Groundwater and rivers have seen their level and flow increase after the multiple depressions which affected France.
Since mid-October, depressions and storms have followed one another, bringing with them torrential rains to part of the territory. Between October 18 and November 12, the country recorded an average cumulative rainfall of more than 215 mm, according to Météo-France, which affirmed, on November 13, “who should have go back to 1993 to observe a cumulative amount of 196.9 mm between September 21 and October 16.”
According to daily rainfall data provided by Météo-France, the heaviest rainfall was measured on October 19 (19 mm over mainland France), November 4 (15.7 mm) and October 26 (14.8 mm).
“If we look before mid-October, we were experiencing a rainfall deficit and particularly dry soil”, explains Simon Mittelberger, climatologist at Météo-France. But since October 17, the date of the first rainfall, the situation has been completely reversed. “In just a few days, we went from a situation worthy of full summer to full winter”, adds the specialist. In October, the deviation from the 1991-2020 average precipitation amounted to +40%. Result: the soil humidity index, which measures the state of the water reserve of the soil over two meters in relation to the useful reserve (water available for feeding plants), skyrocketed in just a few days. Between October 18 and November 6, the deviation from the 1991-2020 normal increased from -42.31% to +29.69%.
Despite everything, this indicator is an average for the entire French territory and can hide disparities. While the weather is very unsettled in the northern three quarters of the country, in the southern quarter, the mercury has sometimes exceeded the average levels for the season by more than 10 degrees since November 15. In these departments, the soil humidity level remains below average. “We find in particular the Pyrénées-Orientales, Bouches-du-Rhône, Aude, Hérault, Gard”explains the climatologist.
More or less reactive water tables
In regions where the rain was intense, it helped moisten the soil. When they are charged, they facilitate and accelerate the journey of water to the water tables. “This is what we call hydraulic conductivity: the wetter the soil, the faster the water infiltration will be”, explains Alain Dupuy, professor of hydrogeology at the Bordeaux Polytechnic Institute (INPB). Wherever it rains, the water tables recharge, but not necessarily at the same speed. As of November 15, 56.58% of groundwater tables had a level above normal, according to calculations made by franceinfo based on data from the Bureau of Geological and Mining Research (BRGM). A month earlier, this level stood at just 20.41%.
“The evolution of the water table level is contrasted depending on the local accumulation of rainfall, but also according to the reactivity of the water tables”, explains the hydrogeology professor. An opinion shared by the BRGM, which recalled, during a press point on Thursday November 16, that inertial layers – the least reactive – can put “up to three months to recharge”. How to explain this? “The water must take labyrinthine paths to reach the water table, sometimes deeper”, illustrates Violaine Bault, hydrogeologist at BRGM. To see the effect of precipitation on these sheets, we will therefore have to wait a few days, or even several weeks.
Other types of layers, called reactive layers, “have already started their recharge since the rains of mid-October”, points Violaine Bault. These underground water bodies, closer to the ground surface and located in terrain favorable to infiltration, recharge and discharge very quickly. In Pas-de-Calais, as in Blendecques, near Saint-Omer, the water table reaches levels so high that “punctual and localized risks of new flooding due to rising water tables are possible”warns the scientist.
Despite “this good news”it is important to remain vigilant, according to Alain Dupuy. “We are only in autumn, it is only the beginning of the groundwater recharge period, he emphasizes. At present, the situation has particularly improved. But if it stops raining in two weeks and there isn’t a drop until March, we will find ourselves in a crisis situation again.”
Watercourses “dependent on the weather”
The observation is the same for surface water. The situation of the rivers has improved significantly since the arrival of the rains in mid-October. Compared to groundwater, the variation in river flow is much more reactive to bad weather. “We are dependent on the weather”, explains the Central Hydrometeorology and Flood Forecasting Support Service (Schapi) to franceinfo. Result: the watercourses located on the western side of the country, Nord-Pas-de-Calais, Haut-Rhin and the Alps record water flows well above the historical average (1990-2020). Some of them are placed on yellow or orange alert by Vigicrues, the information service on the risk of flooding in France. This is particularly the case of the Lys, a river in Pas-de-Calais. An average flow of +563.4% compared to normal was recorded in the town of Delettes, causing significant flooding.
“The rain has been quite copious in these areas and it can be felt in the waterways”details Schapi, who adds that, however, “there are certain nuances depending on the region”. Indeed, the Mediterranean rim as well as Puy-de-Dôme, Loire, Haute-Loire, Ardèche and Drôme are still affected by deficits in surface water. Generally, “we are rather at fairly high levels compared to seasonal norms”, explains Schapi. And what about the months to come? “Rivers have a memory effect: they hold water until everything has flowed into the sea or ocean, but if precipitation persists, the level of the rivers will be maintained.” This is, in fact, what Météo-France is forecasting for the coming weeks. “This rainy scenario should persist over the next quarter”concludes Simon Mittelberger.