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International audience; Drinking water supply in the Lobo catchment is based mainly on surface water. In recent years, the adverse effects of climate change and strong population growth have led to increasing uncertainty about the availability of surface water and growing interest in groundwater in this region. Therefore, to ensure sustainable management of this resource, it is necessary to characterise and understand this groundwater system. This characterization requires knowledge of the geometry and structure of this aquifer system, which is a prerequisite for ensuring future water supply from groundwater resources. This aquifer system, which is the subject of this study, has never been the subject of a study aimed at characterising it and getting to know it better.This study aims to improve the knowledge of this aquifer system by elaborating the geometry of this aquifer system through the study of drilling data and electrical resistivity tomography (ERT). The results show that weathering of the granitic rocks in the Lobo catchment leads to the development of composite aquifers at two levels: a shallow saprolite aquifer overlies in places by a thick layer of ferruginous crust, and a deeper and very thick fractured aquifer. This shallow weathering aquifer, fed directly by precipitation, is tapped by conventional large diameter wells that are generally not immune to seasonal fluctuations. For sustainable management of this resource, an assessment of its recharge should be undertaken in addition to this study of the geometry of this aquifer system. The results of this study of the geometry of the Lobo catchment aquifer system improved the understanding of this aquifer system and will assist in groundwater exploration in this area.

International audience; Determination of groundwater recharge is a major challenge in areas where rainfall is generally abundant. Variability and uncertainty are inherent in the estimation of recharge, and several methods are therefore recommended for its estimation at a regional level. In this study, we evaluated several methods for estimating recharge: the web GIS-based automated hydrological analysis tool (WHAT), water table fluctuation (WTF), hydrograph analyses, a recession curve displacement method, graphical separation, and empirical formulas. The annual recharge estimated by combining direct recharge and base-flow varied from 84 mm in 2019 to 66.4 mm in 2020. The mean direct recharge was about 44 mm in 2018 and 57.3 mm in 2019, representing about 4% and 5% of the respective rainfall. In 2020, this direct recharge was 43 mm, or about 6% of rainfall, around 25% lower than in 2019. Base-flow separation methods and recession curve displacement generally gave low results, whereas modified empirical formulas gave results close to those of the WTF method and were considered more consistent and reasonable. The regression curve displacement method implemented in the United States geologic survey (USGS) RORA program was found to be unsuitable for the study area. However, the other methods presented more reasonable results and could be used to estimate groundwater recharge in the study area.