Purpose <p>Freshwater reservoir drawdown areas are an important yet not well understood source of atmospheric CO₂. Quantifying CO₂ 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 CO₂ emissions and to improve reservoir CO₂ emission budgets.</p> Materials and methods <p>We measured CO₂ emissions from the drawdown area of a German reservoir for three months with hourly resolution using automatic flux chambers. To analyze drivers of CO₂ emissions we monitored sediment temperature and moisture as well as meteorological variables. For comparison we continuously measured CO₂ emissions from the water surface by Eddy Covariance.</p> Results and Discussion <p>CO₂ fluxes showed pronounced diurnal and seasonal variability driven by temperature. Sediment moisture had a negative effect on CO₂ fluxes – low fluxes were observed close to the waterline and after rain events. Inhibition of CO₂ transport by water blocking sediment pores was indicated by a hysteresis between sediment temperature and CO₂ flux, which was amplified under moist conditions. We show that daily CO₂ emissions are overestimated by more than 25% if fluxes are measured only during midday. A comparison with CO₂ emissions from the water surface measured by eddy covariance revealed that drawdown area and water surface contributed equally to the reservoirs CO₂ emissions, despite the much small surface area of the drawdown area.</p> Conclusions <p>Our study clearly shows that CO₂ 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 CO₂ emissions from the drawdown area are essential for precise quantification of greenhouse gas emissions from reservoirs.</p>