Modelling CO2 emissions from water surface of a boreal hydroelectric reservoir

Abstract

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: 0.4-1.3gCm^-2day^-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 ~115gCm^-2yr^-1 with increasing reservoir age over the estimated "engineering" reservoir lifetime (i.e., 100years). 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.

Keywords: Air-water interface; Benthic respiration, modelling; Reservoir lifecycle, inundation; Water column respiration.