Development of an eco-hydrological distance index and improved environmental flow assessment by integrating ecological monitoring and hydrological modeling

: Achieving a good ecological status for rivers is a primary goal under European water protection legislation, and establishing suitable environmental flows (e-flows) is key to reach this objective. Typically, statistical hydrologic methods are used to determine e-flows at the river basin district scale; however, these often overlook water quality and critical flow-ecology relationships, i.e., models linking streamflow and ecological responses. This study integrates ecological status monitoring data with hydrologic models to address the limitations of hydrological methods for e-flow assessment. The new method developed in this study enables a more precise definition of e-flow thresholds and the development of an eco-hydrological distance index (EHDI). The EHDI indicates how closely a river's flow aligns with ecological targets, taking into account catchment pressures. The methodology involves: (i) a water balance simulation using a distributed hydrological model that accounts for human impacts, (ii) regression models to establish good and bad e-flow thresholds based on monitored data, and (iii) the EHDI, which compares actual flow with these thresholds to identify rivers where further water abstraction should be restricted. The application across 11,000 river reaches in Tuscany, (Italy) reveals that many rivers approach the bad e-flow threshold in summer. Instead only a few rivers deviate significantly from ecological targets according to mean annual flow. The findings underscore that statistical-hydrologic methods alone fail to capture the complex dynamics between flow regimes and ecological status, especially under high human pressure. In fact, when human pressures on water quality are significant, the restoration of natural flows would not be enough to achieve ecological status objectives.