Is the restoration of natural flows enough to reach a good ecological status of rivers?

The objective of the European Water Framework Directive WFD (60/2000/EC) is to achieve or maintain at least a good ecological status of surface water bodies also by identifying appropriate e-flows. Water Management Plans issued by the Hydrographic District Authorities, based on the knowledge of the amount of water resources available, define e-flows and permitted water abstractions to support many human activities, e.g. domestic, agricultural etc. The objective of this work is to assess if the theoretical restoration of natural flows in river catchments allows to achieve a good ecological status of rivers. The method is based on the evaluation of natural and actual flows based on a 20-years water balance simulation implemented with a distributed, vector&raster based hydrological model capable of describing anthropogenic alterations, e.g., point abstractions, releases, reservoir regulations, etc. A new dimensionless index of eco-hydrological distance is defined to measure how far is a river flow from its ecological objectives based on actual flow regime and pressures acting on the catchment (e.g., land use, climate). The method is applied to a region in central Italy, with ca. 11,000 river reaches and mediterranean climate, where almost 50% of water bodies currently fails to achieve WFD objectives. The results show that for the summer season, which is the driest one in the study area, most of the rivers are pushed towards the limits of bad ecological status. Moreover, in 48% of rivers the summer natural flows, if restored, will not guarantee a good ecological status, due to the significant eco-hydrological distance from the target e-flows. AckonowledgementsThis study was carried out within the RETURN Extended Partnership and received funding from the European Union Next-GenerationEU (National Recovery and Resilience Plan – NRRP, Mission 4, Component 2, Investment 1.3 – D.D. 1243 2/8/2022, PE0000005). The study also received funding from the Hydrographic District of the Northern Appenines, Florence (Italy) Agreement “Esecuzione delle attività finalizzate alla redazione dei bilanci idrici su base modellistica dei corpi idrici superficiali appartenenti ai bacini toscani del distretto idrografico dell’Appennino Settentrionale ed alla definizione della metodologia da utilizzare per la definizione del deflusso ecologico”.