Dynamics of floodplain-island groundwater flow in the Okavango Delta, Botswana

Abstract

Surface water-groundwater interactions play a crucial role in the hydrology and ecology of the Okavango Delta. The hydrology of the Delta is dominated by the annual arrival of a flood wave which is distributed over an number of branches. Subsequently, the flood water feeds the phreatic aquifers underlying the Delta islands. In order to evaluate the seasonal and long-term dynamics of the surface water-groundwater interactions between the floodplains and the islands, a network of piezometers located in various locations of the Delta was monitored. Groundwater table fluctuations observed for up to 6 years were analysed and modelled using groundwater flow models. The floodplain-island groundwater flow is in general very dynamic and driven by island evaporation and transpiration. A typical small to medium sized island (width <500 m), appear not to be influenced by long-term antecedent conditions. Only on large islands (width >500 m) and at the perimeter of the flooded area is the influence of long-term antecedent conditions apparent. The knowledge gained during this study will be used for the improvement of the hydrological and hydro-ecological model of the Delta, and can be useful for the description of floodplain dynamics in semi-arid regions in general.