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

An environmental flow regime can be used as a specific restoration measure to manage impacts on the river environment resulting from hydrological alterations or loss of structural or functional complexity of the ecosystem. With the entry into force of the WFD (Water Frame Directive), it is now also a measure to help achieve good environmental status. Once the values of its components are known, the regime should be tested to see if it can contribute to the achievement of the proposed objectives. However, the analysis of the validity of the proposal and its subsequent monitoring is not always carried out adequately. This paper presents a four-stage methodology for analysing the validity of a particular proposal prior to its implementation. The four stages include comparison with the natural regime, hydraulic simulation, validity in terms of habitability for fauna, its capacity to create habitat for fauna and a contrast with other proposals. This process is applied to the official environmental flow regime proposal for the third planning cycle in four sections of the middle course of the Tagus River (Spain). The results show that the official environmental flow regime is far from the natural regime, with very low minimum flows that do not contribute to the improvement of the fish habitat or to the conservation of the riverbanks. An alternative regime proposal is presented that gives better results than the official proposal. This proposed methodology can be useful as a control strategy prior to the implementation of an evironmental flow regime.