Effect of evapotranspiration on dissolved inorganic carbon and stable carbon isotopic evolution in rivers in semi-arid climates: The Okavango Delta in North West Botswana

•We measured 379%–500% increase in DIC concentrations spatially across the Okavango Delta.•Evaporated river water lends support to DIC concentration increases from evapotranspiration.•The downriver increase in the δ13C of DIC was not concomitant with DIC concentration increases.•The increases in the δ13C of DIC was controlled by carbon equilibration with atmospheric CO2(g).•Evapoconcentration is not instrumental in transferring river CO2(g) to the atmosphere.

Study regionOkavango Delta, Middle Kalahari, NW Botswana.Study focusWe investigated the effect of evapotranspiration on the evolution of dissolved inorganic carbon (DIC) and stable carbon isotopes of DIC (δ13CDIC) in the Okavango River. We measured the DIC concentrations and the δ13CDIC for samples collected over a 400 km reach of the river in the Okavango Delta during flood conditions and non-flood conditions. In addition, we incubated river samples collected from the proximal portion (Mohembo) and the distal portion (Maun) of the Delta and subsequently evaporated the samples by ∼90% under ambient conditions.New hydrological insightsWe found a 379% and 500% increase in the DIC concentrations and a δ13CDIC increase of 3.9‰ and 6.1‰ in the river during the flood non-flood conditions, respectively. The DIC concentrations of evaporated river samples increased by 535% for the Mohembo and by 850% for the Maun samples. The increase in the δ13CDIC of the evaporated river samples resulted from CO2(g) loss during chemical equilibrium with atmospheric CO2(g) followed by carbon exchange between DIC and atmospheric CO2(g). Although the δ13CDIC increased spatially for the Okavango River, it never reached the value of ∼0‰ expected for equilibration of river DIC with atmospheric CO2(g). The results of the evaporated river samples suggest that isotopic enrichment from equilibration in Okavango River was balanced by respiration and photo-oxidation of carbon-depleted dissolved organic matter.