In 2022, and with greater variability in 2023, all of France's aquifers were affected by drought.

Although the winter of 2023-2024, followed by support from abundant spring rainfall in 2024, enabled most of the aquifers to be effectively recharged, monitoring the state of the aquifers remains crucial in order to anticipate future episodes of drought and better manage the resource.

All the more so as climate change means that climate variability (with more frequent and intense extreme events) is set to increase in the future.

Our hydrogeologists have looked at the subsurface of four emblematic regions, and their groundwater resources.

In 2022, Brittany experienced an exceptionally long and intense drought, with historically low river flows and aquifer levels.

Despite its reputation as a wet region, these historically rare events seem to be becoming increasingly frequent. Why, in a region where it rains regularly, is there now this risk of a lack of water? Climate change is certainly to blame, but it is also due to the specific hydrogeological characteristics of the subsurface in Brittany.

Brittany’s hydrogeological context

Brittany is criss-crossed by 30,000 km of watercourses that are connected to the aquifers found throughout the region. The region's economic and demographic development depends on access to its natural resources, particularly water, which must be managed sustainably.

Its water resources are used for a variety of purposes: drinking water, agriculture and industry. But the region’s needs have tended to increase in recent years, due to demographic pressure exacerbated by summer tourism and the intensification of agricultural uses.

In Brittany, drinking water comes mainly from surface resources (rivers, dams, etc.). Seventy-five percent of water is abstracted from surface water, compared with just 25% from groundwater. For France as a whole, this ratio is reversed: only 36% of water comes from the surface, compared with 64% from groundwater.

Although groundwater accounts for only a minor share of water abstraction, it is a crucial resource. There are almost 30,000 collection points (boreholes, wells, springs and drains) for a variety of uses, mainly agricultural.

Rainfall in Brittany does not always recharge the aquifers

Only part of the rain is re-injected into the water cycle. This is referred to as "effective" rainfall, which is not taken up by evapotranspiration (from the soil and plants), but runs along the surface and seeps into the water table.

Brittany's aquifers are said to be "reactive” as they are very sensitive to the amount of water infiltrated, i.e. the volume of effective rainfall. In this they differ from the so-called "inertial" aquifers found in other regions, which have a multi-year evolutionary cycle.

When geology regulates the water cycle

To understand this difference, we need to look at the geology of Brittany's aquifers. In Brittany, there are no major aquifers, but a mosaic of small, interlocking systems. These are mainly "basement" aquifers, sheltered in ancient hard rock (granite, schist, sandstone, gneiss, mica schist, etc.). In a basement aquifer, water is stored in the rock due to its porosity, and circulates mainly through the network of cracks in the rock. The speed at which water circulates depends on the nature of the rock.

Flows are much slower than in a river, but a large part of the reserve is emptied each year as it feeds surface water courses. More than a reserve, it is a volume in slow transition that allows natural environments to survive the summer. Aquifer levels, at the end of winter, yield information about the future state of rivers over the next two to five months.

Moving from reactive to preventive management

Brittany's geological context means that water management is extremely reactive. Being exposed to the risk of consecutive droughts, most of France's major aquifers can supply water over several years, at the risk of a very significant drop in levels and deterioration of resources over the long term. Conversely, in Brittany, summer resources depend on rainfall in the preceding winter.

The Beauce aquifer, located to the south-west of Paris, is one of the largest underground water reservoirs in France; it supplies the Beauce region, one of the main cereal-producing areas in Europe.

This extraordinary aquifer extends over several départements and two regions, Centre-Val de Loire and Ile-de-France, covering an area of around 14,000 km² with an estimated storage capacity of 20 billion cubic metres per year, or eighteen times the volume of Lake Annecy.

The huge reservoir of groundwater making up the Beauce aquifer meets the requirements for the production of drinking water, as well as for irrigation, industry and feeding rivers. However, this resource faces the dual risks of drought and pollution.

An abundant but vulnerable resource

The unconfined part of the Beauce aquifer (this is the name given to aquifers whose upper level at atmospheric pressure can vary because it is not in contact with an impermeable ceiling) is highly vulnerable to pollution from human sources, particularly nitrates and plant protection products used in agriculture. The aquifer table is unconfined over almost the entire area, except under the Forêt d'Orléans and in Sologne. Preserving the quality of the Beauce aquifer is therefore a major challenge for this region. Several research projects are being carried out to improve our understanding of how the aquifer functions and how pollutants are transferred.

