EVALUATING MANAGED AQUIFER RECHARGE BASED ON WATER SUPPLY SECURITY, RISK, AND SUSTAINABILITY - PALLA ROAD AQUIFER, BOTSWANA

Botswana has a water stressed situation due to the climate and a continuously increasing water demand. Previous studies have shown that Managed Aquifer Recharge (MAR) can substantially increase the water supply security in the regional North-South Carrier water supply system in populous eastern Botswana. This led to more detailed investigations of the Palla Road aquifer, located 150 km northeast of the capital Gaborone, where three different scenarios for full-scale MAR were studied. To thoroughly evaluate the potential of MAR and if it is suitable for improving the water supply security, a comprehensive assessment of the overall sustainability is needed. Hence, the different MAR options were evaluated based on their technical, economic, social and environmental performance relative to a scenario without implementation of MAR. The evaluation was enabled by combined use of a numerical groundwater model, a water supply security model (WSSM) and a multi-criteria decision analysis (MCDA). The numerical model provided input data to the WSSM and the latter determined what scenarios to simulate in the groundwater model. The results from the two models in combination with additional social, environmental and economic studies were used to determine the sustainability scoring of alternatives in the MCDA. The WSSM is a probabilistic and dynamic water balance model where statistically generated time series of source water availability are used in combination with dynamic storages in dams and aquifers. The WSSM also include the infrastructure components of the system and their dynamic reliability, e.g. pipes, pumps and water treatment facilities. The WSSM compares possible supply from the system with the demand to simulate the magnitude and probability of water supply shortages. Using the WSSM the effects of MAR options on water supply security were simulated for a chosen time-period using one-month time steps. Each scenario was also evaluated with respect to expected social and environmental effects using MCDA to address the long-term sustainability of the scenarios. The results provide useful decision support and show that MAR can increase the water supply security. Based on the analyzed scenarios for the Palla Road aquifer, the most sustainable option is based on aquifer storage and recovery with a capacity of 40 000 m³/d.