A New Diagnostic Approach to Assess the Ecological Impact of Droughts on Rivers

In Europe, water management at the river basin district scale requires to satisfy both human supply and ecological preservation to achieve the Water Framework Directive (WFD) objectives. Freshwater resources are essential for human needs and ecosystem balance, but factors like climate extremes, population growth, and pollutant pressures pose serious challenges. Environmental flow (e-flow) is defined as the water flow required to sustain aquatic ecosystems, but traditional methods to establish e-flow thresholds at large spatial scales lack comprehensive ecological relevance. This work discusses the effects of droughts on the ecological status of rivers, focusing on developing diagnostic tools to assess the impact of water scarcity. To address this, the study introduces the Eco-Hydrological Distance Index (EHDI), a metric that integrates hydrological balance, ecological indicators and anthropogenic pressures to evaluate how deviations from e-flow thresholds affect river ecosystems, especially during droughts. Using the Standardized Precipitation Index (SPI) to assess drought severity, the research analyses rainfall data from Tuscany (2001–2020) and compares SPI values with EHDI across different river basins in the Arno River system (Italy). The results reveal a strong correlation between SPI and EHDI, with droughts significantly impacting the ecological status of rivers.  The study identifies critical SPI thresholds, below which river basins risk reaching "bad" ecological status, defined by a substantial loss of biological communities. These thresholds vary across basins due to factors like hydrological conditions, water abstraction, and anthropogenic pressures. This research highlights the need for integrating hydrological and ecological metrics to improve water management strategies at the river basin district scales. By providing tools to predict the ecological impact of droughts, it aims to support sustainable management of water resources and protect ecosystem services essential for biodiversity and human wellbeing.

Acknowledgment: This 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)