The days follow each other and are similar. The umbrella grafted to the arm. Metropolitan France, which has experienced very heavy flooding in recent days, particularly in Pas-de-Calais, was hit between mid-October and mid-November by unprecedented rain over a long period, Météo announced. France this Saturday. Between October 18 and November 16, France recorded an average accumulation of 237.3 mm.
These almost daily rains are welcome to improve the situation of groundwater, the main reserves of drinking water. In France, they only fill up during the winter period: it rains a lot and regularly, which will allow water to infiltrate through the surface soil and into depth. However, as of November 1, two thirds of them were below seasonal norms, announced Thursday, November 16, the Bureau of Geological and Mining Research (BRGM) in a communicated. And contrary to popular belief, the water tables did not fill up much in October compared to September.
October rains “unsatisfactory”
Also as of November 1, 18% of the country’s water tables were at very low levels, said the BRGM. This situation remains close to that of September, when 66% of them were at insufficient levels. But there is good news: their level turns out to be more favorable than in October 2022. At the time, 75% of them were below the monthly averages, after a scorching summer.
If the rains of spring and summer made it possible to maintain or even improve the state of the water tables, the rains that fell in mid-October were, however, insufficient. This is what the BRGM notes, which describes an “unsatisfactory” situation. “Like last year, we had a relatively warm month of September and the beginning of October, vegetation which remained active until November and therefore with difficulty for the rains to infiltrate,” indicated Violaine Bault , hydrogeologist at BRGM. The month of September was the hottest ever measured in France.
What will happen to the rains that fell in mid-November? It is necessary, upstream, to understand the difference between reactive layers and inertial layers. “The levels of reactive aquifers increase very quickly in the event of heavy rain but also drop very quickly when it stops raining. In mid-November, the levels of these aquifers are high or even very high and can cause flooding,” explains the BRGM. Conversely, the level of the inertial layers evolves slowly. “These deep aquifers will take up to three months to recharge, so we have to wait until the end of winter to take stock of the recharge of the aquifers,” underlines Charlène Descollonges, hydrologist and author of Water, Fake or Not (Tana Edition).
Pas-de-Calais: ineffective rain?
Concerning the torrential rains that fell in Pas-de-Calais, “they made it possible to recharge the surface aquifers, but not those in depth. It is still too early to draw an assessment. It also depends on the intensity of the rain , the type of soil, etc., admits Charlène Descollonges. The water tables cannot recharge in three days.”
In addition, when the phenomena are very violent, the water does not have time to penetrate because the ground is waterlogged. It flows and this movement can create flooding. “The ones we see result from heavy and repeated rains on water-saturated soil. Land development and massive drainage of wetlands aggravate the problem,” explains the specialist. Among the obstacles that can prevent water from penetrating into the ground, there is, for example, tar.
If the north-west of France found itself under downpours, the situation is “more unfavorable” in the south of Alsace. The levels of the inertial layers of the Rhône-Saône corridor remain “worrying, from low to very low”, adds the BRGM. Same observation when looking at the site map.Drought Info“. Increased vigilance remains necessary, even if we are only at the beginning of winter.