Flow regime aspects in determining environmental flows and maximising energy production at run-of-river hydropower plants

This study investigates the influence of the river flow regime type on the e-flows releases and hydropower production, constrained by eight hydrologically-based e-flows methods. For this purpose, 20 run-of-river hydropower plants up to 10 MW, from five Iberian Peninsula basins, located in regions with pluvial highly fluctuating, pluvial stable, pluvial winter, and pluvio-nival flow regimes were analysed. We integrated a hydropower model with a hydrological model, and eight e-flows methods to estimate mean daily hydropower production, e-flows, and hydrologic alteration. The results demonstrate little influence on hydropower production and e-flows releases for the pluvial regime type, notably, pluvial stable regime river reaches. Pluvio-nival regime provides unstable hydropower production and comparatively high e-flows alteration. Overall, hydrologic parameters represented by five global indices derived from Indicators of Hydrologic Alteration were affected differently for the e-flows releases regime induced by tested e-flows methods. In general, e-flows methods that involve annual minimum flow and indices of flow duration curve show inconsistent results among all study cases and hydrological regimes types; either they result in high e-flows releases while sharply reducing hydropower production or vice versa. However, so-called dynamic approaches demonstrate consistent results and are more suitable, both in terms of hydropower production and e-flows releases by therefore providing 10-35% more energy production while having little impact in several hydrological parameters. The findings of this study may serve as a starting point to initiate a new discussion on the methods and criteria that should be established regarding e-flows determination at run-of-river hydropower plants.