Environmental flow assessment in transboundary rivers: Challenges and opportunities using big data - A Greek case study

Environmental or ecological flow refers to the minimum flow needed to sustain river- based ecosystems and their services. Evaluating environmental flows is of paramount importance, particularly in light of the rapid changes induced by climate change, anthropogenic pressures, and the continued damming of river courses. The research aims to evaluate the environmental flow of four transboundary rivers in Greece, namely Axios, Strymonas, Nestos and Evros Rivers, using the Tennant and Tessman hydrological methods, and to assess their compatibility with Greek national legislation. The rivers’ runoff is determined using large-scale hydrological models applied at the European scale, which simulate thousands of river basins simultaneously, generating extensive big data datasets. The results demonstrate that Greek legislation underestimates the flows compared to those derived from both hydrological methods. Furthermore, the ecological flow values derived from the Tennant method for winter months generally exhibit lower magnitudes compared to those obtained from the Tessman method, whereas during summer months, there appears to be a convergence in the values derived from both methodologies. The proposed methodology can be applied to any river within the European Union and can serve as a significant roadmap for further advancements in the assessment of environmental flow.