This qualitative aspect should not detract from the quantitative stakes involved. The Beauce limestone aquifer monitoring network comprises around fifty wells and boreholes, fitted with piezometers, which have been monitored since the early 1970s and are managed by BRGM. During successive winter droughts, notably during the periods 1896-1906 and 1988-1993, there was not enough winter rainfall to recharge the water table. The Beauce aquifer has fallen to very low levels, causing small streams and shallow wells to dry up and making borehole pumping more difficult.

In Normandy, the summer droughts of 2003, 2005 and more recently 2022 have put pressure on water use – including the production of drinking water – in western Normandy, which depends directly on river water levels for its drinking water supply.

The east of Normandy has also been affected, although the mechanics are different: it is the succession of winter droughts that has led to a reduced availability of groundwater resources to support river flows in 2017, 2019 and 2023.

Normandy split in two

It has to be said that in geological and hydrogeological terms, the region is full of contrasts, and is even divided in two:

• the subsurface in the eastern part is made up of sedimentary rock, mainly limestone which is part of the Paris Basin. These formations tend to allow rainwater to seep into the subsurface;
• the western part is made up of ancient geological formations (known as basement formations), which correspond to the north-eastern edge of the Armorican Massif (Brittany). This ground, which is less permeable, tends to let rainwater run off the surface. The result is a dense network of rivers.

This has an important consequence for the west of the region: heavy winter rainfall infiltrates very little and runs off easily into the rivers, which can cause major flooding.

The water stored underground is less abundant and depletes quicker. As a result, the rivers that flow over these basement rocks receive little support from aquifers during the summer months: if there is no rain, their flows can become very low in summer. These characteristics make the rivers of western Normandy more vulnerable to summer droughts.

The Bay of Mont-Saint-Michel in 2100

In Normandy, as elsewhere in France, BRGM is carrying out studies on the impact of climate change on water resources, using numerical hydrogeological models, on behalf of local authorities and government departments.

In north-western Europe, climate models predict a rise in temperatures and a seasonal distribution of precipitation that will be much more contrasted than it is today, with wetter winters and drier summers. In southern Europe, climate models are converging on a future that will generally be warmer and drier. In this context, Normandy finds itself in an "intermediate" situation between the North and the South.

In the Bay of Mont-Saint-Michel, five different climate models were used to study the impact of climate change. In the most pessimistic warming scenario, the models all converged on a warmer future (with mean annual temperatures rising by between 3.1 and 3.6°C on average), with greater water losses due to evapotranspiration, and greater seasonal contrasts (wetter winters and drier summers). Annual rainfall is expected to increase by between 4.3% and 9% depending on the area.

What if the 2022 scenario were to be repeated? In the Bouches-du-Rhône region, several dozen communes were placed on heightened drought alert – and therefore subjected to water restrictions – at the end of July 2024.

The Provence-Alpes-Côte d'Azur (PACA) region is particularly vulnerable to the risk of drought, due to its Mediterranean climate and the uneven distribution of water resources across the region. This can lead to a dependence on the hydraulic structures developed to transfer the abundant water from the Alpine areas to the more deficient and densely populated coastal areas.

The large Serre-Ponçon dam, for example, originally designed to generate hydroelectricity, now acts as a reservoir and regulator. It helps to offset water shortages during the summer by gradually releasing reserves that have been built up, in line with the region's needs.

Groundwater still under-exploited

In addition to surface water, 14% of water abstraction for all uses comes from aquifers. For drinking water, this proportion is 50%. Groundwater therefore represents a crucial resource for the future: surface water flows are likely to decrease over the coming decades as a result of climate change.

Climate change in the Mediterranean

The availability of groundwater resources depends on hydroclimatic cycles, which are highly contrasted in the Mediterranean region. On the coast, a change already seems to be detectable, with a significant increase in mean temperatures in recent years and prolonged heat waves during the summer.

The frequency of other extreme weather events, such as heavy rainfall, is also increasing. Conversely, prolonged periods without rain can lead to major droughts, such as the one in 2022, especially when they follow autumns with low rainfall and dry winters.

Climate change could also eventually lead to salinisation of coastal aquifers in the most densely populated areas along the coast. As sea levels rise, the interface between freshwater and saltwater in the subsurface will move inland. This will exacerbate the vulnerability of water catchments by the sea, but also in the plains (often agricultural) and in low-lying wetlands, particularly in the Camargue.

The urgent need to preserve water in the PACA region

According to the Agence de l'Eau, 40% of the regions in the Rhône-Méditerranée basin are under pressure because current needs and abstractions exceed the capacity of available resources. This is a major challenge for the PACA region, which is also subject to strong demographic pressures, particularly from tourism.