Evolution of Environmental Flows Assessment Science, Principles and Methodologies

Environmental flows assessment science focuses largely on defining how modified flow regimes (from water infrastructure and direct water abstraction) can be managed to conserve or restore societally desired ecological conditions in rivers and other wetland systems. The foundations of the science are based on hydroecological understanding and knowledge derived from empirical observation, indigenous (stakeholder) knowledge, and modeling. Flow-ecology relationships are the accepted basis of modern environmental water recommendations, and holistic (ecosystem) approaches have evolved as the main sphere of innovation over the last 40 years. Environmental water applications are increasing in sophistication from the desktop planning level through to advanced river basin and broader landscape water management. New regions are engaging with the environmental water enterprise and advancing the science, increasing rigor, implementation, and policy uptake. This challenging scientific and policy platform faces new challenges to integrate other partially or nonflow-related environmental factors that can limit the ecological outcomes of purely hydrologic restoration. Chief among these are sediment regime alteration, thermal regime alteration, and channel hydraulic features in re-regulated reaches. In addition, rapid human population growth, shifting ecological conditions ("baselines"), and climate change create new challenges for environmental water. The discipline must therefore transition from a historically focused, restore-to-reference perspective to one that aims for adaptation in the face of hydrologic and ecological change and increasing uncertainty. Less reliance on statistical flow-ecology relationships and more emphasis on process understanding of hydrologic controls on ecological dynamics will become increasingly important. Further, the local-scale focus of environmental water science will need to expand to a larger, basin-scale perspective that considers habitat connectivity and species movements that are critical components in attaining freshwater conservation. In summary, we provide seven guiding elements that will likely promote the successful advancement of environmental water science and implementation in the coming years.