Hydrogeological situation on 1 December 2023
The abundant rainfall at the end of October and November had a positive impact on the aquifers. Recharging has begun and levels at 78% of observation points are up.
Rainfall that infiltrated deep into the ground has led to a significant improvement in the state of reactive aquifers and a more moderate improvement in inertial aquifers. The situation is improving considerably: 48% of levels were above monthly averages in November (as opposed to 14% in October). The state of the aquifers varies according to the region. Levels are very favourable in the reactive aquifers of the northern two-thirds and south-west of France, but remain below normal in the aquifers of Corsica, along the Mediterranean coastline, in the Limagne plain, the Rhône-Saône corridor, southern Alsace and the Paris Basin.
Trends and changes during winter will depend primarily on the amount of rainfall. Recharging should continue if areas receive rainfall, meaning the situation should then remain stable or improve. In the event of insufficient rainfall, depletion of the water tables could continue, causing the state of aquifers to deteriorate. The situation will have to be particularly closely monitored in the aquifers along the Mediterranean coast, in the Rhône-Saône corridor and the Sundgau (southern Alsace), which have been weakened by severe low-water levels.
Map of aquifer levels in mainland France on 1 December 2023.
© BRGM
Evolution of the trends observed on piezometers from April to November 2023.
© BRGM
Groundwater trends
The recharge period usually begins with storms at the end of August for reactive aquifers and between October and November for inertial aquifers. In 2023, groundwater depletion continued, due to insufficient rainfall and the fact that the vegetation remained active due to high temperatures. The recharge period began at the end of October, with the vegetation going dormant and the onset of significant rainfall episodes. In October, the recharge period began and trends varied.
In November 2023, the recharging became widespread and concerned all the aquifers. Levels are up at 78% of observation points (as opposed to 41% in October).
Following the rainfall that began in mid-October and continued into November, the aquifers began to recharge between the end of October and the end of November. The time lag between rainfall and the start of the recharge period depends essentially on the responsiveness of the aquifers in question. Low water levels were reached as early as the end of October in the most heavily irrigated areas that have reactive aquifers. Some points in the inertial aquifers of the Paris Basin and the Sundgau (southern Alsace) showed a later low water level, between mid-November and the end of November.
These phenomena are normal for this time of year: vegetation is dormant and a large proportion of the rainfall filters down into the aquifers. It should be noted, however, that the very excessive rainfall has led to increases in water levels in the northern two-thirds and south-west of France, which are all the more rapid and significant because the aquifers there are reactive. This effective rainfall was beneficial in ensuring a high level of aquifer recharge.
Along the Mediterranean coastline, trends were more contrasted. The recharge from the end of October to the beginning of November mainly benefited the high and medium elevation aquifers. In the plains and on the coast, the recharge began at the end of October, but then groundwater began to deplete again. Finally, given the low rainfall, recharging does not appear to have begun in the aquifers of the Roussillon plain and the Corbières massif.
Comparison between 1 December 2022 and 1 December 2023
Map of mainland France showing the state of the aquifers on 1 December 2022 (left) and 1 December 2023 (right).
© BRGM
Evolution of the situation observed on piezometers from April to November 2023.
© BRGM
The groundwater situation
The groundwater situation at the end of the 2022-2023 winter period was unsatisfactory. The spring and summer rainfall helped to maintain, and sometimes even improve, aquifer levels in the areas that received the most rain.
The rain that infiltrated the ground from the second half of October had a significant effect on the state of the aquifers. The overall situation improved in November: The levels recorded at 41% of observation points were below monthly averages, 11% were comparable and 48% were above (respectively 65%, 21% and 14% in October). The situation was more positive than that observed last year, in November 2022, when 70% of the levels recorded were below monthly averages. Only the Languedoc and Roussillon aquifers had levels that are lower than in 2022.
The situation in October 2023 varied greatly, ranging from very low to high levels. Changes in the state of the aquifers between October and November will depend on the total amount of rainfall in recent weeks and the responsiveness of the aquifers.
In the northern two-thirds of the country and in the south-west, the recharge episodes were very beneficial. Reactive aquifers respond quickly to autumn rainfall. Situations are improving considerably and levels are very satisfactory, generally ranging from moderately high to very high. As regards mixed to inertial aquifers, the situation is evolving slowly and is heterogeneous. The state of aquifers is favourable in Artois, eastern Lorraine, the Alsace plain and the Savoyard foothills. The situation remains poor in the inertial to mixed aquifers of the central and western Paris Basin, the Limagne plain, the Sundgau (southern Alsace) and the Rhône-Saône corridor. Worryingly low to very low levels are still being observed locally in the Paris Basin and the Rhône-Saône corridor.
In south-east France, the situation is improving slightly for aquifers in the basement massifs (southern Massif Central) and limestone massifs (Grands Causses, Cévennes and Provence), and has not changed for aquifers in the plains and along the Mediterranean coast. Levels are generally below monthly norms. Levels are a cause for concern in the aquifers of Roussillon, the limestone of the Corbières massif and the alluvial deposits of the Languedoc coast. Rainfall is still far from sufficient to offset the deficits accumulated since 2022.
