SPATIAL AND TEMPORAL CONTROLS ON SOLUTE BEHAVIOR IN RIVERS IN ARID WATERSHEDS: THE OKAVANGO RIVER IN THE OKAVANGO DELTA, NW BOTSWANA

Solute transport and solute cycling models for rivers in arid watersheds require a good understanding of sources of solutes, solute transfer to rivers and the processing of solutes in rivers. Because solute stores vary in space across arid watersheds and the hydrology is highly variable, we investigated the hourly time series of total dissolved ions (TDI) for 2 years at four stations along the Okavango River flowing through the Okavango Delta (Delta) in semi-arid Botswana. Our objectives were to: (1) document the spatial and temporal variations of solutes and (2) determine the processes controlling variations in solute concentrations in the Okavango River. We observed downriver enrichment in solute concentrations associated with progressive evaporation of river water as it transits across the Delta. At a temporal scale, the solute concentrations at each station showed sub-seasonal to seasonal variations that were chiefly controlled by annual pulse flooding and seasonal rains. The annual pulse flooding and seasonal rains caused variable spatial and temporal hydrologic connectivity between the Okavango River and solute stores in the river floodplains, hundreds of thousands of salt islands and isolated evaporated wetland pools in the Delta. The concentration-normalized water level (C-NWL) relationship for the Okavango River showed that the behavior of solutes in the river which varies across the Delta is a function of the availability of solutes in their temporal stores, variable hydrologic connectivity between the river and solute stores in the local watershed and contribution of solutes from the upper watershed in Angola. Our findings indicate that the temporal hydrology, river connectivity to solute stores in the watershed and evapotranspiration jointly control the solute behavior in the Okavango River at variable spatial and temporal scales. Our findings will inform solute transport and solute cycling models for rivers in arid watersheds.