The days follow each other and resemble each other. In Ile-de-France, the rain has been falling almost daily for a week. Ditto in the Nord-Pas-de-Calais, yet on drought alert despite rainy episodes. The reason: accumulated rainfall that is not enough to make up the deficit. “The risk of worsening of the state of the environment and the tension on the supply of drinking water remain high. In particular in the catchment area of the Yser, the continuous reduction in the flow of the Yser puts the environment at risk natural aquatic”, writes the prefecture of the North in a press release published Friday, July 28.
Same scenario in Limousin. The rain deficit in July will be 40 to 50% compared to a “normal” year, said Météo-France on Monday at the microphone of France Blue, even if the situation has nothing to do with what we experienced last year. If the French have the image of a wet summer, the figures from Météo-France show the opposite. In his latest bulletin published this Monday, July 31, the institution notes that on “average over the country, the rainfall for the month of July 2023 is deficient by around 10%. The drying out of the soil most often continued during the month of July, except over a broad northwest quarter […] Precipitation accumulations are deficient over a large southern half of the country. On Corsica and more locally on the Mediterranean rim, the accumulations are less than 5 mm.
From North to South, precipitation remains very heterogeneous depending on the territory: not the slightest drop of water in certain Mediterranean cities such as Nîmes or Perpignan when other regions near the Channel were correctly watered with + 32% in Abbeville, + 21% in Caen, + 12% in Brest, + 10% in Rennes, always reports Météo-France. “Also note thunderstorms which gave good accumulations of rain as in Burgundy with 62 mm, i.e. an excess of + 19%”, continues the meteorological service.
Several types of drought
Once the idea of a rainy summer has been swept away, what about the rains that have fallen in recent days? “Right now, the rains will solve the problems of agricultural drought. When it rains in summer, the water will be used by all the vegetation (trees, crops, etc.), explains to L’Express Françoise Vimeux, climatologist and research director at the Institut de recherche pour le développement (IRD) We distinguish between three types of drought: meteorological drought (rain deficit), agricultural drought (the first two meters of soil) and hydrological drought (the groundwater and watercourses). The recent rain has reduced the agricultural drought.”
Heavy rains for farmers, although the situation remains heterogeneous. “This year, the yield forecasts for field crops would be better than in 2022 but this is less the case for some farmers, especially in the south of France who have been suffering from the drought which has been raging for several months, even for more than a year like the Pyrénées-Orientales”, declares Charlène Descollonges, hydrologist engineer, author of the book Water, fake or not? Sometimes the situation is not the same within a department. “In Golden Coastit is very good in the south of the department, both in terms of quality and quantity […] In the North, crops have suffered more from the drought”, explains Clément Babouillard, representative of the agricultural union of the FDSEA 21 to our colleagues from France 3.
In summer, the rain does not recharge the water tables
But behind the greener fields, the depths of the soils are far from being regenerated. “For small streams as for rivers, we will quickly return to summer base flows because the water will run off, will infiltrate a little but will not effectively recharge the water tables which play a role in supporting the low water level. rivers during this period [NLDR : fonction d’alimenter en eau les cours d’eau pendant les périodes de bas débits, soit en été pour la majeure partie des cours d’eau français]”, specifies the hydrologist Charlène Descollonges. According to the last report of situation of groundwater tables of the Bureau of Geological and Mining Research (BRGM), the level of the water tables was in deficit on 66% of the territory. comparison, last year at the same time, 75% of them were in deficit.”And it is difficult to hope for an effective recharge of the water tables before the fall”, assures the expert.
In France, groundwater only fills up during the winter period. It rains a lot and regularly, which will allow water to infiltrate through the surface soil and in depth. “Summer and winter rains are not used for the same thing. Summer rains moisten the soil and are used by vegetation. Winter rains, elels, fill the groundwater, underlines Françoise Vimeux “In addition, the summer rain prevents us from using water, for example to water our gardens. It must be understood that our groundwater is an important freshwater resource and a resource for irrigation.”
To fully understand the impact of rain, several parameters must be taken into account. Firstly: the intensity of the rain and the evaporation demand of the atmosphere which depends on the temperatures. “There is a difference between a rain that does one millimeter per hour, and a rain that does 30. A very fine rain will have time to infiltrate the soil, if the soil can absorb (if it’s bitumen, the water can’t infiltrate so it runs off)”, specifies Charlène Descollonges. When there is intense rain, the water does not have time to infiltrate the ground and there is no effective recharge of the watercourses.
“In the case of abnormally high temperatures, with heat waves, there is greater evaporation from the soil and vegetation,” she continues. The water will therefore be taken up into the atmosphere and will not be able to recharge the water tables and waterways. Although summer rain does not have the same virtues, it is still important for stemming surface dryness and moistening vegetation. But still it is necessary that these rains are maintained in August. What does Météo-France say? “Our seasonal forecast envisages a warm month of August (+1.5°C) with precipitation slightly above normal in connection with a stormy tendency which could be quite marked from the southwest to the northeast.” So the ground should become drier again.