Analysis of reference evapotranspiration (ET0) trends is essential for understanding the impacts of climate change on water resources. Thus, despite the continuous rise in temperatures, a decrease in evapotranspiration is noted in some parts of the world. This contrast is called the "evaporation paradox" and is thought to be related to the variation in wind speed, relative humidity and solar radiation. The objective of this work is to analyze, based on the Reanalysis weather data from the NASA Langley Research Center (LaRC) POWER project, the annual and seasonal trends of ET0 and climate variables at the scale of the Senegal, Gambia and Casamance rivers basins over the period 1984-2019. Mann Kendall's test and Sen slope were used to analyze trends in ET0 and climate variables. Results show that on annual scale, ET0 increases significantly in 32% of the Senegal basin and decreases in less than 1% of it. In contrast, in the Casamance and Gambia basins, the annual ET0 drops by 65% and 18%, respectively. On an annual scale, temperature and relative humidity show an increasing trend over all basins while wind speed and radiation decrease significantly. This confirms the existence of the "evaporation paradox" in the three basins. This phenomenon is explained by the increase in relative humidity and the decrease in wind speed.