According to forecasts for the month of August, France should experience a third strong heat wave, unprecedented, with a scorching peak that will spread throughout France from Wednesday August 10.
What is the risk of drought threatens groundwater - we speak in this regard of hydrogeological drought - in metropolitan France?
This risk is defined by the possibility of reaching, by August-September 2022: low to very low levels, a rather rare situation; levels below alert thresholds defined by prefectural decrees on representative piezometers.
The slick forecasts turn out to be quite pessimistic.
The risk of drought (in dark red on the map, "very high risk") is indeed proven in the South-East and concerns the karstified limestone aquifers of Provence, the alluvial aquifers and tertiary formations of the Côte d' Azure. This risk also affects the Centre-West, including the Jurassic limestone layers of Charentes, Brenne and Poitou; the Cenomanian sand aquifer of Perche and Maine and the Seno-Turonian chalk aquifer of Touraine.
In these sectors, groundwater restrictions have already been put in place and the situation is unlikely to improve in the coming weeks.
The "high" risk sectors (in light red on the map) are home to groundwater whose levels are already below normal. Their situation could deteriorate rapidly, as a result either of the absence of sufficient rains, or of heavy demand from pumping.
Finally, the levels of groundwater identified as at “very low” risk (in dark green on the map) to “low” (light green) should remain satisfactory throughout the summer. These are inertial aquifers that are not very sensitive to the current year's drought, or reactive aquifers that have benefited from satisfactory recharge.
Tensions could however appear locally, in the event of strong solicitation of groundwater by withdrawals.
In addition, the data from this map were compiled at the departmental level with data on the rainfall deficit, soil drought and hydrological drought by the Ministry of Ecological Transition, in order to define the territories at risk of drought.
Close-up on the state of the aquifers on May 1, 2022
The groundwater situation at the end of the recharge period, i.e. on the 1er May 2022, depends on the situation of the aquifers at the start of the recharge period (between October and November 2021), the rains infiltrated at depth during the fall and winter of 2021-2022 (apparent recharge) and the cyclicity of the aquifers .
The year 2021 was characterized by an excess 2020-2021 recharge period and by several unusual episodes of recharge from May to July.
In the fall of 2021, the groundwater situation, then at the start of the recharge period, was favorable with levels above normal (see on the left map above, the blue and green sectors). Only Corsica, Provence and the Montpellier-Nîmes region showed levels below normal, due to the lack of significant recharge since 2020 (on the map on the left, the yellow and orange sectors).
The aquifers recharge mainly during the fall and winter, when the vegetation is dormant. However, the apparent recharge 2021-2022 was in deficit on most of the aquifers (see on the top map above, the shades of red). At the scale of the territory, it was only 70% of the normal apparent recharge. The recharging period ended in January-February 2022 over a large part of the territory, i.e. 2 to 3 months in advance. Only the aquifers in the south of Occitania benefited from a slightly excess recharge (on the top map, the shades of blue).
The profit temporarily made in the aquifers thanks to the excess recharge of 2020-2021 has therefore faded with the rainfall deficits recorded during the winter and spring of 2021-2022.
However, the impact of this recharge deficit is different depending on the cyclicity of the aquifer, that is to say its reactivity to the infiltration of rain.
The inertial sheets (chalk, tertiary formations and volcanic formations) have a multiannual cyclicity (see on the bottom map above, the green color). Their inertia, characterized by slow flows, makes it possible to maintain slightly degraded levels at the end of winter despite the deficit recharge.
On the contrary, the reactive aquifers with annual cyclicity (alluvium, Jurassic and Cretaceous limestones, Triassic sandstones and basement) are very sensitive to the deficit of effective rainfall (see on the bottom map above, the color blue). Their situation therefore deteriorated rapidly at the end of winter and during spring.
Consequently, only the inertial and reactive aquifers having benefited from a recharge close to normal (75 to 125%) show levels close to, or even higher than normal, on 1er May 2022 (on the right map above, the green and blue sectors). A large part of the aquifers are recording moderately low to very low levels, a consequence of the deficit recharge (on the right map above, the orange and red sectors).
How the Drought Map is Developed
The development of the drought risk map is based on the initial state of the groundwater after the winter recharge period 2021-2022, on the Meteo France seasonal forecasts, on seasonal forecasts from hydrogeological models and on the expertise of BRGM regional hydrogeologists.
From spring and during summer, the rise in temperature, the resumption of vegetation, and therefore the increase in evapotranspiration, limit the infiltration of rain into the groundwater. Between May and October, barring exceptional rainfall events, drainage usually continues and levels continue to fall until autumn. The evolution of the groundwater situation in the coming months of 2022 will therefore depend on effective rains that will manage to infiltrate deeply, water withdrawals and the resistance of the groundwater to drought.
Concerning inertial sheets, precipitation should not generate a significant recharge, except for very exceptional rainfall events. The situation of the inertial layers should deteriorate slowly over the next few months, or more quickly in the heavily stressed sectors.
On reactive aquifers, in the event of insufficient precipitation, drainage should continue and the situation should then continue to deteriorate. Significant rainfall episodes may nevertheless cause momentary refills, thus making it possible to support the levels (temporary pause in their fall) or even, very occasionally, to cause a rise in the levels.
The seasonal forecasts of Météo France favor a scenario warmer than normal over the whole territory and drier than normal over the southern half.
Consequently, the drought risk map was constructed on a pessimistic assumption, considering that the groundwater situation should continue to deteriorate over the coming months. However, it is not excluded that oceanic disturbances cross France and generate episodes of significant recharge. Irrigation campaigns could also influence the groundwater situation.
In order to protect our water resources, it is essential to follow measures to save this vital resource. These measures make it possible to guarantee water levels in the aquifers sufficient to supply the population with drinking water and to regulate the flow of the rivers.
Violaine bault, Hydrogeological engineer, BRGM et Marc Laurence, Researcher in hydrogeology, BRGM
This article is republished from The Conversation under Creative Commons license. Read theoriginal article.