CO emissions from German drinking water reservoirs.

Globally, reservoirs are a significant source of atmospheric CO. However, precise quantification of greenhouse gas emissions from drinking water reservoirs on the regional or national scale is still challenging. We calculated CO fluxes for 39 German drinking water reservoirs during a period of 22years (1991-2013) using routine monitoring data in order to quantify total emission of CO from drinking water reservoirs in Germany and to identify major drivers. All reservoirs were a net CO source with a median flux of 167gCmy, which makes gaseous emissions a relevant process for the carbon budget of each reservoir. Fluxes varied seasonally with median fluxes of 13, 48, and 201gCmy in spring, summer, and autumn respectively. Differences between reservoirs appeared to be primarily caused by the concentration of CO in the surface water rather than by the physical gas transfer coefficient. Consideration of short term fluctuations of the gas transfer coefficient due to varying wind speed had only a minor effect on the annual budgets. High CO emissions only occurred in reservoirs with pH<7 and total alkalinity <0.2mEql. Annual CO emissions correlated exponentially with pH but not with dissolved organic carbon (DOC). There was significant correlation between land use in the catchment and CO emissions. In total, German drinking water reservoirs emit 44000t of CO annually, which makes them a negligible CO source (<0.005% of national CO emissions) in Germany.