contributed to groundwater recharge up until May. Groundwater levels continued to fall during the summer, but often at a slower pace due to rainfall and low abstraction pressure.
In September 2024, falling groundwater levels remained the dominant trend, at 70% of indicator sites (86% in August). This phenomenon is common in the summer and early autumn. Until the vegetation becomes dormant, the rainfall is rarely effective enough to result in groundwater recharge, as the water that does manage to infiltrate the ground is entirely taken up by the vegetation.
However, the conditions for recharge events are present in many areas, as wet soils and low temperatures reduce evapotranspiration and the water needs of vegetation. As a result, September rainfall may have had an impact on groundwater levels.
Rising or stable groundwater levels were mainly observed at groundwater sites that respond quickly to rainfall: alluvial aquifers, Jurassic and Cretaceous limestone aquifers and aquifers of the basements of the southern Paris Basin, eastern and southern Massif Central, south-western Aquitaine Basin and south-eastern Corsica. Groundwater levels were observed to have stabilised or risen in the southern Paris Basin and Bas-Dauphiné aquifers, which are very slow to respond to rainfall, as the summer rainfall percolates into the ground slowly.
Lastly, groundwater levels were still falling over a large portion of the country, but only slightly in many of the aquifers whether with a rapid or slow response to rainfall, indicating that they are about to begin rising again in the coming weeks as the recharge period restarts. The declining trend in groundwater levels is still very marked in Roussillon and western Languedoc, due to the lack of rainfall.
Comparison between 1 October 2023 and 1 October 2024
Map of France showing the state of the aquifers on 1 October 2023 (left) and 1 October 2024 (right).
© BRGM
Evolution of the situation observed on piezometers from October 2023 to September 2024.
© BRGM
Overall groundwater situation
After a highly abundant recharge in 2023–2024, the groundwater situation at the end of winter was very satisfactory over a large portion of the country. The rainfall in the spring and summer helped to maintain, and even improve, the groundwater situation. The only exceptions were in the Pyrénées-Orientales, Aude, Cap Corse and the eastern Corsican plains where groundwater levels remained poor, due to the 2023–2024 recharge deficit and little to no rainfall during the spring and summer.
Groundwater levels were mostly above the monthly norm in September 2024: 17% of observation points were below the monthly norm, 10% were similar and 73% were above the monthly norm (compared with 17%, 13% and 70%, respectively, in August 2024).
The groundwater situation was much better than that observed last year, in September 2023, when 66% of recorded levels were below the monthly norm. The situation in Pyrénées-Orientales, in Aude and in eastern Corsica bucked the trend, with groundwater levels lower than in 2023. September 2024 was the second wettest September month for groundwater in the last 30 years (after September 2001, when 81% of the levels observed were above normal for that month).
There was little change between August and September 2024. The situation is gradually improving for responsive groundwater bodies that have received effective rainfall: Lorraine, Alsace, Jura, northern two-thirds of the Massif Central, south-western Aquitaine Basin, Provence, Côte d'Azur and Corsica. Groundwater levels deteriorated slightly in the Champagne and the southern Massif Central groundwater bodies, which typically respond rapidly to rainfall. In these areas, groundwater levels were affected by the lack of effective rainfall that was experienced in September. Finally, groundwater levels continued to fall in Roussillon and in Aude, following a lack of rainfall over several months.
In September, the groundwater situation remained very satisfactory over a large portion of the country, with many levels recorded as moderately high to very high. This can be explained by a higher-than-normal recharge in 2023–2024 and the significant contribution of rainfall during the spring, as well as occasional episodes of rain in the summer. Locally, there were very high levels usually reached only once in 20 years, particularly in the Jurassic limestone in Berry and south of the Poitou ridge.
In some areas, lower-than-average monthly groundwater levels were recorded locally. Unsatisfactory levels, ranging from moderately low to low, were still being recorded at a few indicator sites that are very slow to respond to rainfall, in the central-western part of the Paris Basin (Normandie chalk and Beauce), Sundgau, Bresse and Dombes, and north of the Drôme. This is because the groundwater bodies have very long response times and/or because of a more pronounced rainfall deficit than usual during the winter recharge period. Moderately low to low levels were also observed in the Côtes d’Armor basement as this area received low levels of rainfall during the spring and summer. Following the 2023–2024 recharge deficit, groundwater levels in the Auvergne volcanic region were gradually catching up thanks to spring and summer rainfall, but some levels were still low under the Puys mountain range. Groundwater levels in Provence reflected the contrasted rainfall patterns of these last few weeks, and some groundwater sites still showed lower-than-average levels.
