Spatial and temporal controls on the solute behavior of rivers in arid watersheds: The Okavango River, NW Botswana

Abstract

Solute cycling models for rivers in arid watersheds must identify solute sources and their spatial and temporal connectivity to rivers to assess the processes that control the solute behavior in these rivers. In this study, we collected hourly time series of total dissolved ions (TDI) concentrations, water levels, water temperatures and air temperatures at select stations distributed longitudinally in a river in an arid watershed. Our objectives were to: (1) document the spatial and temporal variations in solute concentrations and (2) determine the processes controlling the spatial and temporal variations in solute concentrations. We conducted this investigation over a two-year period at four stations along the Okavango River flowing through the Okavango Delta (Delta) in the middle Kalahari Desert, Botswana. At the spatial scale, we observed progressive downriver enrichment in TDI concentrations associated with evapotranspiration of river water because of the 5 months river transit across the hot Delta. At a temporal scale, the TDI concentrations increased seasonally from evapoconcentration and sub-seasonally from solute transfer from the watershed to the river. We analyzed sub-seasonal TDI concentration perturbations using the slope (β) of the relationship between TDI concentrations and normalized water level (C-NWL) relationships. We find that at low discharge during flow recession and at the beginning of rising discharge from flooding, the solute enrichment anomalies with β > 0 are related to hydrologic connectivity between the river and solute stores in the river floodplains, hundreds of thousands of salt islands and isolated evapoconcentrated wetland pools. Our findings indicate that hydrologically-driven river connectivity to solute stores in the local watershed and evapotranspiration jointly control the solute behavior at variable spatial and temporal scales in rivers in arid watersheds. We anticipate that our findings will inform solute transport and solute cycling models for rivers in arid watersheds.