Why did the rains of the last few weeks not necessarily protect us from drought for the whole summer?

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It will not come as a surprise to those who have wiped out rain after rain in recent weeks. June 2021 ranks among the five wettest months of June over the period 1959-2021, Météo France indicated in its hydrological situation bulletin published last Thursday.

Blame it on very frequent and very active rain and storm passages “which were not very localized as we are used to with this type of episodes but which, on the contrary, concerned a large part of the country, says Michèle Blanchard, climatologist at Météo France. From Hauts-de-France to New Aquitaine as well as from the Grand Est to the north of Auvergne-Rhône-Alpes.

Up to more than 200 mm of precipitation

“On average, over the country and over the month, the excess rain was 50% higher than normal [calculée sur la période 1980-2021], continues Michèle Blanchard. We expected 70mm of rain, we got 102.2. »But, in some places, the accumulations of precipitation reached 200 to 250 mm, from the Allier to the Lot, from Vienne to the north of the Gironde, on the Jura Massif, the north of the Alps as well as, very locally, on Seine-et-Marne, Haute-Marne, the south of the Vosges and Burgundy. With records broken here and there. Especially in Hauts-de-France, reported France 3.

And then, to this wet month of June, we must add May, also well watered, just as is, for a large part of the metropolis, this first part of July. These last hours still, on Twitter, Météo France warned against an increase in rains in the departments of the North-East.

Summer rains which only slightly reach the water tables

We could also see the glass half full and tell ourselves that all this fallen water shelters us from the drought for the rest of the summer to come, the forecasts of Météo France still counting on the months of July, August. and September drier and warmer than normal over much of France. It missed. “At this time of the year, the rains which infiltrate the soils, even abundant, have little impact on the level of the water tables,” explains Violaine Bault, hydrogeologist at the Bureau of Geological and Mining Research (BRGM), the national geological service in charge of monitoring groundwater. Much of it is captured by growing vegetation, which therefore drinks a lot of water. The rest must first cross the ground, very dry in May because it had not rained for three months then. Very little water therefore reaches the aquifers, not in sufficient quantity in any case to fill the emptying of the latter towards their natural outlets. Rivers, oceans, wetlands… ”

Result: the last BRGM map summarizingthe state of the groundwater on July 1, published this Monday, is dotted with triangles pointing downwards *, indicating that the level of the water tables is generally lower. Nothing unusual at this time of year. The heavy rains of recent weeks are not without positive consequences. “They have allowed, in many sectors, that this drop in level is slower than usual,” continues Violaine Bault. Quite simply because we had less to use the groundwater to water crops, gardens, fill swimming pools, etc. The streams are also at a higher level than usual in relation to the water tables, so the emptying is slower. “

An overall satisfactory level of groundwater … for the moment

Add to this an autumn-winter 2020-2021 marked, for the second year in a row, by a strong recharge of groundwater over a large part of the territory, and you obtain a situation on July 1 that is generally satisfactory in terms of the hydrological situation. in a city. With levels close to normal over a large northern and western part of the territory, indicates the BRGM. Sometimes even high on the Aquitaine basin.

But it is only for this region that the BRGM considers that “the situation should remain satisfactory during the summer”. Elsewhere, even when the water tables are currently in the green, “the situation can deteriorate rapidly during the summer in the event of a prolonged meteorological drought”, indicates Violaine Bault. “The most responsive water tables we have in mainland France – those in particular of the Armorican massif (Brittany) and the Massif Central (Limousin and Auvergne) – take five to six weeks to go from a green level as we have today to a level that is starting to turn orange-red, ”she explains.

Much more complex situation on the Mediterranean rim

The Mediterranean rim, the Alps and the Pyrenean foothills, for their part, face the summer in a less favorable situation. “These regions have already been very little affected by the June rainfall,” begins Michèle Blanchard. In the PACA region, for example, the accumulations of precipitation were 50% lower than normal. We expected 60 mm of rain, there were 32. ”Violaine Bault goes back even further to the famine around the Mediterranean. “The region experienced significant precipitation at the end of September and the beginning of October until it caused flooding,” she recalls. Then, they had hardly anything during the whole winter. It fell in May, but that was not enough to reload the water tables. “

On the Mediterranean coast, the water tables thus show a “low level”, indicates the BRGM, which expects the state to deteriorate further during the coming weeks. The situation could become tense in certain sectors, he said in his bulletin published on Monday. “The level of the water tables is also moderately low in Corsica, even if the situation is a little more contrasted according to the sectors, specifies Violaine Bault. It has rained on the reliefs in recent weeks, in particular. “

Anyway, these regions of the south of France “will have to be watched closely,” continues Violaine Bault. In particular, always, on this Mediterranean coast where the water tables are very often requested in the summer because of the influx of tourists that this region receives. ” “Drought” prefectural decrees have already been taken to slow down the stress on the water tables. The entire Mediterranean rim and the Rhône corridor are already vigilant [information et incitation des particuliers et professionnels à faire des économies d’eau]. In the Var, the Bouches-du-Rhône, the Hérault, the Eastern Pyrenees, the Haute-Garonne, several sectors have even been placed on alert, with water use restrictions already in force.





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