Understanding evapotranspiration and its long-term trends is essential for water cycle studies, modeling and for water uses. Spatial and temporal analysis of evapotranspiration is therefore important for the management of water resources, particularly in the context of climate change. The objective of this study is to analyze the trend of reference evapotranspiration (ET 0) as well as its sensitivity to climatic variables in the Senegal River basin. Mann-Kendall's test and Sen's slope were used to detect trends and amplitude changes in ET 0 and climatic variables that most influence ET 0. Results show a significant increase in annual ET 0 for 32% of the watershed area over the 1984-2017 period. A significant decrease in annual ET 0 is observed for less than 1% of the basin area, mainly in the Sahelian zone. On a seasonal scale, ET 0 increases significantly for 32% of the basin area during the dry season and decreases significantly for 4% of the basin during the rainy season. Annual maximum, minimum temperatures and relative humidity increase significantly for 68%, 81% and 37% of the basin, respectively. However, a significant decrease in wind speed is noted in the Sahelian part of the basin. The wind speed decrease and relative humidity increase lead to the decrease in ET 0 and highlight a "paradox of evaporation" in the Sahelian part of the Senegal River basin. Sensitivity analysis reveals that, in the Senegal River basin, ET 0 is more sensitive to relative humidity, maximum temperature and solar radiation.