Modelling outflow from the Jao/Boro river system in the Okavango delta, Botswana

The mean annual input, including both surface flow and rainfall, into the Okavango Delta(Botswana) is about 16 billion m(3). Between 96 and 98% of this returns to the atmosphere by evapotranspiration, only 2% leaving the delta by surface outflow through the Thamalakane and Boteti Rivers. It remains unclear how much is infiltrating into the groundwater below the delta. A severe problem is the extreme variability of the outflow. which is of great importance for the water supply of Maun and downstream areas along the Boteti where groundwater is scarce and usually saline. Previous modelling of the surface outflow, using over twenty years of rainfall, evapotranspiration and stream flow data, has revealed fluctuations in outflow which could not be explained with one single parameter set. Attempts were made to interpret this behaviour in terms of flow system changes occurring within the delta, or simply as an effect caused by the poor quality of rainfall measurements. This paper shows that substantial model improvement is possible when both long and short term antecedent climatic conditions are taken into account, The time span of the long term conditions, which is in the order of 10 years, seems to be expressed by shallow groundwater level variations in the lower delta, while the short term conditions, with a memory of one year or less, are linked to a combination of peak rain and flood events. The modelling therefore suggests that the outflow variations observed during the period of record are neither caused by physical changes in the delta nor result as an artefact of errors in the rainfall data, They are indicated to be entirely the result of climatic variations. (C) 1997 Elsevier Science B.V.