Mesoscale extreme rainfall events in West Africa: The cases of Niamey (Niger) and the Upper Ouémé Valley (Benin)

In West Africa, a sharp decrease in rainfall has occurred in conjunction with an increase in flood damage since 1970. The material damage and loss of life resulting from floods highlights the undeniable vulnerability of populations to this threat and illustrates the importance of addressing the evolution of hazardous precipitation caused by intense rainstorms. This work aims to improve our knowledge of the behaviour of extreme rainfall in West Africa by studying the sub-hourly, hourly and daily evolution of the most extreme rainfall events, a topic that is especially important to those interested in studying the links between heavy rainfall and flash flooding or inundation. This study analyses the classes of extreme rainfall events in two distinct climatic areas within West Africa using the meteorological scales relevant to rainfall processes. The study is based on two precipitation datasets recorded by dense networks of rain gauges set up within the meso-sites of Niamey (Niger, Sahelian area) and the Upper Ouémé Valley (Northern Benin, Soudanian zone) from 2000 to 2010 and 1998 to 2010, respectively. The Gumbel distribution was used to analyse the frequency of the maximum rainfall series for durations varying from 5 min to 24 h. The reliability of this model was examined, and the Intensity-Density-Frequency (IDF) curves derived from it were used to estimate the critical rainfall intensities at each site. The results returned exceeded frequencies that were useful for the isolation and classification of extreme rainfall cases using temporal characteristics. The climatological results confirm the existence of a latitudinal gradient in the mean annual rainfall and number of extreme events at the mesoscale. The classification methods illustrate clear distinctions between local, meso and synoptic scale events derived from convective systems over the Sahel. In contrast, Soudanian climate conditions lead to a nesting of the phenomena involved in the formation of cloud systems, making it difficult to classify rain events in that area. However, we were able to utilize the duration of rainfall events within this zone to discriminate between types of convective systems that cause extreme rainfall. For both areas, the proportion of precipitation in an extreme event compared to total yearly precipitation served as a suitable additional criterion used to objectively identify extreme precipitation event types

Spatio-Temporal Analysis and Water Quality Indices (WQI): Case of the Ébrié Lagoon, Abidjan, Côte d’Ivoire

For decades, the Ébrié Lagoon in Côte d’Ivoire has been the receptacle of wastewater effluent and household waste transported by runoff water. This work assesses the spatio-temporal variability of the Ébrié lagoon water quality at the city of Abidjan. The methodological approach used in this study is summarized in three stages: the choice and standardization of the parameters for assessing water quality for uses such as aquaculture, irrigation, livestock watering, and sports and recreation; the weighting of these parameters using the Hierarchical Analysis Process (AHP) of Saaty; and finally, the aggregation of the weighted parameters or factors. Physicochemical and microbiological analysis data on the waters of the Ébrié lagoon for June and December of 2014 and 2015 were provided by the Ivorian Center for Anti-Pollution (Centre Ivoirien Anti-Pollution, CIAPOL), and the concentrations of trace elements in sediments (As, Cd, Cr, Pb, Zn) were used. The aggregation of standardized and weighted parameters allowed the calculation of the Water Quality Indices (WQI) by usage for each bays of the lagoon. The results show that in both 2014 and 2015, the waters of the Ébrié lagoon were generally of poor quality for the different uses examined in this study (aquaculture, irrigation, livestock watering, and sport and recreation) with an accentuation in 2015. However, some bays of the lagoon have waters of dubious to satisfactory quality. This study contributes an improved evaluation of the Ébrié lagoon waters