THE SPATIAL AND TEMPORAL VARIATIONS IN THE STABLE ISOTOPES OF HYDROGEN AND OXYGEN AND SOLUTES IN THE OKAVANGO RIVER IN THE OKAVANGO DELTA, NW BOTSWANA: IMPLICATIONS FOR SOLUTE CYCLING IN RIVERS IN ARID WATERSHEDS

We investigated the spatial and temporal effects of evapoconcentration on solutes by assessing the behavior of stable water isotopes and solutes. Investigations of solute behavior from solute concentrations and stable water isotopes will allow for assessing variations in solute concentrations caused by evaporation and other processes such as watershed solute influx and instream processes. We measured the stable isotopes of hydrogen (δD) and oxygen (δ¹⁸O) and total dissolved ions (TDI) concentrations every one to two months for one year at nine stations along the Okavango River flowing through the Okavango Delta (Delta) in semi-arid Botswana. We aim to use the spatial and temporal variations in solute concentrations and the δD and δ¹⁸O composition to elucidate the role of evaporation in controlling river solute behavior. We observed progressive downriver enrichment in the δD, δ¹⁸O and TDI concentrations across the Delta. The δD and δ¹⁸O of river samples plotted along the Okavango Delta Evaporation Line (ODEL) indicating evaporation of river water. On a temporal basis, the δD and δ¹⁸O decreased due to pulse flooding in the dry season. The TDI concentrations increased pre-flooding especially in the lower Delta. Near peak flooding, the TDI concentrations decreased, and stayed low during the flood recession. Least squares regression models shows that the mean δD and the d-excess were well correlated with distance providing evidence of evaporation across the Delta. A least square regression model of the mean TDI concentrations vs. d-excess was negative and well correlated, indicating that evaporation is the dominant process causing spatial solute enrichment in the river. We observe that the TDI concentrations vs. d-excess for Okavango River samples from published studies lie below the regression model of the mean TDI concentration vs. d-excess. We also found that some of the data fall along the TDI concentration vs. d-excess regression model constructed for evaporated water from Mohembo at the inlet to the Delta, while the rest were scattered around an evaporation model for water from Maun at the outlet of the Delta. We posit from the results of previous studies that the temporal effects of river hydrology drives the episodic solute enrichment and dilution behaviors in the Okavango River. Our findings provide insights useful for solute cycling models for rivers in arid watersheds.