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

Abstract Study region Okavango Delta, Middle Kalahari, NW Botswana. Study focus We investigated the effect of evapotranspiration on the evolution of dissolved inorganic carbon (DIC) and stable carbon isotopes of DIC (δ 13 C DIC ) in the Okavango River. We measured the DIC concentrations and the δ 13 C DIC 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 insights We found a 379% and 500% increase in the DIC concentrations and a δ 13 C DIC 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 δ 13 C DIC of the evaporated river samples resulted from CO 2(g) loss during chemical equilibrium with atmospheric CO 2(g) followed by carbon exchange between DIC and atmospheric CO 2(g) . Although the δ 13 C DIC increased spatially for the Okavango River, it never reached the value of ∼0‰ expected for equilibration of river DIC with atmospheric CO 2(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.