The Long‐Term C Footprint Trajectory and Emissions Attribution of a Large Boreal Reservoir (Paix Des Braves (Eastmain‐1), Québec)

Abstract Hydropower is considered essential in meeting the increasing demand in low carbon energy in the context of climate change. Greenhouse gas emissions (GHG) by hydroelectric reservoirs have nevertheless become a major concern to the energy sector. The challenge lies in developing robust estimates and models that account for temporal and spatial heterogeneity of emissions, and validating projections, because comprehensive studies are scarce due to their complexity and cost. Here we present the long term (2005–2022) GHG trajectories and other aspects of the biogeochemical functioning of the Eastmain‐1 (or Paix des Braves) reservoir in boreal Québec, Canada. The study considers both seasonal and spatial heterogeneity of diffusive, ebullitive and downstream CO 2 and CH 4 emissions by combining discrete sampling with continuous, automated monitoring instrumentation. CH 4 fluxes represented only 1.2%–6.8% of the total gross emissions, and CO 2 diffusive emissions represented 80.1%–90.0% of the total emissions in CO 2 equivalents. Our results confirm the projected decreasing trend in dissolved CO 2 over the initial years after flooding, a pattern i.e. not observed for CH 4 concentrations. Total reservoir emissions after 17 years are on the order of 434 10 3 tCO 2 eq yr −1 (i.e., ∼2 gCO 2 eq m −2 .d −1 ) very close to those initially modeled right after impoundment. Based on a comparison with regional lakes, we estimate that only 45% of the current diffusive GHG emissions are attributable to the impoundment itself. This leads to current generation efficiencies of around 38 to 41 gCO 2 eq kW h 1 .