CO 2 emissions from German drinking water reservoirs estimated from routine monitoring data

Abstract

Globally, reservoirs are a significant source of atmospheric CO 2. However, precise quantification of greenhouse gas emissions from drinking water reservoirs on the regional or national scale is still challenging. We calculated CO 2 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 CO 2 from drinking water reservoirs in Germany. All reservoirs were small net CO 2 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 CO 2 annually, which makes them a negligible CO 2 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 CO 2 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 CO 2 emission only occurred in reservoirs with pH < 7 and total alkalinity < 0.2 mEq l –1. Annual CO 2 emission correlated exponentially with pH, making pH a suitable proxy for CO emission from German drinking water reservoirs.