seven questions to understand the problems

The construction site for the Sainte-Soline (Deux-Sèvres) mega basin was restored on Tuesday, November 8, ten days after a large demonstration against this project, which was approved in December 2018 and intends to allow the irrigation of agricultural crops. summer and dry season. These reservoirs, protected by the government, were declared by the Minister of Agriculture, Marc Fesneau, ” positive and better compared to the current situation or ten years ago However, environmentalists and some scientists denounce the ecological aberration associated with water monopolies.

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What is a mega basin?

Among opponents of these reservoirs, some generally speak of “basins” or “mega-basins,” but the official term preferred by advocates of these projects is “storage” or “replacement reservoir.” It is a crater-like structure surrounded by a mound several meters high, covered with plastic tarpaulin for waterproofing.

Its function is simple: to draw water from shallow water tables in the winter when rains are more frequent, store it outdoors, and use it to irrigate crops in the summer. It is a solution for farmers to ensure productivity especially during times of drought and water constraints.

The basin envisioned for Sainte-Soline could hold up to 628,000 cubic meters of water, the equivalent of about 250 Olympic swimming pools. The Coop de l’eau des Deux-Sèvres, which brings together the farmers behind the project, believes it will be possible. “Reduce direct debits by 70% (…) in the summer”. Indeed, during this season, farmers pump water tables, exacerbating soil drying due to lack of water.

Reservoirs can form an important part of the country’s irrigation. In Deux-Sèvres, the sixteen planned basins should eventually cover just over 6 million cubic meters, or 20% to 30% of the total volume used for irrigation in the department.

How many equivalent projects are there in France?

There is no national account. There are twenty-five basins operating on the outskirts of the Marais Poitevin Regional Nature Park. The Deux-Sèvres Basin is planning sixteen new basins in the same region, including the Saint-Soline basin, the second to be built after the Mauze-sur-le-Mignon basin. Another large-scale project, approved by the prefect of Vienna on November 3 without the support of environmental organizations involved in the consultation, involves the installation of thirty new reserves in his department by 2028.

A map maintained by the association Les Uprisings of the Earth lists many more basins and megabasins in France, some of which have already been “disarmed” by militants. Two regions appear on this collaboration and warrior map: the Marais Poitevin and the Alps, where it is most often used for artificial snow in winter sports resorts. According to the site, an accurate calculation is more difficult because some basins are built illegally Reporter, In the Sea of ​​Charente.

Why are beds a problem?

For environmental organizations, mega-basins are a symptom of a water-intensive agricultural system that seeks palliatives for recurring droughts and maintains the illusion that modest adaptation to climate change is sufficient.

Opponents of the projects decry future basin-fed crops as water-intensive, citing corn in general. In this regard, the prefectures of Vienna and Deux-Sèvres, the carriers of two large projects for the construction of reservoirs, remain uncertain. An observatory on agricultural practices and actions benefiting aquatic and terrestrial biodiversity has been established to measure. “products, volumes used and measures implemented within the framework of individual obligations of operators”, however, this information is not yet available on their website.

For Jean-Francois Soussana, a member of the High Council for Climate, these reservoirs are even “adaptation”. In July, the scientist gave the following explanation to the deputies: “We’ll need it [à l’horizon de 2050], and I think agricultural actors are aware of changes in production systems to reduce irrigation needs and achieve a better balance with the climate as they develop. »

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Do they pose a risk to the environment?

Upstream of the Sainte-Soline project, an impact study was conducted by the Bureau of Geological and Mining Surveys (BRGM), a public agency for soil and subsurface research. It is concluded that the reduction of withdrawal from rivers due to the use of basins in summer can increase their flow by 5%-6%, while in winter the flow decreases by 1%. For Christiane Lambert, president of the National Federation of Farmers Unions (FNSEA), this is an effective adaptation to frequent summer droughts and rarer and more intense rains. In the south of the Vendée, there is an increase in the groundwater level after the installation of replacement reserves.

But the BRGM study does not take into account other effects of creating several hectares of stagnant water bodies, such as the possible appearance of bacteria that affect water quality. The Vienna prefecture rejects this argument, reminding that this water is filtered and intended for irrigation, not for swimming.

Another problem is evapotranspiration, where evaporation is stronger over a wide open area than in groundwater aquifers. This phenomenon is difficult to estimate: the projects in Deux-Sèvres and Vienna are based on research that estimates evaporation at 4% of the total volume; FNSEA’s Christiane Lambert puts forward the 7% figure when CNRS research director emeritus Christian Amblard, interviewed by Franceinfo, estimates the rate of evaporation at between 20% and 60%.

How are withdrawal limits determined?

Withdrawals are permitted by prefectural orders, regardless of whether or not a replacement supply is available. They can be reduced or banned, for example, during drought.

In Vienna, the total amount that can be withdrawn (…) will only be withdrawn if the minimum thresholds are met.

Contrary to what government spokesman Olivier Vera told MPs on November 3, water stands are not only pumped when there is water. “already” from rainwater. Sampling is carried out in winter (1er November to March 31) and any withdrawal made to fill the replacement basin during this period replaces the withdrawal of the same amount made during the summer. The total volume that can be drawn in Vienna is not the “excess” volume of water. It is set at 80% of the average volume withdrawn during 2003-2012, which will only be withdrawn if the minimum thresholds are met. However, he kept in touch The worldThis was reported by the Vienna prefecture. “Even if we enter the “winter” period, the existing storage stocks cannot be replenished until today (…) After the severe drought this year.”

This summer season, irrigation was prohibited in the natural environment in Deux-Sèvres. “Most Sensitive Cultures”. The first reserve of Mauzé-sur-le-Mignon, filled in the winter of 2021-2022, can nevertheless be used without restrictions.

Sainte-Soline basin area protected by gendarmes, November 2, 2022.

How are projects funded?

Each water supply costs more than 1 million euros. Most of the projects are financed from state funds. In the case of the Clain basin protocol, in Vienna this covers two-thirds of the project, mainly through the Loire-Bretagne Water Agency, which has to finance 42%. The rest is provided by the cooperatives that benefit from the water. In the current version of the project, 80% of the amount will be borrowed and paid due to the added value of the water price calculated at 0.20 euros per cubic meter. Therefore, larger consumers will return a larger amount.

Public funding is rejected by opponents of the projects, such as Nicolas Girod, national spokesman for the Confederation paysanne de l’eau, “don’t (…) water privatization.

What counterparties are required from farmers?

Substitute watershed projects may be subject to environmental obligations. This is the case for sixteen planned in the Marais poitevin and thirty in the Clain basin. In this latest memorandum of understanding, the farmers benefiting from the installation have undertaken to buy “Measures for the protection of land and water biodiversity and landscape”, for example, the creation of 30 kilometers of hedges, a 20% rate of conversion to organic farming, a 20% high environmental value level 3 (HVE 3) certificate or even a 50% reduction in pesticides (this is already in line with the national objective of the Professional Ecophyto plan).

The prefecture of Vienna, a signatory to the Clain basin protocol, believes that these projects are a key lever for action to change farmers’ practices because, unlike other programs that are not purely voluntary, commitments are enforced.

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