The concept of defining environmental flow regimes to balance the provision of water resources for both human and environmental needs has gained wide recognition. As the authority responsible for water resource management within England, the Environment Agency (EA) uses the Environmental Flow Indicator (EFI), which represents an allowable percentage deviation from the natural flow to determine where water may be available for new abstractions. In a simplified form, the EFI has been used as the hydrological supporting component of Water Framework Directive classification, to flag where hydrological alteration may be contributing to failure to achieve good ecological status, and to guide further ecological investigation. As the primary information source for the EFI was expert opinion, the EA aims to improve the evidence base linking flow alteration and ecological response, and to use this evidence to develop improved environmental flow criteria and implementation tools. Such tools will be required to make predictions at locations with no or limited ecological monitoring data. Hence empirical statistical models are required that provide a means to describe observed variation in ecological sensitivity to flow change. Models must also strike a balance between generic and local relationships. Multilevel (mixed effects) regression models provide a rich set of capabilities suitable for this purpose. Three brief examples of the application of these techniques in defining empirical relationships between flow alteration and ecological response are provided. Establishment of testable hydrological–ecological relationships provides the framework for improving data collection, analysis, and ultimately water resources management models.
Conflict of interest: The authors have declared no conflicts of interest for this article.