Groundwater: report on changes in aquifer levels in 2022-2023

Situation at the start of the hydrological year

The lowest water levels in 2022 were recorded between mid-August 2022 and January 2023, sometimes 2 or even 3 months late. More specifically, the lowest water levels in the most reactive aquifers were recorded between mid-August and the end of November 2022. Many reactive to mixed aquifers in the east of the country reached low-water levels between mid-August and mid-September, with the first storms at the end of the summer halting the fall in levels: this was the case for the alluvial aquifers in the Alsace plain, Jurassic limestone aquifers in Berry, Côte-des-Bars and Jura, aquifers in the eastern Massif Central, alluvial aquifers in the Rhône, Saône and Alpine valleys, aquifers in Languedoc and alluvial aquifers in Corsica. The low-water period shifted to November 2022, thus several weeks late, for the reactive aquifers in the west: the basement aquifers from Brittany to the Vendée, the limestone aquifers from the Seuil du Poitou (Poitou threshold) to the Causses du Quercy and the basement aquifers in the Limousin. For the more inertial aquifers, the lowest water levels occurred between November 2022 and January 2023 for the Paris Basin aquifers and between November 2022 and mid-December 2022 for aquifers in the Rhône-Saône corridor. 

The state of the aquifers during the low-water period in 2022, at the start of the 2022-2023 hydrological year, was unsatisfactory. Most levels were below normal. Rainfall deficits recorded in the 2021-2022 hydrological year and intense abstraction of groundwater in the spring and summer of 2022 led to severely low levels in most aquifers. The 2022 low-water period proved to be the most intense ever recorded for certain aquifers. For example, the springs in the karst limestone formations of the Vaucluse and the centre of the Var were starting to run dry. The aquifer in the Var's alluvial and tertiary formations became disconnected from surface water and no longer played its role of regulating the river's flow. Only a few aquifers have maintained levels comparable to their averages, in particular the alluvial aquifers of the Garonne, the Dordogne and their main tributaries.

Analysis of the 2023 low-water period

Generally speaking, the summer drop in levels is linked to low rainfall, high evapotranspiration and/or vegetation activity, as well as greater demands on groundwater in certain areas (pumping). Generally, from mid-spring through to autumn, rainwater seeping into the soil is usually taken up by vegetation. In this case, drought or rainfall have little effect on aquifer levels. An improved groundwater situation with higher aquifer levels will require significant rainfall, damp soil and vegetation that is more or less dormant. Abundant rainfall can only have an observable effect on reactive aquifers, resulting in a slowdown in groundwater discharge or even an increase, often temporary, in levels. These occasional inflows have a beneficial effect, as they postpone the start of the depletion period until the spring and sustain aquifer levels in summer. During the discharge period, the fall in levels can be greatly accelerated in areas where there is heavy demand (abstraction) on groundwater resources.

At the end of the discharge period, the aquifers reach their lowest level of the year, during the ‘low-water period’. These low-water levels are generally observed in October and November. But they may occur earlier in reactive aquifers, with end-of-summer storms towards the end of August or during September, marking the start of recharging.

Evolution of aquifer depletion

During the spring and summer of 2023, there were alternating wet periods, which could lead to recharge episodes, and dry periods, during which depletion occurred. There was enough occasional rainfall to generate recharging and sustain aquifer levels. However, the effect of precipitation on the aquifers diminished as the summer progressed. As vegetation began growing again and temperatures rose, the rainfall that seeped into the soil first moistened it and was taken up by the vegetation. The rainfall was therefore only beneficial for aquifers in areas that had enough rain to meet their needs and ensure that the rain infiltrated deep into the ground.

