Globally, reservoirs are a significant source of atmospheric CO₂. However, precise quantification of greenhouse gas emissions from drinking water reservoirs on the regional or national scale is still challenging. We calculated CO₂ fluxes for 39 German drinking water reservoirs during a period of 22 years (1991–2013) using routine monitoring data in order to quantify total emission of CO₂ from drinking water reservoirs in Germany and to identify major drivers.

All reservoirs were a net CO₂ source with a median flux of 167 g C m^− 2 y^− 1, which makes gaseous emissions a relevant process for the carbon budget of each reservoir. Fluxes varied seasonally with median fluxes of 13, 48, and 201 g C m^− 2 y^− 1 in spring, summer, and autumn respectively. Differences between reservoirs appeared to be primarily caused by the concentration of CO₂ 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 CO₂ emissions only occurred in reservoirs with pH < 7 and total alkalinity < 0.2 mEq l^− 1. Annual CO₂ emissions correlated exponentially with pH but not with dissolved organic carbon (DOC). There was significant correlation between land use in the catchment and CO₂ emissions. In total, German drinking water reservoirs emit 44000 t of CO₂ annually, which makes them a negligible CO₂ source (< 0.005% of national CO₂ emissions) in Germany.