The rain is back. After a third heat wave that affected all of France, thunderstorms have been raging since Sunday in several regions. Météo France placed five departments in the south of France on orange alert for thunderstorms on Tuesday 16 August.
These rains in large quantities were expected throughout the territory, while the country is experiencing an episode of historic drought. The groundwater level is particularly low and several municipalities were deprived of water in July, the driest month ever recorded by Météo France.
But these precipitations, short and intense, are not necessarily a good omen. Franceinfo explains why they will not help curb the drought.
Because it would take rains more spread out over time
A lot of water in a short time. This is what Météo France forecasts for Tuesday. During the afternoon in the south of the Massif Central (Tarn and Aveyron) as well as in Aude, “we expect local accumulations of 20 to 40 mm in less than an hour”notes the forecasting institute in its latest bulletin.
Storms must reach Hérault and Gard at the end of the afternoon with intense precipitation. “We can locally exceed 100 to 120 mm”, warns Météo France. These storms should shift towards the Provence-Alpes-Côte d’Azur region overnight.
However, the rhythm of these precipitations is not adapted to properly hydrate the soil.“We prefer rains that fall moderately over a long period of time than rains that fall very intensely over a very short time, as was the case this Sunday when the rain did not have time to penetrate the floor”details on TF1 Patrick Galois, forecaster at Météo France.
“The drought has been going on for a while, the plants are suffering, and it will take small regular rains to hope that this vegetation will start again”confirms with BFMTV Romaric Cinotti, vegetation fire referent at Météo France.
Because the rain deficit over the year is too great to be filled so quickly
It has rained very little in France since the start of the year. After a very dry winter, rainfall was rare in the spring, and the scorching summer did not help the situation. July 2022 was the driest month since records began in 1959, with an 85% rainfall deficit.
Balance sheet #JULY2022
️ Average temperature: 23.2°C.
2.1°C warmer than normal, 3rd hottest July since the beginning of the 20th century, behind July 2006, July 1983, tied with July 2018.
️ The driest month of July, period 1959-2022.➡️https://t.co/BNACTFOc2e pic.twitter.com/1zf2K9H4qQ
— Meteo-France (@meteofrance) August 2, 2022
“The cumulative rainfall since the beginning of the hydrological year (September 2021) shows values below normal by 10 to 50% over the entire country”, figured Météo France in its last hydrological bulletin, on August 8. In this context, the one-off storms that hit France are far from making up for a long-term deficit.
Because rain runs off dry soils which absorb less water
Paradoxically, water penetrates less well in dry soils. This phenomenon is illustrated by a video from the British University of Reading. Researcher Rob Thompson’s experiment consists of spilling a glass filled with water on three different types of soil: wet, normal and dry. Result: it is much more difficult for water to seep into the dry ground.
In this experiment Dr. Rob Thompson of @UniRdg_Met shows just how long it takes water to soak into parched ground, illustrating why heavy rainfall after a #drought can be dangerous and might lead to flash floods. @R0b1et @UniRdg_water pic.twitter.com/zbb3xLTXdK
—Uni of Reading (@UniofReading) August 10, 2022
A phenomenon which is explained by the dead grass, which no longer drinks, and the hardness of the soil. “Part of the plant cover is dead and no longer absorbs water. And then the land hardens, the structure of the soil changes, and instead of having something loose, we have a hard surface”summarizes Jérôme Lecou, forecaster at Météo France at West France.
Worse, heavy rains on dry ground can promote flooding, because the water runs off without penetrating the ground. “After a period of drought, dry soils are no longer able to properly absorb rainfall and this can cause floods, floods and landslides”warns the water information center.
Because this episode of rain will not be enough to recharge the water tables
It will be necessary to wait for winter to see the water tables recharge. “The soil is in first position in the face of the rain, it is the one that is used first. This will allow a restart of plant life., explains hydrologist Vazken Andreassian to franceinfo. But on the other hand, groundwater must wait: it is used last.”
“It would take several episodes of this type to recharge the water tables a little durably”, analyzes Yves Tremblay, hydrologist at the Hydrosciences laboratory in Montpellier interviewed by franceinfo. Next winter will still have to be rainier. At the risk of being confronted with the same problems of drought, with water tables already at their lowest.