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

A significant increase in dissolved inorganic carbon (DIC) and stable carbon isotopic composition (δ13C DIC) was observed in the Okavango River, with DIC concentrations rising by 379% during flood conditions and 500% during non-flood periods. Evaporation experiments on river samples demonstrated even more pronounced increases, with DIC concentrations surging by 535% and 850% for proximal (Mohembo) and distal (Maun) portions of the delta, respectively. The δ13C DIC increase was attributed to CO2 loss during chemical equilibrium with atmospheric CO2, followed by carbon exchange between DIC and atmospheric CO2. However, the river's δ13C DIC never reached the expected value for full equilibration (~0‰), suggesting that isotopic enrichment is balanced by respiration and photo-oxidation of carbon-depleted dissolved organic matter. This study highlights the complex interplay between evapotranspiration, DIC dynamics, and stable carbon isotopes in semi-arid river systems.

Atekwana, E. A., Molwalefhe, L., Kgaodi, O., & Cruse, A. M. (2016). 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. Journal of Hydrology: Regional Studies, 7, 1–13. https://doi.org/10.1016/j.ejrh.2016.05.003