Modelling the sustainable yield of groundwater rersources in river-bed aquifers in arid and semi-arid areas

The aim of the project is to assess the sustainable groundwater yield of shallow alluvial aquifers in northeastern Botswana, where such aquifers may provide an important additional water resource for rural communities. Field studies were undertaken to establish the aquifer geometry, hydraulic and storage properties of the river-bed sediments, and the aquifer response to recharge from flood flows during the rainy season and subsequent depletion during the dry season. The aquifer depends on recharge from flood flows mainly through the arrival of the first flood event, and interannual variability of the duration of the rainy season is therefore of importance for determination of the sustainable groundwater yield. Abstractable yields depend, in parts on groundwater movements within the weathered basement. A groundwater modelling study has been undertaken to assess the sustainable groundwater yield in relation to variations in the duration of the dry season. This has also highlighted the need for a better understanding of the relationship between the sand river aquifer and the underlying weathered basement. A daily rainfall-runoff model has been developed to simulate the temporal variability of recharge from flood flows, where recharge to the river-bed aquifer has been included as a transmission loss component within the rainfall-runoff model. This has allowed the variability of the dry season duration to be investigated by simulation of surface flows over a 70-year period. Improved spatial rainfall representation and inclusion of the transmission loss component within the rainfall-runoff model have also resulted in improved simulation of surface flows in the sand river catchments of Botswana.
Ph.D. study by Niels Jørgen Andersen carried out at Imperial College, UK, and funded by Council for Development Research, Denmark.