INTERMITTENT HYDROLOGIC PERTURBATIONS CONTROL SOLUTE CYCLING AND EXPORT IN THE DISTAL OKAVANGO DELTA IN BOTSWANA

We investigated the concentrations of solutes in the Okavango River in the distal portion of the Okavango Delta in semi-arid Botswana. Our objectives were to (1) document the temporal variations in solute concentrations, (2) determine the processes controlling solute transfer to the river and (3) estimate the solute out-flux from the Delta. We measured the concentrations of the total dissolved solids (TDS), major cations and silica and the δ²H and δ¹⁸O at sub-weekly intervals for one year. The TDS and major cation concentrations were anomalously high during the rainy season and just before the arrival of the annual flood pulse. The anomalous increases in the solute concentrations are due to dissolution and mobilization of precipitated salts stored in the floodplains and hundreds of thousands of islands scattered across the Delta, as well as flushing of remnant flood water that occurs as isolated evaporated wetland pools of higher salinity. The flood pulse and rains connect the river to solute stores in the watershed and transfer solutes to the river that flushes them out of the Okavango Delta. We estimate that of the 12,037 Mg/y of dissolved solutes flushed out of the Delta, 67% was removed by pulse flooding (6 months) and 30% by the rains (3 months). Removal of solutes by pulse flooding and rains is an important mechanism that keeps the Delta’s water resources fresh. We conclude that hydrology, although intermittent, is important in controlling solute cycling in rivers in arid environments.