Application of a Presumptive Standards Approach to Evaluate Environmental Flows in a Highly Regulated River Basin

ABSTRACT The presumptive standards approach to environmental flows offers a method to develop interim guidelines for ecological and social‐cultural flow needs. The approach is based on deriving acceptable percent‐of‐flow limits based on naturalised flows (the absence of depletion or alteration of flow), and it can be an effective precursor to a full assessment of environmental flows, particularly when used to highlight seasonal differences. In this study, we analysed naturalised flows modelled for the period 1979–2010 using the MESH community‐based modelling approach for 31 gauges in the Saskatchewan River basin in western Canada differentially affected by hydropower dams or irrigation withdrawals. We assessed deviations of measured runoff from naturalised runoff and estimated presumptive standards for social‐ecological protection. Downstream of irrigation withdrawals, measured runoff was lower than naturalised during the open water season, reflecting water use for agriculture. Downstream of hydropower dams, runoff hydrographs were flatter, with lower runoff than naturalised during open water and higher runoff than naturalised during ice‐on; deviations during ice‐on were most predominant. Presumptive standards were established to allow for a 20% or 30% deviation from median naturalised runoff, representing high and moderate social‐ecological protection, respectively. Half the gauges were within or bordered on the 20% or 30% sustainable flow boundary during the open water season, but only five gauges fell within those boundaries during the ice‐on season. The deviations from naturalised runoff have implications for habitat quality, environmental cues, and social‐cultural flow needs. As climate change continues to impact water availability in these systems, the impacts of flow alteration on ecological and social‐cultural flow needs may be magnified.