Combining meteorological and streamflow information for defining minimum environmental flow requirements in Mediterranean catchments

The compliance of environmental flows following the recommendations of the Water Framework Directive (WFD) is a crucial aspect in the management of reservoir water allocation. This issue is especially challenging in catchments with long and recurrent drought periods, such as those affected by the Mediterranean climate. There are still some gaps to be addressed by managers and water authorities. For instance, the correct definition of minimum environmental flow (MEF) to be fulfilled. These MEF values, which are set in the River Basin Management Plan (RBMP), commonly vary slightly throughout the year. Sometimes, this assumption is not accomplished because these values are above those that would have been observed under natural conditions. Therefore, in these regions it is decisive to adapt the MEF requirements to the local hydrometeorological seasonally.       This work proposes the combination of historical streamflow and precipitation data to assess the viability of using seasonal hydrometeorological patterns in the definition of the MEF rates. For that, several monitored water bodies were analysed. Streamflow information from gauging stations and precipitation data were selected in the two main river basin districts in the South of Spain: the Guadalquivir River Basin, and the Andalusian Mediterranean Watersheds River Basin. First, for each case, we reviewed the compliance of the MEF rates analysing when these threshold values were or not achieved at the monthly scale.  In addition, we analysed the seasonal variability in terms of both precipitation and streamflow and compared these outcomes with the seasonal variations of the MEF. This analysis was carried out during 10 hydrological years, from September 2010 to August 2020. According to our results, most of the locations were below 28% of accomplishment during the summer months. This percentage increased when the period analysed was the winter. However, this percentage was below 50% in some locations. On the contrary, only in those locations which are fed by mountain catchments, the accomplishment was fulfilled in 73-100% during the whole year. The joint seasonal analysis of precipitation and streamflow highlights that the MEF established in the RBMP were oversized in most of the cases, overlooking the precipitation patterns. This work showed that a revision of the MEF values set by the RBMP is required. That is especially significant in locations where seasonal variations of the MEF are null or imperceptible. Our outcomes will help to set the basis for the design of a new methodology when defining MEF. Hence, this new approach will consider not only water quantity but also hydrometeorological seasonal variability as the main step to truly address water management from the perspective of the WFD.      Acknowledgments: This work has been funded by the project TED2021-130937A-I00, ENFLOW-MED “Incorporating climate variability and water quality aspects in the implementation of environmental flows in Mediterranean catchments” with the economic collaboration of MCIN/AEI/10.13039/501100011033 and European Union “NextGenerationEU”/Plan de Recuperación, Transformación y Resiliencia.