Groundwater: report on changes in groundwater levels in 2023–2024

Situation at the start of the hydrological year

The 2023 low-water period marks the beginning of the 2023–2024 hydrological year. The low-water period began a little late due to a lack of rainfall and the vegetation remaining active at the beginning of autumn owing to the warm weather. The time lag between rainfall and the start of the recharge period depends essentially on how responsive individual groundwater bodies are. The 2023 low-water period lasted from the end of October to mid-December for the least responsive groundwater bodies and during the second half of October for the more responsive ones. A few responsive groundwater sites reached the low-water point between August and September 2023, as late summer storms stopped the decline of groundwater levels in the South-West, the Alsace plain, around the Cévennes and in the Hérault and Orb valleys. Low-water points were recorded very late – between February and the beginning of March 2024 – at groundwater sites along the French Riviera (Côte d’Azur). Groundwater sites in the Roussillon plain and Corbières Massif did not reach a low-water point, as levels continued to fall during the autumn and winter of 2023–2024.

The overall picture during the 2023 low-water period was relatively poor. In the least responsive groundwater bodies of the Paris Basin and the Rhône-Saône corridor, groundwater levels were generally below the norm, from moderately low to low. The groundwater situation was worse for Sundgau (southern Alsace) and the Dijonnais, and for Bresse and Dombes. Levels were above normal in the groundwater bodies of Artois and the Savoyard foothills as these areas benefited from several recharge episodes during the summer of 2023. The situation was variable as regards the more responsive groundwater bodies. Levels were satisfactory, from moderately low to moderately high, in western France over an area spanning from Artois and Champagne to Nouvelle Aquitaine. This was due to several recharge episodes during the spring and summer of 2023, which helped improve or sustain groundwater levels at these sites. In eastern France, in the Massif Central and the South-East, responsive groundwater bodies showed low to very low levels. The 2023 low-water period was particularly marked in the groundwater bodies of the Roussillon plain, limestone formations of the Corbières Massif, and the alluvial deposits of the Hérault and Orb rivers and the French Riviera. Cumulative rainfall infiltration during the 2022–2023 hydrological year was far from sufficient to compensate for the cumulative rainfall deficit since the previous hydrological year.

Analysis of the 2024 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-April until autumn, water seeping into the soil is usually completely taken up by vegetation. In this case, drought or rainfall have little effect on groundwater levels. An improved groundwater situation with higher groundwater levels will require significant rainfall, damp soil and vegetation that is more or less dormant. Abundant rainfall can only have an observable effect on the more responsive groundwater bodies, resulting in a slowdown in groundwater discharge or even an increase in levels, which is often temporary. These occasional inflows have a beneficial effect, as they postpone the start of the discharge period to spring and sustain groundwater levels in summer. During the discharge period, the fall in levels can be much steeper in areas where there is heavy demand (abstraction) on groundwater resources.

At the end of the discharge period, groundwater bodies reach their lowest level of the year, known as the “low-water period”. These low-water levels are generally observed from October to November, and until December for the least responsive groundwater bodies. But they may be reached sooner in the least responsive groundwater bodies with end-of-summer storms as of the end of August or during September, marking the start of the recharge period.

Groundwater trend during the discharge period

The 2024 discharge period was unusual in that it was short and not very significant. The discharge period began gradually as of April 2024. However, there was sufficient cumulative rainfall in the spring to keep the soil moist and meet the water needs of vegetation. Consequently, rain was able to filter deep into the ground and maintain levels or even recharge certain groundwater bodies up to June. Levels continued to fall during the summer of 2024, but often at a slower rate due to occasional rainfall events and negligible groundwater abstraction. It should be noted that the effect of precipitation on the groundwater bodies 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 groundwater in areas that had enough rain to meet these needs and ensure that the rain percolated deep into the ground. Groundwater levels stopped falling a little earlier than usual, between September and October 2024.

Groundwater levels began falling later than usual – between April and June 2024 – at the least responsive sites. Usually, they keep falling continuously until the autumn when the vegetation becomes dormant and the first significant rainfall occurs, and spring and summer rainfall has little impact on the least responsive groundwater bodies. However, given the high cumulative rainfall and wet soils of the summer of 2024, some of the rain was able to infiltrate deep into the ground. In addition, abstraction, particularly for irrigation, may have been limited. Groundwater levels thus continued to fall more slowly than usual during the summer of 2024, and the groundwater situation continued to improve very slowly. Several of the least responsive groundwater sites in the Paris Basin and the Rhône-Saône corridor even changed classes between the 2024 high-water period and the 2024 low-water period.