In the case of reactive to mixed aquifers, their trends and situation changed during the spring and summer of 2023, depending on the recharge episodes and depletion periods. At the beginning of spring, water levels were rising and the situation improved considerably in aquifers in the northern third of the country (Massif Armoricain, the Channel coast and Grand-Est), thanks to rainfall in March and especially April. In the southern third of the country, the rains in May moistened the soil and watered the vegetation, which enabled the June rains to infiltrate deep into the subsurface. However, their impact on aquifers varied, depending on local rainfall totals. In July and August, precipitation over the northern third of France resulted in stable or rising water levels during the month of August. This recharge episode remained limited, however, but helped to sustain aquifer levels. In addition, the rains enabled irrigation to be reduced to a greater or lesser extent, thus easing the pressure on groundwater resources.

The levels of the inertial aquifers continued to fall throughout the summer of 2023 and the state of the aquifers deteriorated very slowly. The spring and summer rains had little effect. This is normal, as these aquifers are not very sensitive to meteorological events during the summer. However, for the inertial aquifers of the Paris Basin and Artois, although the abundant rainfall in July and August was insufficient to reverse the trend, it did significantly slow their depletion.

On the Mediterranean rim, the rainfall which had infiltrated deep into the ground during the spring and summer of 2023 was insufficient to make up for the deficits accumulated since the beginning of 2022. The heavy rainfall in May and June did little for the aquifers. The rainfall was unevenly distributed and fell during sometimes violent storms, which were not conducive to infiltration into the subsurface. In addition, high temperatures encouraged evapotranspiration and increased plant water requirements. However, these rains did allow some inflow peaks to have an impact and they did improve the situation for the Provence aquifers. Several aquifers remained under stress, with persistent low to very low levels throughout the summer, namely aquifers on the Roussillon plain, aquifers on the Languedoc coast and on the Côte d'Azur. In the Roussillon aquifers, restrictions on the use of groundwater prevented a very worrying situation from deteriorating further.

Groundwater situation at the end of the depletion period

The 2023 low-water levels (lowest water level) were observed to be a little later than usual, during the second half of October, for the reactive and mixed aquifers, and spread out between the end of October and mid-December for the inertial aquifers. The recharge period began rapidly and at the same time in virtually all the aquifers. This was due to heavy rainfall from mid-October and into November over almost the entire country. There were exceptions in a few sectors: low-water levels occurred between the end of August and mid-September in the extreme south-west, during August on the Alsace plain water table and in December for the Bas-Rhône aquifers. Finally, the aquifers of the Roussillon plain and the Corbières massif were still being depleted in January 2024.

The depletion period is usually characterised by a gradual deterioration in the state of the aquifers. However, the 2023 discharging period was atypical, with several recharge episodes interspersed. These recharge episodes improved or sustained the general state of the reactive aquifers, as well as that of inertial aquifers in a few areas. The state of the aquifers during the low-water period in 2023 was very heterogeneous, depending on the local rainfall totals recorded between autumn 2022 and summer 2023 and on the cyclic fluctuations of the aquifers in question.

With regard to inertial aquifers, there was little change in levels between the high-water and low-water periods in 2023. They generally remained below normal, from moderately low to low. The situation was more unfavourable in the aquifers of the Plio-Quaternary and Miocene formations of Sundgau (southern Alsace) and the Dijon, Bresse and Dombes areas. Only the aquifers in the Artois area and Savoyard foothills had levels above normal. These sectors benefited from several recharge episodes during the summer.

The situation also remained variable as regards the reactive aquifers. The levels were satisfactory, from moderately low to moderately high, in the west, in the Champagne region and from the Artois area to Nouvelle Aquitaine. This was due to several recharge episodes during the spring and summer, which helped to improve or sustain the state of the aquifers in those areas. Over the rest of the country, the levels in reactive aquifers remained moderately low to low. The situation remained poor around the Mediterranean coast. The 2023 low-water period was particularly intense in the aquifers of the Roussillon plain, the limestone formations of the Corbières massif, and in the alluvial aquifers of the Hérault and Orb rivers and in the Côte d'Azur. The accumulated rainfall infiltrated during the 2022-2023 hydrological year was far from enough to compensate for the deficits accumulated since the previous hydrological year.