The situation is still very uncertain, with low to very low groundwater levels in Roussillon and western Languedoc (Aude, Hérault and Orb). On the Roussillon plain and in the Corbières Massif, the groundwater situation is a cause for great concern. The levels have been receding for more than two years, sometimes reaching historic minimum levels. The groundwater situation remained very variable across Corsica, with worryingly high to very high levels along the western coast, and moderately low to very low levels in Cap Corse and the eastern plains. In these areas, while the rainfall led to some recharge events, there was not enough rain for groundwater levels to fully recover and make a visible difference to the groundwater situation.
Aquifers that are in a very good state
The situation at many groundwater sites was very satisfactory, with high to very high levels compared to historic levels for September:
- The least responsive and slightly responsive groundwater bodies of the chalk and Tertiary formations along the Artois-Picardie coast, in Burgundy and the Gâtinais, Sologne and Sancerre, and from Brie to Tardenois showed high to very high groundwater levels.
- Many of the responsive groundwater bodies along the edges of the Paris Basin, the Jura, the Massif Central, the southern Armorican Massif and the Aquitaine Basin received substantial rainfall in the spring and summer.
Aquifers in a poor state
The situation of several groundwater bodies was relatively poor, with low to very low levels compared to September in previous years, due to a severe lack of rain over these last few months or years:
- The status of groundwater bodies in north-eastern Corsica remained poor, with moderately low to very low levels locally despite the recent rainfall.
- Alluvial groundwater levels in Aude, Hérault and Orb were low to very low.
- Groundwater levels in the multi-layer Roussillon aquifer and in the karst limestone of the Corbières Massif remained very low, due to a lack of rainfall for more than two years.
How do aquifers fill up and how do they empty?
Groundwater levels vary throughout the year, from high levels in winter (when vegetation does not absorb rainwater) to low levels in summer (the traditional depletion period).
The fate of rainfall varies greatly depending on the time of year and the condition of the ground surface on which it falls. Usually, the groundwater-recharge period takes place from early autumn (September-October) to early spring (March-April), a six-month period during which vegetation is dormant (with low evapotranspiration) and rainfall is generally more abundant. If the winter is dry, groundwater recharge is very low.
From spring through summer, rising temperatures coupled with the regrowth of vegetation and thus increased evapotranspiration, limit the infiltration of rainfall into aquifers. Between May and October, unless there are exceptional rainfall episodes, aquifer depletion usually continues and levels will keep decreasing until the autumn.
Why do some aquifers recharge faster than others?
Groundwater flows at different rates depending on the porosity (percentage of gaps/cracks in the rock) and permeability (capacity to allow water to circulate, i.e. interconnectivity between these gaps/cracks) of the aquifers. The larger the gaps and the more interconnected they are, the faster the water will flow, for both refilling and depletion.
It takes a given volume of water different periods of time to travel the same distance, depending on the kind of rock formation:
- a few years in a porous formation,
- a few months in a cracked formation,
- and a few days, or even a few hours, in a karst formation.
Inertial and reactive aquifers: what are they?
The impact of the winter recharge varies according to the cyclic nature of the aquifer, i.e., its reactivity to rainfall infiltration.
We refer to aquifers that are:
- reactive (when they are composed of sand, gravel, karst limestone or weathered granite formations). These aquifers are characterised by their rapid reaction times: they can recharge during heavy summer rainfall, but are also highly sensitive to drought. Their levels can therefore vary very quickly over the course of the same season.
- inertial (when composed of chalk, non-karst limestone, sandstone formations). Their reaction times are slow. They can have multi-annual cycles, meaning that they require a long period to recharge or empty.
Cyclicity of aquifers in mainland France.
© BRGM
Forecasts
The Météo-France seasonal forecasts for October, November and December predict drier conditions across western France. No clear pattern emerges for rainfall in the north and east of the country or for temperatures.
The beginning of autumn is generally a pivotal period between the end of the summer decline (with falling levels) and the beginning of the winter recharge (with rising levels). A reversal of the trend will depend on rainfall infiltration, which in turn depends on precipitation levels and vegetation dormancy, groundwater abstraction levels and how fast groundwater bodies respond. Several low-pressure fronts, accompanied by heavy rain, have been observed and are expected in October. In addition, the vegetation gradually becomes dormant and its need for water diminishes. The conditions seem to be right for the recharge period to gradually become general across all groundwater bodies during October. The speed with which rainfall impacts groundwater levels will vary, depending on the responsiveness of individual groundwater bodies. However, groundwater levels may continue to fall in areas where there is a lack of effective rainfall.