Groundwater levels in the responsive groundwater bodies remained stable overall during the 2024 discharge period. The groundwater-level trend and situation evolved depending on recharge events and the times when discharge was dominant. Over most of the country, the status of the most responsive groundwater bodies remained better than the monthly norm. This is because effective rainfall during the spring of 2024 and occasionally during the summer of 2024 helped sustain the levels. In addition, the rains enabled irrigation to be reduced to a greater or lesser extent, thus easing the pressure on groundwater resources. The groundwater situation remained poor in the Pyrénées-Orientales, Aude, Cap Corse and the eastern plains of Corsica, following insufficient recharging in 2023–2024 and little or no rainfall during the spring and summer of 2024. These areas remained under pressure, with persistent low to very low groundwater levels during the summer. For the Roussillon sites, restrictions on groundwater use were established to prevent the situation from getting worse as it was already a matter for concern.

Groundwater situation at the end of the discharge period

September and October are generally a transitional period for groundwater. The heavy rainfall and onset of vegetation dormancy enabled the water to filter deep into the ground. This resulted in a reversal of the trend in terms of groundwater levels. In recent years, the recharge period has been pushed back as a result of the vegetation remaining active for longer, owing to the mild temperatures and insufficient effective rainfall. In 2024, the conditions for recharge events were present from the end of the summer in many areas: the wet soils and low temperatures observed limited evapotranspiration and caused the vegetation to become dormant in October. The winter recharge started between September and October 2024 throughout the country, which is a little earlier than usual.

The first recharge events were observed during the second half of August 2024 with summer storms. In certain localised areas, the decline in groundwater levels slowed because of the contribution of occasional effective rainfall. This precipitation resulted in temporary recharge events, mainly at the responsive sites that received rainfall, namely in the southern Alsace plain, south-western Aquitaine Basin, southern Massif Central (from the Cévennes to the Grands Causses) and Corsica. The 2024 low-water mark was reached slightly earlier, generally during the first half of September over much of the southern half of the country and in Corsica, and between the end of September and October over the northern half but also in Roussillon, Languedoc and the southern Massif Central.

During the discharge period, the groundwater situation often deteriorates gradually. However, 2024 was unusual in that rainfall infiltration managed to sustain the groundwater levels, and abstraction pressure was probably relatively light. These recharge episodes improved or sustained groundwater levels in more and less responsive groundwater bodies alike. Groundwater levels were generally higher than usual during the 2024 low-water period.

In the least responsive groundwater bodies of Artois-Picardy, the Paris Basin and the Rhône-Saône corridor, the situation gradually improved over the 2023–2024 hydrological year. The 2024 low-water levels recorded were usually moderately high to high. They were very high in the Artois-Picardy Basin, in Champagne, Brie and Tapenois. Relatively normal to moderately low levels persisted in the groundwater bodies of the Bresse and Dombes formation and Sundgau, because they are typically slow to respond.

The responsive groundwater bodies in the northern two-thirds of the country and South-West had higher groundwater levels than usual, with low-water levels generally high to very high. Low-water levels in a few of the less responsive groundwater bodies and those that received less rainfall during the spring and summer of 2024 were recorded as moderately high to normal. Very high low-water levels were observed mainly in the highly responsive Jurassic and Cretaceous limestone aquifers in the Boulonnais, the Jura mountains, around the Paris Basin and around the Aquitaine Basin. The situation was similar for the responsive groundwater bodies of the Vosges sandstone in Lorraine, the basement of the southern Massif Armoricain and the southern Massif Central, Plioquaternary formations in the Aquitaine Basin, and alluvial deposits of the upper Garonne, Alpine valleys, Bas-Rhône and Durance rivers.

In many areas around the Mediterranean and in Corsica, the situation remained uncertain for part of the summer of 2024. There were variations during the 2024 low-water period, depending on the amount of rainfall infiltration in the spring and summer of 2024. The 2024 low-water period was particularly marked in the groundwater bodies of the Roussillon plain, limestone formations of the Corbières Massif, and the alluvial deposits of the Aude river. Cumulative rainfall infiltration during the 2023-2024 hydrological year was far from enough to offset the cumulative rainfall deficit of the previous two hydrological years.