Observations on the hydrology and geohydrology of the Okavango Delta

The Okavango Delta is situated within two major grabens, one containing the Panhandle and the other the Delta itself. The main graben is underlain by between 200 m and 300 m of largely unconsolidated medium to fine sand. The Delta is an alluvial fan, which is divided longitudinally into two by horsts which form Chief9s Island and the Duba Island cluster. Permanent swamp, sustained by base flow of the Okavango River, is developed in the Panhandle, upper fan, and portion of the fan to the east and northeast of Chief9s Island. Seasonal flood water enters the Panhandle via the Okavango River, but because of the permeable nature of channel margins and the elevated water surface in the channels, especially at the apex of the fan, it is rapidly leaked from the channels. Most of the seasonal flood water flows to the west of Chief9s Island where the bulk of the seasonal swamps are located. The extent of annual inundation is variable, and is strongly influenced by the magnitude of flood discharge and local rainfall, and to a lesser extent by antecedent conditions and evapotranspiration. Seasonal flooding has created a recharge mound beneath the Delta, but analysis of ground water chemistry and isotopic characteristics indicates that the Delta is essentially hydrologically closed, with no ground water outflow, and only limited surface water outflow. There is no large-scale lateral flow of ground water and water movements in the system are essentially vertical. Recharge occurs through the flood plains, and water is lost to the atmosphere by evaporation and especially by transpiration, particularly from islands. Soluble salts are accumulating in the deep ground water. This study has emphasized the need for more data on the aerial extent of seasonal flooding, on the ground water recharge, and on the contributions of evaporation and transpiration to water loss.