Spatial patterns of diffusive greenhouse gas emissions from cascade hydropower reservoirs

Greenhouse gas (GHG) emissions from reservoirs have received increasing attention in recent years. Despite extensive studies in single reservoirs, GHG emission patterns in cascades of multiple reservoirs, which are becoming increasingly common worldwide, remain unknown. This study investigated the spatial patterns of diffusive carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) emissions, as well as their total CO₂-equivalent (CO₂-eq), for a cascade hydropower system in the heavily dammed upper Mekong River, China. Results demonstrated that GHG emissions in cascade reservoirs were higher than that in the upstream channel since the accumulated sediments fueled microbes for GHG production. In cascade reservoirs, CO₂ made the largest contribution (58.6%–84.8%) to total CO₂-eq, while the contribution of N₂O was marginal. Deep reservoirs emitted less CO₂, which was attributed to higher CO₂ consumption by phytoplankton. Reservoirs formerly occupying the most upstream position for the longest period of time in the cascade emitted the most CH₄, perhaps due to accumulations of river borne sediments. The total CO₂-eq generally increased with distance downstream except within deep reservoirs. These findings indicate that, with respect to mitigating GHG emissions, the deepest, most upstream reservoir should be constructed first in the configuration of cascade hydropower reservoirs, and less sediment will enter downstream reservoirs, which have higher CO₂-eq emissions.