SOLUTE TRANSPORT IN A SEMI-ARID ENDORHEIC BASIN, OKAVANGO DELTA, BOTSWANA: A TIME SERIES INVESTIGATION

Solute flux, storage, and transport prediction are critical for the preservation and management of freshwater resources in arid and semi-arid regions (drylands). Drylands support over two billion of the world’s populations and characterize the depositional climate of large sections of the earth’s rock record, yet, there remains limited understanding of the primary mechanisms governing solute flux, and transport in drainage systems within such regions. The characteristic low water fluxes in arid regions, as well as the sparsity of hydrological records, present a challenge for field monitoring and experiments. However, advances in automated and high-frequency sampling technologies provide us with an opportunity to investigate solute chemistry and transport at microscales in semi-arid regions. In our study, we employ high-frequency data loggers to evaluate the relationships between solute flux and transport, with hydrology and geomorphology in the Okavango Delta, a freshwater wetland set in a semi-arid endorheic basin. We hypothesize that changes in hydrology caused by the arrival of the flood pulse and local rains drive temporal variations in solute concentration. While geomorphology, particularly the presence of tree islands, and climate are spatial drivers of solute concentration variations. The latter (arid climate) has been shown to induce significant water losses via evapotranspiration, a process which in turn affects the hydrochemical composition of the delta. Our results show that the Delta is a massive solute sink, which receives an annual solute load of 266000 tons and expels an annual solute load of 46000 tons through its main outlet. Solute behavior is characterized by variable solute concentrations in the rising versus falling limb of the hydrograph, to produce hysteretic solute-discharge patterns. We observe anomalous shifts in solute concentration with changing hydrological conditions from the flood pulse and local rains. These changes in hydrological conditions cause flushing and dilution patterns at different times of the rising limb at the inlet. This study provides a greater understanding of the time-scales of solute flux and transport in arid environments, as well as the primary mechanisms driving solute variation. This knowledge is transferable to close disciplines such as ecology, environmental science, arid studies, and transboundary water basin management.