Modelling CO2 emissions from water surface of a boreal hydroelectric reservoir

Highlights: • Hydroelectric reservoirs shaped by flooding terrestrial organic carbon emit CO₂. • A daily time-step reservoir biogeochemistry model was developed. • The 1-D model predicted CO₂ fluxes well compared to eddy covariance measurements. • The annual effluxes steeply declined in the first three years after flooding. • Physical and biogeochemical processes co-determine the CO₂ flux pattern. To quantify CO₂ emissions from water surface of a reservoir that was shaped by flooding the boreal landscape, we developed a daily time-step reservoir biogeochemistry model. We calibrated the model using the measured concentrations of dissolved organic and inorganic carbon (C) in a young boreal hydroelectric reservoir, Eastmain-1 (EM-1), in northern Quebec, Canada. We validated the model against observed CO₂ fluxes from an eddy covariance tower in the middle of EM-1. The model predicted the variability of CO₂ emissions reasonably well compared to the observations (root mean square error: 0.4–1.3 g C m^{− 2} day⁻¹, revised Willmott index: 0.16–0.55). In particular, we demonstrated that the annual reservoir surface effluxes were initially high, steeply declined in the first three years, and then steadily decreased to ~ 115 g C m^{− 2} yr⁻¹ with increasing reservoir age over the estimated "engineering" reservoir lifetime (i.e., 100 years). Sensitivity analyses revealed that increasing air temperature stimulated CO₂ emissions by enhancing CO₂ production in the water column and sediment, and extending the duration of open water period over which emissions occur. Increasing the amount of terrestrial organic C flooded can enhance benthic CO₂ fluxes and CO₂ emissions from the reservoir water surface, but the effects were not significant over the simulation period. The model is useful for the understanding of the mechanism of C dynamics in reservoirs and could be used to assist the hydro-power industry and others interested in the role of boreal hydroelectric reservoirs as sources of greenhouse gas emissions.