Abstract: New environmental policies establish the need to maintain the ecological integrity of freshwater ecosystems. The hydrologic regime is a key element in determining river processes and therefore the definition of environmental flow regimes (EFR) is essential to achieve this goal. The EFR can be broadly defined as the water required to sustain freshwater and estuarine ecosystems, and the human livelihood that depend on these ecosystems. Nevertheless, the role of the EFR and the methods to calculate them has not been clearly stated in many countries. This paper sets out a procedure to calculate EFRs, which includes not only a minimum flow, but also a temporal variability of this flow. The procedure integrates the results of hydrologic and habitat simulation methods in a temporal scale that takes into consideration the natural hydrologic seasonality while providing a certain level of flexibility to regulate water resources still meeting the requirements of the Spanish Water Planning legislation. The results highlighted the advantages of using different methodological approaches to calculate EFRs. Therefore, this study concludes the validity of a relatively simple hydrologic method for defining minimum environmental flows at a period of maximum hydrologic stress, but also the need to consider different approaches to take into account as many ecosystem elements as possible.

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