CO₂ emissions from German drinking water reservoirs estimated from routine monitoring data

Abstract. 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.
All reservoirs were small net CO₂ sources with a median flux of 167 g C m^–2 y^–1, which makes gaseous emissions a relevant process for the reservoirs carbon budgets. In total, German drinking reservoirs emit 44000 t of CO₂ annually, which makes them a negligible CO₂ source in Germany. Fluxes varied seasonally with median fluxes of 30, 11, and 46 mmol m^–2 d^–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 variable wind had only a minor effect on the annual budgets. High CO₂ emission only occurred in reservoirs with pH < 7 and total alkalinity < 0.2 mEq l^–1. Annual CO₂ emission correlated exponentially with pH, making pH a suitable proxy for CO emission from German drinking water reservoirs.