Admittedly, it has rained for the past two months. A good part of the French can attest to this. The Bretons in the lead. Ile-de-France residents too. But did this precipitation improve the groundwater situation in France?
At the end of winter, the situation outlined by the Bureau of Geological and Mining Research (BRGM), the public body in charge of their follow-up, had cause for concern. To the drought of last summer was added a winter drought aggravating a little more the state of the water tables. “As of April 1, 73% of them were below seasonal norms,” recalls Christophe Béchu, Minister for Ecological Transition.
Improved “reactive” aquifers
This Wednesday, the BRGM is publishing a new status report, this time based on the levels recorded on May 1. If the cumulative rainfall has been excessive over a large part of the territory over the past two months, the impact on the groundwater is very heterogeneous. If only because it rained unevenly on the territory. Above all, “at this time of year, the rains go first to the vegetation and thus recharge the water tables little”, insist both Christophe Béchu and Christophe Poinssot, interim director general of the BRGM.
All the same. Some aquifers show rising levels. Those called “reactive”, on which heavy rains (or the reverse) have rapid effects, in particular because they are shallow. These include the Grand-Est, Bourgogne-Franche-Comté and northern Massif Central aquifers, for which the recharge continued in April, notes the BRGM. The same goes for the Armorican Massif, Brittany and Vendée aquifers, which also benefited from excess rains in March and April, even if the attenuation of the “effective” rains over the last two weeks of April that these aquifers are displayed with falling or stable levels on the new BRGM map. However, this northwest quarter of the country is displayed in dark blue which reflects a high level of groundwater.
Concern for the Rhone corridor and the Mediterranean rim
So much for the good news. They should not make us forget that 68% of the water tables remain at moderately low to very low levels, including 20% at very low levels, against 19% in March. “At the same time last year, the ratio was 58%, and we all remember how the season ended,” recalls Christophe Béchu.
Christophe Poinssot draws attention to the areas where the water tables are at critical levels, indicated by the color red. “They have increased significantly compared to last year,” he points out. The entire Rhône corridor and the Mediterranean rim are displayed in this color. The situation is also worrying in the Paris basin, “where the levels are generally very low compared to all the months of April in previous years”, reports the BRGM.
Why so, when there are many more since April? “We are, this time, on an inertial layer with very slow flows, which makes it not very reactive to what is happening on the surface, explains Christophe Poinssot. The light rains that fell this winter and those, certainly more important, in recent weeks, were not enough to catch up with the drop in levels that began this summer. And unless it rains all summer, which is unlikely, this downward trend is likely to continue, he said.
Territories with very probable risks of drought
“From this first map of groundwater levels on May 1, then follows a second, which anticipates the risk of drought by the end of the summer. This risk is considered very likely over a large part of the Paris basin, Deux-Sèvres and Vienne, as well as all of southern France, from the Hautes-Pyrénées to the Alpes-Maritimes, going up to the department of Ain. .