[PULA Project] Extreme rainfall and floods in arid regions: replenishment or contamination of water resources?

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While recent research suggests that recharge of groundwater resources in arid regions, including Sub-Saharan Africa, only occurs during extreme rainfall events, these conditions are also associated with the mobilisation of contaminants into surface water and aquifers. Scarcity of clean surface water has already put tremendous pressures on water resources in arid developing countries. Projections of growing populations and more pronounced climatic extremes suggest that sustainable approaches to water resources are urgently needed, but there are key deficiencies in understanding of 1/the mechanisms and timing by which water stores (surface and ground) recharge and get contaminated as a result of heavy rainfall and 2/the associated trade-off between increasing water storage but also potentially of poorer water quality.The current exceptional ongoing rainy season in Botswana provides an urgent opportunity to study the effects of heavy rainfall on water resources in arid countries. The last heavy rainfall occurred nearly two decades ago and the exceptionally wet conditions have activated potential migratory routes for leachate from dumpsites to ground and surface water. Such point source pollution provides ideal opportunistic conditions for large-scale tracing of flow and contaminant pathways in arid environments.The proposed work aims to take advantage of this unique prospect to monitor the immediate effect of heavy rainfall and floods on water resources and their transitional hydrologic readjustment towards the dry period, and elucidate whether these events support either or both resources replenishment and contamination. The densely populated Gaborone reservoir catchment is of particular interest because water demand and pollution sources are high and preferential flow pathways in the fractured bedrock result in fast groundwater recharge and contamination. Erosion of shallow soils affects sediment and contaminant transport, as well as geomorphological conditions which in turn impact on recharge patterns.We will utilise the unique opportunity to evaluate the crucial link between water quantity and quality, often investigated separately, through an integrated approach that combines hydrologic monitoring, temporal geophysical imaging, geomorphological mapping and natural tracer experiments (leachate and stable water isotopes) with modelling. Experiments will span the ongoing wet season and its transition to the following dry season. Water monitoring and sampling will focus on existing ground and surface water points across the catchment as well as dams and potential pollution sources at different spatial and temporal scales. Integrating the datasets into the modelling framework will allow for characterising and quantifying the contribution of extreme rainfall events to water resources replenishment and contamination.The project team is composed of UK and African scientists with specialisations in hydrogeology, surface water hydrology, geophysics and sedimentology and water governance stakeholders. Research will benefit from a hydrometric monitoring in place and in-kind sample collection pre-award, which will be expended during the project.The overall research outcomes will be 1/a greater understanding of the key processes governing flood-induced water resources replenishment and contamination in arid developing regions and 2/an increased ability of water resources stakeholders to adapt management and policy to the foreseeable increase in hydrological extreme events.The new science generated will impact both the academic community interested in new methods and knowledge with regards to hydrological processes under changes, and the governance sector in arid (sub-Saharan) regions in need of adaptive water management strategies able to cope with both short and long term effects of climate change. Students and project partners will receive training and early career researchers will build their research portfolio.