Temporal dynamics of CO2 emissions from dry reservoir sediments

Abstract Purpose Freshwater reservoir drawdown areas are an important yet not well understood source of atmospheric CO 2 . Quantifying CO 2 emissions from drawdown areas is challenged by considerable spatial and temporal variability. In this study we aimed to assess temporal variability to understand the drivers of drawdown area CO 2 emissions and to improve reservoir CO 2 emission budgets. Materials and methods We measured CO 2 emissions from the drawdown area of a German reservoir for three months with hourly resolution using automatic flux chambers. To analyze drivers of CO 2 emissions we monitored sediment temperature and moisture as well as meteorological variables. For comparison we continuously measured CO 2 emissions from the water surface by Eddy Covariance. Results and Discussion CO 2 fluxes showed pronounced diurnal and seasonal variability driven by temperature. Sediment moisture had a negative effect on CO 2 fluxes – low fluxes were observed close to the waterline and after rain events. Inhibition of CO 2 transport by water blocking sediment pores was indicated by a hysteresis between sediment temperature and CO 2 flux, which was amplified under moist conditions. We show that daily CO 2 emissions are overestimated by more than 25% if fluxes are measured only during midday. A comparison with CO 2 emissions from the water surface measured by eddy covariance revealed that drawdown area and water surface contributed equally to the reservoirs CO 2 emissions, despite the much small surface area of the drawdown area. Conclusions Our study clearly shows that CO 2 fluxes between reservoir drawdown area and atmosphere have a large diurnal variability in the same range as seasonal variability and within system variability. High temporal resolution measurements of CO 2 emissions from the drawdown area are essential for precise quantification of greenhouse gas emissions from reservoirs.