System-Wide Greenhouse Gas Emissions From Mountain Reservoirs Draining Permafrost Catchments on the Qinghai-Tibet Plateau

Reservoirs influence the global climate by exchanging greenhouse gases (GHGs) of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) with the atmosphere. Few studies, however, quantify emissions of all three GHGs from reservoirs, particularly in permafrost-affected mountain regions where ecosystems are highly vulnerable to climate change. This study presents three-year direct measurements of CO₂, CH₄, and N₂O concentrations and fluxes upstream, within, and downstream from two reservoirs draining permafrost catchments on the Qinghai-Tibet Plateau, including periods of reservoir drawdown. Comparing GHG fluxes across space and time exhibits a general pattern of lower fluxes at the two reservoirs relative to up- and downstream channels. Ebullitive fluxes contributed to 36.7% and 9.4% of total CH₄ and N₂O fluxes, respectively. CO₂ has no response to drawdown, but CH₄ and N₂O display synchronous drawdown-associated increase within the reservoir, constituting 57.5% and 32.8% of the annual reservoir emissions in just 2 months, respectively. Riverine emissions from up- and downstream channels accounted for an outsized fraction (55.5% for CH₄, 17.3% for CO₂ and 16.5% for N₂O) of the system-wide GHG budget. Compared with global reservoirs, the two reservoirs have high CO₂ and N₂O but low CH₄ fluxes in CO₂ equivalents. Upscaling shows that the two reservoirs emit the same magnitude of carbon as thermokarst lakes, and four times higher N₂O than Finnish lakes on an areal basis. This article shows that alpine reservoirs draining permafrost catchments are unrecognized atmospheric sources in current reservoir GHG inventories, but also emphasizes the importance of system-wide emissions when assessing total GHG evasion from reservoir systems.