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

Abstract

This study investigated hourly time-series of total dissolved solids (TDS), discharge, air temperature and water temperature in the Okavango endorheic basin at the inlet and outlet of the Okavango Delta (Delta) in semi-arid Botswana. We hypothesized that solute delivery from watersheds to rivers in arid environments is controlled by the temporal activation of surface and subsurface flow pathways by rain and flooding, and that hydrological perturbations are more important than evapotranspiration in solute cycling. The 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. 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 rainwater with solute stores in the floodplains, islands and isolated saline wetland pools controlled temporal solute loading in the Okavango River. The concentration-discharge relationships show that solute delivery to the river varies across the Delta and is a function of the spatial variability of solute stores, solute availability in the stores and the heterogeneous activation of the hydrologic flow pathways connecting the river to the solute stores. Of the 266,160 Mg/y of dissolved solids delivered to the Delta, 46,980 Mg/y (18%) is removed via the main Maun-Boro river channel and outlet, and 220,000 Mg/y is retained. Annually, 87% of the dissolved solutes removal through the Maun occurred during the flood pulse, 4% during the local rainy season and 9% at other times. Our findings indicate that hydrologic perturbations and temporal river connectivity to watershed solute stores modulate solute transport in rivers in arid environments.