Tradeoff between economic and environmental costs and benefits of hydropower production at run-of-river-diversion schemes under different environmental flows scenarios

With the boom of hydropower industry, it has never been more important than now to re-assess approaches to a sustainable hydropower projects management. We evaluated the environmental and economic costs and benefits (namely environmental and economic effects) of implementing different hydrology-based environmental flow restrictions (i.e., environmental flows scenarios) at nine run-of-river power plants involving flow diversions, located at pluvial, nival and stable river reaches in Spain. For each scenario, economic effects were inferred from energy production figures, whereas environmental effects along the bypassed river reach came from the deviations of the ecologically meaningful flow attributes from the unaltered flow regime. We found a linear inverse relationship between both effects, which were the lowest for the pluvial type reaches. Imposing significantly low constant flows resulted into the highest energy production but at the highest environmental costs, whereas the opposite happened when using more complex environmental flow methodologies which also imposed flow fluctuations and annual floods, and restricted ramping rates. The ecologically meaningful flow attributes were not similarly affected in all tested environmental flows scenarios. We present a simple approach to define the environmental flow standards at run-of-river-diversion schemes that guarantee the right balance between rivers conservation and energy production, and which are valid for all rivers sharing similar flow regime patterns. In such definition it is possible to address specific ecological goals by prioritizing the naturalness of certain key flow attributes.