Modelling of groundwater recharge in the megacity of Lagos, Nigeria: preliminary results using WetSpass-M

Lagos, Nigeria is one of the World's fastestgrowing cities. The increasing water demand has led to depletion and deterioration of the groundwater resources contained in its highly productive coastal aquifers. Groundwater recharge processes and drivers, which dictate groundwater replenishment, and their changes associated with rapid urbanization, are yet to be quantified and understood. We used the spatially distributed WetSpass-M model to estimate the spatiotemporal patterns and rates of recharge between 2000 and 2020 in the 5440 km(2) coastal aquifer system underlying the megacity of Lagos. The influence of expanding urban areas, from 807 km(2) in 2000 to 1183 km(2) in 2020, coincide with reduction in the simulated recharge by 50% in 20 years. The study also revealed diffuse and focused recharge processes associated with, respectively, the urban centres and areas closer to water bodies signalling strong groundwater-surface water interactions. The preliminary findings from this modelling work provide directions for exploring sustainable water management options including Managed Aquifer Recharge (MAR) in Lagos and other fast-growing coastal megacities in the world.