The mean inflow, both surface water and rain, into the Okavango Delta (Botswana) is about 16 billion m? per year. Approximately 96% of this returns to the atmosphere by evapotranspiration, only 2% leaving the delta surface by outflow through the Thamalakane and Boteti Rivers. The remaining 2% is thought to infiltrate into the ground water below the delta. The solute content of the surface outflow, however, is lower than expected from water mass balance. The delta therefore acts as a solute trap under present climatic conditions.