Reference evapotranspiration (ET0) is an essential parameter for hydrological modeling, irrigation planning and for studying the impacts of climate change on water resources. This work evaluate 20 alternative methods of estimating ET0 in order to adapt them to the climatic context of the 3 mains basins of Senegal where very little climate data is available. The methods of Valiantzas 1, Doorenboss & Pruitt and Penman are the most robust for the estimation of ET0 in this context.

Runoff simulation is complex in poorly gauged and data-scarce hydrosystems of the West African Sahel. The results of this study showed that multi-sites calibration without nested sub-catchments is the best modelling scheme for capturing the hydrological response of the Sahelian catchments. This approach could therefore be applied to obtain regionalized parameter values for the West African Sahel region from small catchments distributed across all climatic zones.

The results showed that there is an variability in the spatial distribution of extreme indices with an upward and downward trend of dry and wet rainfall periods in West Africa in both historical and projected periods. Thus, the results revealed that the spatio- temporal variability of extreme rainfall can have repercussions on the hydrological functioning of watersheds, water availability and water-dependent activities.

The analyze of the trends of ET0 at the scale of the Senegal, Gambia and Casamance river basins using reanalyze data of NASA/POWER over 1984–2019 shows that ET0 increases significantly in 32% of the Senegal basin and decreases in less than 1% of it. In the Casamance and Gambia basins, the annual ET0 drops by 65% and 18%, respectively. Temperature and relative humidity show an increasing trend over all basins while wind speed and radiation decrease, confirming the so-called "evaporation paradox".

Rainfall causes material damage and human losses in Abidjan, Côte d'Ivoire, every year. This study shows that the period of the 60 wettest days of the long rainy season is more exposed to the risk of flooding than the rest of the year because the rains are more spatially extensive and heavier. The increase in the number of floods in recent years is probably due to the recovery of rainfall after the drought of the 80s associated with the sharp increase in urbanization of the District.