Why is it already too late to recharge groundwater? With Simon Mittelberger, climatologist at Météo France

Summer has not yet started and already, France — metropolitan France, in any case — lack of water. Farmers are suffering. Our electricity production too. The first water restriction measures have been taken. The map published this Wednesday, May 18 by the Hydrological Anticipation and Monitoring Committee (Cash) confirms this. The situation is set to continue. Over the whole country. Since no department is guaranteed to escape the drought in the coming weeks. And no less than 22 departments are even classified as risk ” very probable “ by the end of this summer 2022.

However, let us remain measured in the interpretation of this map. “It gives indications in relation to the normal and the situations usually encountered”specifies the press release from Cash. “This year, we talk a lot about the drought, also because the contrast with the past two years is strikingadvances Simon Mittelberger, climatologist water resource monitoring specialist Weather report France. And at the origin of the current situation, there is first of all a precipitation deficit I’winter last “.

In the southeast, for example, some stations recorded a meteorological drought of more than 40 days at the beginning of the year. A meteorological drought? This is how scientists call a deficit of precipitation prolonged. Over the period from 1^er December 2021 to February 14, 2022, this deficit reached 80%, on the side of Sète or Montpellier. “But it is the whole of France which is experiencing a particularly dry start to the year. Between January and April, we saw a rainfall deficit of around 20%”, teaches us Simon Mittelberger. Neither more nor less than 4^e largest deficit since records began in the late 1950s.

But a dry winter is not enough, on its own, to soar the drought risk. “The lack of rain has continued this spring. And the strong heat that we experienced in mid-May only made the situation worse. » The record for the longest period of warmer than normal spring has even just broken. It had held since 2007. The high temperatures had then already been maintained for no less than 38 days. Today, “Across France, the soils are in a situation that we usually encounter rather at the beginning of July”.

Soils, groundwater and rivers

“To estimate the drought risk — it is rather a question here of so-called “agricultural” drought, which reflects the state of the soil, up to only 1 to 2 meters deep —, we rely on precipitation and temperature forecasts. Both are equally important”, explains the climatologist. And these forecasts are not reassuring. According to Météo France, for the months of May, June and July 2022 — an update on June, July and August is expected soon — one scenario at a time “warmer and drier than normal” is most likely, especially in the southern half of the country. “But what plays a major role is the initial state. But this year, at the end of winter, soil moisture was already well below normal. » In the Paca region, in particular, the soils are extremely dry. “A drought like we only see once every 25 years or so”underlines Météo France.

Are we to understand that the most important thing is that it rains enough in winter? “Winter precipitation — the rains that fall between September and March — are decisive for recharging the groundwater», Simon Mittelberger reminds us. And in the matterthe Bureau of Geological and Mining Research (BRGM ) confirms this. The lack of rain since September 2021 has had a strong impact on the state of groundwater, which rapidly deteriorated from February 2022. This year, the emptying of groundwater even began two to three months in advance. Result today “the situation is particularly worrying, with low to very low levels locally”. A little more so when we know that certain aquifers feed and support the waterways during the summer.

Taking into account the forecasts of Météo France, “the emptying should continue and the levels remain low during the spring and summer”. Even if oceanic disturbances accompanied by heavy precipitation could recharge the most reactive aquifers. It will remain temporary. “From spring onwards, precipitation can no longer really recharge the water tables. The rise in temperatures facilitates evaporation and the resumption of vegetation limits the infiltration of rain into the soil., explains the climatologist. We understand better why it is so to speak impossible to make up for a winter deficit.

Especially since the start of the campaigns forirrigationusually doesn’t help matters. “This is where spring and summer precipitation have a major role to play. For agriculture. They bring water to the plants and thus avoid irrigation. » Because irrigating means drawing water from available stocks. The rivers or underground waters. “Soils are more reagents . A rainy episode, even if it is stormy, can improve the situation. Punctually anyway. »

Global warming, drought facilitator?

Having said all this, the question that remains is the following. Do we owe this drought to global warming ? “We cannot attribute a particular event to climate change in this way.warns Simon Mittelberger. What can be said, based on past records and projections for the future, is that since the 1960s, the soils have become increasingly dry. With an even more marked drying up in summer”. In question, more to the rise in temperatures – because the hotter it is, the more the plants take water from the soil –, than a drop in precipitation. “At this level, we observe very little change in the annual average. But all the same a slight increase in precipitation in winter — the uncertainty is important here, because it will depend on the evolution of the stormsover Europe — and a drop in the summer. » According to experts, it is therefore possible that the filling of water stocks for the coming winters will not be sufficient to meet the growing needs due to increasingly hot and dry summers.

And Météo France more widely confirms that “The frequency of droughts has increased since the early 2000s”. with episodes “more intense and longer”. As well as a “proportion of territory hit each year increasing from 5% in the 1960s to 10% more recently”. However, predictions remain difficult to make. They do not only depend on water supply. “The impact of droughts will depend a lot on the uses and the evolution of certain practices”, underlines the researcher. Of some agricultural practicesparticularly water-intensive. But we’ll talk about that in a future topic.