Spanish Supreme Court ruling affects Tagus–Segura water transfers

Spain’s Supreme Court ruled on May 6 that the EU Member State has to comply immediately with national and European laws to guarantee ecological flow requirements along protected stretches of the Tagus river. This ruling will affect the water transfer between the Tagus and Segura rivers. After an extremely wet spring the water transfer, named the Tajo – Segura Trasvase, was fully reactivated in April.

The judgment, issued on 6 May and published in Spain’s official state bulletin on 6 June, is reshaping the legal and political landscape surrounding the Tajo–Segura water transfer. Ecological flow requirements refer to the minimum amount of water that must remain in a river to maintain its ecological health. These flows are necessary to protect habitats, species, and the overall functioning of aquatic ecosystems. In the case of the Tagus, the ruling specifically targets water bodies located within legally protected areas, including those designated under the EU’s Natura 2000 network. The court found that deferring the enforcement of these flow levels violated environmental law and that their implementation cannot be postponed.

Trasvase Tajo-Segura reactivated

The decision comes at a time of unusual hydrological conditions. In late March 2025, following one of the rainiest months on record in the central Spanish plateau, water levels in the Entrepeñas and Buendia reservoirs rose above the threshold needed to activate the Tajo–Segura transfer. On 24 March, the Confederación Hidrográfica del Tajo announced that the reservoir had reached 714.19 metres above sea level. The high water levels authorised a total transfer of 180 cubic hectometres of water between April and June.

Comply with European and national laws

The Spanish Supreme Court has partially upheld the appeal brought by the environmental groups Plataforma en Defensa de los Ríos Tajo y Alberche and GRAMA, challenging provisions of the Hydrological Plan for the Tagus River. The Court declared null and void the sections of the plan that allowed for the staggered implementation of ecological flow targets in 19 water bodies located along the central axis of the Tagus, between the Bolarque and Valdecañas reservoirs. The ruling emphasised that ecological flow requirements are not a future objective but have been legally binding since 2009, following the implementation of the EU Water Framework Directive and corresponding Spanish legislation. The court concluded that it is unlawful to delay full compliance with these flow targets until 2027, as previously scheduled under the phased regime of the hydrological plan. In its reasoning, the court stressed that the water bodies in question are protected. Many of them form part of the Natura 2000 network, and are already in a poor ecological state. Any delay in achieving ecological flows in such areas, the Court ruled, would constitute a breach of both national and EU law.

Debate over ruling amongst autonomous regions

The Supreme Court’s ruling and the resumption of water transfers has reignited debate between Spain’s autonomous communities. Castilla-La Mancha’s president, Emiliano García-Page, described the ruling as sealing an irreversible environmental position, suggesting the Tajo–Segura transfer ‘is history in its current form’. He noted that the Supreme Court’s decision affirms that the system can no longer operate under ecological standards and must now be considered as a complementary resource, not a primary supply, with cases like desalination taking precedence.

Water transfers essential for other regions

Murcia’s regional authorities and irrigation unions have defended the continuation of the transfer. They argue the ruling does not prohibit phased ecological flow implementation in non-protected areas and that the infrastructure remains legally operational. In 2021, regional governments in the southeast, including Murcia, protested against cuts to the transfer system. By contrast, leaders in the regions of Murcia, Valencia and parts of Andalusia, which benefit from the transfer for irrigation and drinking water, have warned of the ruling’s potential consequences. Murcia’s regional water minister, Sara Rubira, rejected the ruling’s alleged political criteria for determining ecological flows. She stated the Murcia government is considering the decision’s legal implications, including filing a regional appeal. The secretary-general of the PSOE in Murcia, Francisco Lucas, reiterated that solving water issues demands rigour, not political posturing or emergency laws, and insisted the Tajo–Segura transfer will not be dismantled. Furthermore, irrigation unions such as SCRATS contend the ruling applies only to protected stretches of the river and does not necessarily preclude continued water transfers under current rules. They have expressed concern, however, that new flow requirements could reduce available volumes in the future.

Government will ‘rigorously enforce’ the ruling

Spain’s Secretary of State for the Environment, Hugo Morán, confirmed on 11 June that the government would rigorously enforce the Supreme Court ruling. He stated the Ministry for Ecological Transition (MITECO) is already working on measures to ensure compliance, which will involve revising water management plans and potentially amending the rules governing the Tajo–Segura system.