Intermittent hydrologic perturbations control solute cycling and export in the Okavango Delta

We measured the concentrations of the total dissolved ions (TDI), dissolved silica, major cations and the δD and δ¹⁸O at sub-weekly intervals for one year in the Okavango River at the outlet of the Okavango Delta (Delta). Our objectives were to (1) document the temporal variations in the concentrations of solutes in the Okavango River, (2) determine the processes controlling the transfer of solutes to the river and (3) assess the temporal solute load and outflux from the Delta. We found that the TDI and major cation concentrations in the river were anomalously high during the rainy season and 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 on hundreds of thousands of islands scattered across the Delta wetlands, as well as from ‘flushing’ of remnant evaporated flood water of higher salinity trapped in isolated wetland pools. Overland flow generated by local rains and flooding connect the river to the solute stores in the watershed. The temporally activated hydrologic flow pathways transfer solutes to the river that flushes them out of the Delta. The solute load in the river was higher during the rainy season and during pulse flooding, and mimicked the discharge hydrograph. We estimate that 17,838 Mg/y of dissolved solutes was flushed out of the Delta, with 67% removed during pulse flooding (6 months) and 30% during the rainy season (4 months). The transfer of solutes from the watershed to the river during pulse flooding and the rainy season, and solute export from the Delta is an important mechanism that keeps the Delta's surface water resources fresh, which is critical for supporting a freshwater wetland ecosystem. Our results highlight the importance of intermittent activation of hydrologic flow pathways in controlling solute cycling in this and other arid watersheds.