THE EFFECTS OF EVAPORATION on THE CHEMISTRY OF THE OKAVANGO RIVER, NORTHWESTERN BOTSWANA

The Okavango River delta, located in northwestern Botswana, is the world’s largest continental alluvial fan, covering more than 22,000 km². The Okavango River delta is also located in a semi-arid region where >96% of river water influx into the delta is lost by evapotranspiration, as the river flows to the distal end of the delta. The effects of a long transit time for river water and evapotranspiration on water chemistry is unknown, although increases in solute mass in the river water from the proximal to the distal end of the delta has been observed. It is not also clear if, in addition to evapotranspiration, interactions between river water, vegetation and suspended and bed load sediments are also important in the river’s solute mass balance.

To isolate the effects of river water-vegetation-sediment interaction on riverine solute evolution, we investigated the effects of evaporation on water from the Okavango River delta. The experimental objective was to assess changes in water chemistry due to direct evaporation. These results can serve as a basis for evaluating the contribution of vegetative evapo-concentration and river water-vegetation-sediment interaction to surface and groundwater chemical evolution. Water was collected from the proximal (Mohembo) and distal (Maun) portions of the delta and evaporated in open containers outdoors under ambient conditions. Temporal measurements of pH, oxidation reduction potential (ORP), total dissolved solids (TDS), alkalinity, silica, dissolved inorganic carbon (DIC) and major ions were made. The values of all measured parameters increased with continued evaporation. Although the initial concentrations of the parameters for the two samples were different, all trends in parameter enrichment were similar, and linearly related to the extent of evaporation. Comparisons of the solute concentrations in the evaporative experiment with spatial river water samples from the Okavango River and shallow groundwater show similar relations. Thus, we infer that vegetative evapo-concentration is a primary mechanism in the evolution of solutes in surface and shallow groundwater in the Okavango River delta.