Aquifers that are in a very good state
Several aquifers were found to be in a very good state, with levels being moderately high in relation to the levels recorded for November in previous years:
- Aquifer levels in the Jurassic limestone of the Boulonnais region and in the Cenomanian chalk marl of the Artois-Picardie coast are due to a very high 2022-2023 recharge and exceptional rainfall in recent weeks;
- The Jurassic and Cretaceous limestone aquifers in Poitou, Charentes, Périgord, the Angoumois basin and the Causses du Quercy reacted quickly to the heavy rainfall in October and November;
- The aquifers from the basement of the Vilaine basin to the Vendée bocage as well as in the Limousin and the Chataigneraie plateaux are very sensitive to the excess rain that has fallen since mid-October;
- The aquifer in the plioquaternary formations of the Aquitaine Basin was strongly recharged in October and November.
Aquifers in a poor state
Many aquifers were in a poor state, with very low levels compared to those recorded in November of previous years, owing to an extreme rainfall deficit over the last few months or years:
- The Sundgau Pliocene gravel aquifers show very low levels, due to several successive winter recharges of low intensity and due to their highly inert recharge systems;
- The inertial Plioquaternary and Miocene aquifers in the Dijonnais, Bresse, Dombes and Nord Isère areas had very low levels, due to their highly inert recharge systems and several insufficient recharges over successive winters;
- Alluvial groundwater levels in the Cote d'Azur, Hérault, Orb and Aude rivers are low, with insufficient rainfall to generate significant recharging;
- The situation of the aquifers in the Roussillon multi-layer formation and the karst limestone of the Corbières massif remains extremely poor, with very low levels. Limiting abstraction during the spring and summer reduced the pressure on the levels, but also reduced inputs from gravity-fed irrigation.
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 seasonal forecasts from Météo-France for the months of December 2023 and January and February 2024 predict higher temperatures across the country and wetter-than-normal conditions in a large part of northern France. No clear scenario has been established concerning rainfall around the Mediterranean coastline.
The trends and changes over the next few weeks will depend exclusively on the amount of infiltrated rain, and consequently the rainfall totals, and the reaction time of the aquifers (reactivity/inertia). In the event of there being normal to surplus effective rainfall, recharging should continue. The state of aquifers should then persist or gradually continue to improve. If there is insufficient precipitation, the infiltrated rain will not compensate for the output volumes (natural outlets and abstractions). Aquifer discharge in the affected areas could start again and if so, the situation will deteriorate, slowly in the inertial aquifers and rapidly in the reactive aquifers.
In the aquifers of the northern two-thirds and south-west of France, the initial 2023-2024 recharge is much more than is needed. The rainfall forecast for December should lead to further recharge episodes, and the seasonal forecasts are optimistic.
As far as reactive aquifers are concerned, the start of winter recharge means that we can expect satisfactory levels at the end of the winter. However, the situation may also deteriorate rapidly if there is insufficient rainfall. Levels in the summer of 2024 will depend on abundant recharge during the winter that continues into the spring, in order to postpone the start of the depletion period.
The state of the aquifers in the Artois, Paris Basin, Sundgau and Rhône-Saône corridors should continue to improve gradually over the coming weeks as autumn rainfall slowly filters down into them. In the longer term, excess rainfall will be needed throughout the winter to return to levels comparable to or above normal. It is difficult to foresee a replenishment of groundwater reserves between now and the spring, unless there is a great deal of excess rainfall in the coming months, in areas which had very low levels in November.
Trends and developments in aquifers around the Mediterranean are highly uncertain and will depend on cumulative rainfall over the coming months. It will be possible to return to above-normal levels by the end of winter 2023-2024 if there are significant and well-distributed rainfall events over the coming months. It will be more difficult for aquifers which had very low levels in November to achieve normal levels. It would appear difficult to envisage a sustainable replenishment of the Roussillon aquifers between now and the spring of 2024.
At the beginning of the recharge period, it is extremely important to give water reserves that are in a fragile state the time they need to recover, in order to ensure their long-term sustainability. The only way to protect the aquifers, and thus maintain the continuous link between groundwater and surface water, is to limit water abstraction. The 2023-2024 recharge period will determine next year's aquifer levels. During the winter and early spring, when most recharge occurs, the situation in all aquifers across the country will need to be monitored, particularly those that experienced severe low-water periods in 2023.
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.
BRGM monitors groundwater levels and quality in mainland France. Discover the actions carried out by the French geological survey and the resources and databases available on groundwater in France.
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Map drawn up by BRGM on 11 December 2023, based on data from the ADES database, acquired up to 30 November 2023. Data source: ADES database (ades.eaufrance.fr) / Hydroportail (hydro.eaufrance.fr) / Background map © IGN. Data producers and contributors: 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).
This map shows the global indicators reflecting the average fluctuations of the aquifers. They are based on point indicators collected at groundwater monitoring points (by means of piezometers).
The "Aquifer levels" indicator compares the current month’s figures with those of the same months in the entire record, i.e. at least 15 years of data and sometimes up to as much as 100 years of data. It is divided into 7 classes, from the lowest level (in red) to the highest (in dark blue).
The grey areas correspond to areas without unconfined aquifers, i.e. with an impermeable or semi-permeable layer above the aquifer, and/or sectors with a very low density of measuring points. This last case primarily concerns mountainous areas with small, heterogeneous aquifers.
The "Evolution of levels" indicator reflects the variation of the water level of the past month compared to the two previous months (stable, increasing or decreasing).
These global indicators reflect general situations and trends and do not take into account possible local disparities.