Winter-filled substitution reservoirs with agroecological conditions (contested)

Mechanism

Shallow groundwater is pumped in winter, when aquifers and rivers are high, into large lined open-air reservoirs. In summer, connected farms irrigate from the stored water instead of abstracting from the stressed aquifer, which is the substitution logic. Access is tied by protocol to agroecological commitments (reduced pesticide use, practice changes) as the public counterpart for largely public financing.

Where it fits

Intensively irrigated arable basins where summer abstraction visibly stresses aquifers and rivers, and where winter recharge is (or is assumed to be) abundant. In France this is the model of the substitution reservoirs (opponents call them mégabassines) promoted by irrigator cooperatives with roughly 70 percent public funding.

This approach is contested

This solution should be read as contested, and the evidence honestly cuts both ways.

Supporters cite:

• BRGM hydrogeological modelling indicating limited impact of winter pumping on the aquifer.
• Court validation: after litigation, France's Conseil d'Etat confirmed the legality of 12 of the 16 Deux-Sevres reservoirs in October 2025, and the Loi Duplomb (August 2025) created a presumption of major public interest for agricultural water storage, upheld with reservations by the Conseil constitutionnel.

Opponents cite:

• Evaporation losses from large open-air storage.
• Capture of a common resource: a minority of farms get secured access to water pumped from a shared aquifer, financed about 70 percent publicly.
• Maladaptation lock-in: securing irrigation for water-intensive crops such as maize instead of driving adaptation of cropping systems.
• Biodiversity harm: the Bordeaux administrative court of appeal ruled 4 of the 16 Deux-Sevres reservoirs illegal in December 2024 for lack of a protected-species derogation (the little bustard).

Implementation path

• Hydrogeological modelling of winter pumping impacts, published and independently reviewable.
• A signed protocol binding reservoir access to verifiable agroecological commitments.
• Full protected-species and environmental permitting before construction, not after.
• Above all, an allocation process that non-connected farmers and the wider public perceive as fair.

Trade-offs and the honest lesson so far

Beyond the technical disputes, the clearest lesson from the French experience is political: infrastructure-first responses without perceived allocation fairness generate maximal conflict. The Deux-Sevres program produced one of France's most violent environmental confrontations, years of litigation, and structures that may be legal but remain socially contested. Whatever the hydrology says, a storage project that a basin's other users see as a private capture of a common resource will be fought as one.