Time-series variability of solute transport and processes in rivers in semi-arid endorheic basins: The Okavango Delta, Botswana

We investigated hourly time-series of total dissolved solids (TDS) in the Okavango River at the inlet and outlet of the Okavango Delta (Delta) in semi-arid Botswana. We estimated the solute flux into and out of the Delta. Our objectives were to (1) document the temporal variations of solutes (represented by TDS) in the Okavango River and determine the processes that control the variations in solute concentrations and (2) quantify the solute load into and out of the Delta. The solute concentrations in the Okavango River varied from sub-weekly to monthly time frames, with a semi-annual increase controlled by annual pulse flooding and local rains. Variable interaction of floods and rains with solute stores in the floodplains, hundreds of thousands of islands and isolated saline wetland pools controlled temporal solute loading in the Okavango River. The concentration-discharge relationship for the Okavango River showed that the timing of solute delivery to the river which varies across the Delta is a function of the spatial variability of solute stores, availability of solutes in the temporal stores and the heterogeneous activation of the hydrologic flow pathways that connect the river to the solute stores. Of the 329,200 Mg/y of dissolved solids delivered to the Delta, 68,240 Mg/y (21%) is removed and 260,960 Mg/y is retained, making the Delta a solute sink. Annually, of the dissolved solutes removed from the Delta, 87% are removed during the annual flooding, 4% during the local rainy season and 9% during other times. Our findings indicate that hydrologic perturbations and temporal river connectivity to watershed solute stores play a significant role in modulating solute transport processes in rivers and solute cycling in arid watersheds. Our findings inform sampling regimes, water quality monitoring and modeling of solutes in rivers in arid watersheds.