The situation will need to be closely monitored at groundwater sites currently showing moderately low to very low levels (Roussillon, western Languedoc, north-eastern Corsica). The current state of groundwater in those areas is worrying, and the 2024 low-water period is expected to be very severe. The 2024–2025 recharge period will need to be monitored more closely. The recharge will need to be particularly prolonged and abundant to replenish the groundwater resource over the long term at the sites with the lowest levels.
Any changes in groundwater levels over the coming weeks will depend mainly on cumulative rainfall. The situation can change rapidly for the groundwater bodies with fast responses and more gradually for those with slow responses. However, it is already clear that the 2024 low-water mark (minimum annual water flow) is likely to be particularly high over a large portion of the country, and forecasts are optimistic, showing no winter groundwater drought where groundwater levels are currently above the monthly norm (the vast majority of the country).
Given the current high to very high levels, an early and particularly abundant groundwater recharge could have a significant impact on surface environments. The main risk currently concerns groundwater bodies that respond rapidly to rainfall, as they could be a contributing factor to overland flow and fluvial flooding where they limit rainfall infiltration and water discharge, or where groundwater flooding occurs and they discharge back to streams, surface-water systems and springs. In this scenario, the alluvial groundwater bodies would no longer play a buffer role and would no longer reduce the impact of heavy rainfall and fluvial flooding. On the other hand, the risk of flooding from rising groundwater levels in the least responsive groundwater bodies is still very uncertain, as it depends on cumulative rainfall over the entire 2024–2025 recharge period. Changes in groundwater levels that respond very slowly to rainfall will therefore need to be monitored particularly closely during the autumn and winter, with a greater risk in 2025.
For the moment, particular attention should be paid to the more responsive groundwater bodies, which reached very high levels in September and could soon reach historic highs. Increased monitoring at these groundwater sites is recommended over the coming weeks/months, particularly where they are prone to groundwater flooding, such as for the following groundwater bodies:
- Jurassic limestone in Berry and south of Bourges;
- Upper Jurassic limestone in Charentes, north of Angoulême and Cognac;
- locally in the Limousin plateau basement; and
- locally in the Plio-Quaternary formations of the Aquitaine Basin.
Next groundwater tables status report
Our groundwater report is now published every month, in the middle of the month.
The next issue will be published in mid-November 2024.
State of groundwater: monitoring by BRGM
Groundwater is a widely used resource: in metropolitan France, it accounts for nearly two-thirds of drinking water consumption and more than one-third of agricultural water consumption. It is also widely used in the industrial sector. Groundwater tables depend on cyclical recharges.
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Carte établie le 10 octobre 2024 par le BRGM, à partir de données de la banque ADES acquises jusqu’au 30 septembre 2024.
Source des données : banque ADES (ades.eaufrance.fr) / Hydroportail (hydro.eaufrance.fr) / Fond de carte © IGN. Producteurs de données et contribution : APRONA, BRGM, Conseil Départemental de la Vendée, Conseil Départemental des Landes, Conseil Départemental du Lot, EPTB Vistre Vistrenque, Parc Naturel Régional des Grandes Causses, Syndicat Mixte d’Etudes et de Travaux de l’Astien (SMETA), Syndicat Mixte pour la protection et la gestion des nappes souterraines de la plaine du Roussillon (SMNPR).
Cette carte présente les indicateurs globaux traduisant les fluctuations moyennes des nappes. Ils sont établis à partir des indicateurs ponctuels relevés au niveau des points de surveillance du niveau des nappes (piézomètres).
L'indicateur "Niveau des nappes" compare le mois en cours par rapport aux mêmes mois de l’ensemble de la chronique, soit au minimum 15 ans de données, et jusqu'à plus de 100 ans. Il est réparti en 7 classes, du niveau le plus bas (en rouge) au niveau le plus haut (en bleu foncé).
Les zones grises correspondent à des secteurs sans nappes libres, c'est-à-dire avec une couche imperméable ou semi-perméable au-dessus de la nappe, et/ou des secteurs comportant une très faible densité de points de suivi. Ce dernier cas concerne notamment les zones montagneuses dont les nappes sont petites et hétérogènes.
L'indicateur "Évolution des niveaux" traduit la variation du niveau d'eau du mois échu par rapport aux deux mois précédents (stable, à la hausse ou à la baisse).
Ces indicateurs globaux rendent compte de situations et de tendances générales et ne tiennent pas compte d'éventuelles disparités locales.