Cyclone in Normandy, fallen trees in Paris, flooded villages… Violent thunderstorms caused damage in western and northern France this weekend, where more than forty departments were placed on orange alert. The south of the country and the Mediterranean are not spared either: since mid-May, thunderstorms have been hitting there almost daily. But, despite these torrential rains, during which the equivalent of three months of precipitation can fall in an hour, the drought remains.
“We were starting from a very low level with last year’s drought, recalls Yves Tramblay, research director at IRD and at the Hydrosciences laboratory in Montpellier. However, the longer a drought lasts, the more it spreads in the different compartments of the soil, and the longer it takes to resolve. »
Caused by an absence or lack of rain, meteorological drought is the first to be felt, but also the first to improve. Thanks to the storms, Gard and Vaucluse were thus able to temporarily lift their water restrictions, while Alsace, where it has not rained for 40 days, remains on red alert.
The storms also improved the so-called agricultural drought. When a meteorological drought lasts and is combined with a rise in temperature, water evaporates from the soil and misses vegetation and crops. On this aspect too, the situation has clearly improved in the south of the country. On the other hand, it continues to be precarious in the north, which has not yet caught up with the month without rain caused by a blocking high over the British Isles.
Above all, groundwater remains subject to problematic drought. 66% of them lack water, according to the organization responsible for monitoring them, the BRGM, which writes that in “May, the precipitation remained insufficient to generate episodes of recharge and improve the state of the aquifers. Levels are mostly down. The situation remains unsatisfactory over a large part of the country. »
The aquifers of the Pyrénées-Orientales, Provence-Alpes-Côte-d’Azur and the Lyon region thus show particularly alarming levels. “Groundwater is recharged by infiltration of water into the ground,” summarizes Fabienne Trolard. The rain has to cross the surface, then the earth, to reach them, which takes time. Aquifers can therefore take months, and in some cases even years, to recharge. »
The recent storms are therefore far from enough, especially since they do not arrive at the right time. Between March and June, 80% of the rain is captured by growing vegetation, and partly evaporates instead of penetrating the soil. It is winter rains, which fall when dormant vegetation has little need, that benefits groundwater. However, we experienced a very dry winter, marked by 32 days without rain, which also followed a historically dry year 2022.
“The rains that fell between September and March are below normal for the season in a large part of the country, and 2022 has been very dry, underlines Yves Tramblay. The water deficit is therefore likely to persist for several consecutive years. This is part of the long-term climate change scenarios established by the IPCC. With the rise in temperatures, this kind of situation will occur more and more frequently, and be more and more intense, we currently have a glimpse of it. In addition, storms can damage vegetation: “durum wheat crops, which are reaching maturity, are particularly affected,” explains Fabienne Trolard, research director at Inrae and member of the Chamber of Agriculture. Their ears have been flattened by the storms, which will complicate the harvest. »
The two specialists therefore call for a new way of managing water resources, especially since Meteo-France estimates that rainfall will drop by 10 to 15% in ten to twenty years in Western Europe. .
“We must greatly improve the recycling of wastewater and the efficiency of agricultural irrigation, insists Fabienne Trolard. We also need to survey groundwater. By applying the tools used to observe oil to them, we could understand their mode of operation and their rate of renewal, in order to manage them with precision. We absolutely have to adapt quickly. »
