Fig. 1.

Pathways of CO₂ emissions to the atmosphere from a river–reservoir system including the downstream emission hotspot and their quantification for the Zambezi River–Kariba system. (A) Reservoir-related CO₂ emissions differentiated between emissions across the surface (F_CO2 surface) of the standing water body and emissions that occur downstream of the dam resulting from degassing at the turbines (F_CO2 turbines) or through evasion of the remaining excess gas in the downstream river (F_CO2 downstream). The magnitude of the latter depends on the stratification of the reservoir and hydropower operation (positive fluxes are from the waterbody to the atmosphere). The release of hypolimnetic CO₂-oversaturated water together with hydropeaking (D and E; photos taken 3 km downstream of Kariba Dam) generates carbopeaking, subdaily fluctuations of the CO₂ flux through the river’s surface. (B) During low flow, the lower water–air gas exchange velocity and the smaller water–air interface reduce the outgassing. Vice versa, (C) during high flow the higher turbulence generating higher water–air gas transfer velocity and the larger water–air interface enhances CO₂ outgassing. Multiintake reservoirs can further enhance carbopeaking by causing fluctuations of CO₂ concentration in the outflow.