COS 8-6
Meaningless mean discharge in environmental flow management
River ecosystems are highly dependent on and responsive to hydrologic variability over multiple time scales (e.g., hours, months, years). Fluctuating river levels present a key challenge to river managers, who must weigh competing demands for freshwater. Environmental flow recommendations and regulations seek to provide management targets balancing socio-economic outcomes with maintaining ecological integrity. Often, flow management targets are based on average river conditions over temporal windows such as days, months, or years. The objective of this study is to highlight the potential pitfalls of mean-based environmental flow regulations. Three case studies are presented at annual (Middle Oconee River, Athens, Georgia, USA), monthly (Rio Sabana, Sabana, Puerto Rico, USA), and daily (Oconee River, Watkinsville, Penfield, Georgia, USA) intervals to demonstrate the consequences of mean discharge as a single-minded target for flow management. Each case study presents the ecological setting, source of hydrologic alteration, hydrologic analyses at multiple time scales, and pitfalls of management decisions based on mean discharge at inappropriate time intervals.
Results/Conclusions
Each case study shows that all regulations are being met by water operators; however, the intent of the environmental flow target is not met in light of hydrologic variability. Hydrologic variability within the regulatory time scale (i.e., hourly fluctuation within a daily mean) leads to over-allocation of the water resource in all three case studies. I close by recommending three steps to avoid the pitfalls associated with mean-based environmental flow regulations. First, regulations should be developed in a nested temporal hierarchy (e.g., an instantaneous flow target coupled with a daily average). Second, conditional permitting should be pursued to avoid over-allocation of water during drought. Third, non-stationary hydrologic conditions from climate and land use change introduce key uncertainties which could be addressed through an iterative regulatory process (e.g., mandatory permit renewal on 10 year time scales). Mean conditions have been and will continue to be important tools for management recommendations, but meaningless means create a potential stumbling block to meeting the spirit of environmental flow management.