Questioned, Friday, April 14, on the question of water sharing during the program "Extra local", on Public Senate, the deputy La France insoumise (LFI) of Seine-Saint-Denis Aurélie oublie, agricultural engineer from training, criticized the cultivation of maize, lamenting that "25% of the water consumed in France is consumed by the cultivation of maize".
Ms. Trouva's reasoning is relatively simple: agriculture accounts for 57% of water consumption in France, and "half of the irrigated areas are for corn". This is equivalent to a quarter of the total consumption.
This calculation has been strongly criticized for its simplism, even qualified as misleading, in particular by the journalist of the daily L'Opinion, Emmanuelle Ducros, on Twitter. She deplores a "figure that came out of nowhere and totally false", and offers a counter-argument to denounce a "canard".
Ms. Ducros (who did not respond to our requests) explains in her tweets that three-quarters of corn crops are not irrigated and that a quarter are, suggesting that MP LFI has confused this figure and the share of water consumed. She then adds that agricultural irrigation accounts for only 9% of freshwater withdrawals in France.
But the journalist probably confused two notions:
In 2019, the last year for which figures are available, 31.5 billion cubic meters of fresh water were withdrawn in France, and irrigation represents a good 9% of this total, as evidenced by official statistics (dating from 2017).
But the share of agriculture in water consumption (61.5%) is significantly higher than its share in withdrawals (10%), since it consumes about 80% of the water it withdraws, according to the government's data and statistical studies service (SDES). This is not the case for other uses, such as the operation of nuclear power plants or the supply of canals, which withdraw large quantities of water but return it almost entirely.
It is conventionally estimated that around 88-90% of the water used by agriculture is for irrigation. The rest, 10% to 12%, is used for watering livestock and cleaning facilities. The real weight of irrigation in water consumption in France was therefore, in 2019, approximately 54%.
The share of maize in this total is more difficult to estimate. According to Agreste, the statistical service of the Ministry of Agriculture, corn, all types combined (grain, fodder, seed), represents 39% of irrigated areas in France in 2020. Given its water needs per hectare of crop, substantially equivalent to those of wheat and higher than those of market gardening (the second and third most irrigated crops in France in hectares), it does not seem unreasonable to think that the irrigation of maize weighs for a little less than half of the volume of water consumed by irrigation in France.
If the 25% cited by Aurélie Trouvé is a quick calculation, the order of magnitude is correct. The real figure is probably somewhere between 21% and 24%, although it is difficult to determine precisely. And as analyzed by Marc Dufumier, agronomist and honorary professor at AgroParisTech, contacted by Le Monde: "it is not so much the overall figure for the liter that matters as the question of the origin of the water: is it rainwater or did we get it? »
Corn growers often praise the qualities of this plant, which requires little water to grow. It's true: corn is a so-called "C4" plant, which photosynthesizes very efficiently and loses less water than other plants. According to CNRS data frequently quoted by the agricultural world, 1 kilogram of grain maize requires approximately 454 liters of water to be produced, while barley requires 524, wheat and potatoes 590, soybeans 900 and upland rice 1,600. Fodder maize, which is harvested, ground and then stored in a silo and which is intended for animal feed, only consumes around 238 liters of water per kilogram produced.
The problem is not so much that corn consumes a lot of water, but the period when it consumes it, because it is a summer plant: its seedlings are planted between April and May, and its growth takes place between June and August. , a time of year when the plant is particularly sensitive to water stress and when rainfall is lowest, while freshwater resources are the rarest. "The real problem with corn is that it requires water in the summer. It is a tropical plant, and in the tropics the hot seasons are the rainy season. With us, the hot seasons are the seasons when it rains the least,” comments Marc Dufumier.
This is the reason why maize has great irrigation needs: in 2021, according to Agreste, 35.12% of cultivated areas of grain maize were irrigated, compared to 4.8% of wheat areas, and 4 % for other cereals.
Irrigation of maize has a major economic interest for farmers. On average, between 2016 and 2021, the yield of non-irrigated grain maize was 9.23 tonnes per hectare, compared to 10.95 tonnes per hectare for irrigated grain maize, a gain of 18.6%. In 2020, this yield gain resulted in the production of approximately 1.2 million tonnes more corn.
However, such irrigation is not without consequences. The sub-basins in the south-west of the country (Mayenne-Sarthe-Loir, Charente, the Aquitaine and Charente coasts), where a good part of the grain maize is grown, are particularly vulnerable to summer water stress. "They have low renewable water resources, while facing high summer consumption," notes a 2021 SDES report. In these basins, agriculture consumes more than 90% of the water available in summer, and "is primarily intended for grain crops, predominantly maize," the report said.
Global warming and the increase in the frequency of droughts in France risk aggravating the situation by generating conflicts of use around fresh water in these territories if this irrigation continues at the same level. Average freshwater resources have already declined by 14% between 1990 and 2018, and this decline is likely to continue. The increase in temperatures will promote evapotranspiration, especially during the recharge period which runs from September to February, which may shorten it significantly.
In the long term, such episodes of drought are likely to multiply and weaken the soils earlier and earlier in the year. New climate projections released in the fall of 2022 show that warming could reach 3.8°C by the end of the 21st century. "It is certain that such warming will lead to more frequent, long and severe heat waves, significant drying of the soil and a sharp drop in water resources," noted Julien Boé, research director (CNRS) at the European Center for research and advanced training in scientific computing, and one of the study's authors.
