The third heat wave of the year, which began on July 31, lingers over the whole of France and further accentuates the drought, which now affects almost all of the territory, and has reached a historic level since mid-July.
Ground drought not seen since 1958
The soil moisture index, which is one of the indicators of drought, has been below the normal level (average value over the period 1991-2020) since January 19, and the situation has continued to deteriorate since.
Since July 17, this index has reached the record level of drought on the same day in previous years. And since August 6, it has even exceeded the absolute national drought record observed since 1958, beyond the level of the very dry summer of 1976.
A lack of rain at the worst time
For Christian Viel, climatologist at Météo-France, this exceptional situation results from the combination of two main factors: “The first rainfall deficit took place at the worst time, when the water tables recharged during the winter, from September to March, and was followed by an extremely dry spring with very high temperatures, which leads us in very dry conditions now. »
The year 1976, which was marked by the greatest drought in France, experienced a greater rainfall deficit than in 2022, with 294.2 millimeters of rain until August 7, against 332.5 mm this year in mean. However, compared to 1976, greater heat has largely contributed to the drying out of the soil: « The temperatures of spring 2022 [avril à juillet] were 1.7°C higher on average than those of spring 1976, which amplified the phenomenon of evaporation”, adds Christian Viel.
No department is spared
The whole of the metropolitan territory is concerned by this lack of moisture in the superficial soils. The vast majority of metropolitan departments (72 out of 96) are in a situation of soil drought more than twice as high as normal (more than 50% on the map) and even nearly three times drier in Normandy, in the Provence-Alpes-Côte d’Azur region and in Corsica.
Unprecedented usage restrictions
To preserve the resource, 93 metropolitan departments adopted, on August 8, measures limiting the use of water. These measures taken by prefectural decrees are limited in time and circumscribed on a given perimeter. They span four levels of severity.
The number of departments in which the highest “crisis” level has been established is gradually increasing. On the same date, 69 were affected by at least one local decree to reserve water for priority uses (health, civil security, drinking water, sanitation and animal watering), ie seven more than the previous week. In these perimeters, the watering of green spaces is prohibited and water withdrawals for agricultural irrigation are also prohibited, with the exception of certain sensitive crops.
A “slow and limited” return to normal
For Christian Viel, “we are currently tending towards an extreme value of the soil humidity index, and we are reaching its minimum since the heat wave and the dry period should end at the end of the week to give way to a drop in temperatures and a gradual, albeit limited, return of precipitation next week”.
Soil humidity should thus gradually rise, but the phenomenon “will be slow and limited” warn the meteorologist. In fact, the water that infiltrates the ground constitutes only a part of the precipitation; the rain will first be absorbed very quickly by the vegetation and evaporate just as quickly with a ground that is still very warm. “Even if there is precipitation, we are starting from such low levels that it will take time for a return to normal. »