Proposal for analysis of minimum ecological flow regimes based on the achievement of technical and environmental objectives: Tagus River basin case study (Spain)

An environmental flow (e-flow) regime is designed to mitigate the effects of hydrological changes on rivers and to achieve good environmental status of a river as required by the Water Framework Directive (WFD). Once the components of an e-flow regime have been defined, it is important to verify that it meets its objectives, ideally through field sampling. However, field sampling is often avoided due to its high cost and time demands. This paper presents a methodology for validating a proposed river flow regime prior to implementation, allowing its suitability to be assessed without the costs and delays associated with monitoring and ecosystem response studies. The method includes a hydrological process that compares the official flow regime with the natural regime, and a hydraulic simulation process that tests the flow regime in terms of habitability for fauna. The method is tested on four reaches of the Tagus River, which have undergone significant hydrological alterations, using two different environmental flow regime proposals: one official and the other proposed by the authors. The results show a significant reduction in the annual inflows under the official e-flow regime compared to the natural ones (about 25% of the natural total), as well as a simplification of the seasonal flow variation in all river reaches. In addition, the study shows that at the minimum flows proposed in the official regime, three of the stretches would experience conditions that may impede fish migration. This method of inspecting e-flows can be useful for testing mitigation measures for hydrological changes in the very early stages of their application.