Potential evapotranspiration trends in West Africa from 1906 to 2015

In this study, Climate Research Unit monthly observations were used to assess the trends of potential evapotranspiration (PET) over West Africa from 1906 to 2015. Trends and changes in PET during the 110-year study period and two reference climatological periods (1931-1960 and 1961-1990) were examined. Nonpara metric trend test of the Mann-Kendall and Kolmogorov-Smirnov was employed to find the changes in trends of PET and their significance or otherwise. The contributions of some meteorological parameters (air temperatures, precipitation and cloud cover) to the observed trends in PET were also examined. Results of long-term data analysis showed mixed trends in PET in the three designated zones (Guinea, Savanna and Sahel) but notable significant increasing trends (0.165 mm per year) at p= 0.1 in Sahel. However, very sharp differences in PET were observed in both reference periods across the zones with significant decreasing trends (at p = 0.01) in PET during the first period but increasing significant trends (at p= 0.1) during the second. In spite of this pattern of variations, general reductions in PET (-0.6% to - 1.2%) were observed, which were found to be triggered by decrease in temperature (-0.13 to - 1.07%) and vapour pressure (-0.02 to -0.33%) as well as increase in cloudiness (0.01-0.05%). However, the magnitudes of changes in PET were found to be insignificant in all the zones. Maximum temperature, more than other dominant climatic parameters, contributed significantly (Theil-Sen's regression: 23.5% <= beta <= 50.3%; p = 0.01) to observed variations in PET.