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. Nonparametric 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% ≤ β ≤ 50.3%; p = 0.01) to observed variations in PET.
This study evaluates the capability of a Climate-Crop Modelling System (RegCM3-GLAM) in simulating the regional climate and crop yields (maize, rice, cowpea, and groundnut) over Nigeria. Daily climatic data obtained from a Regional Climate Model (RegCM3) simulation was used as the input data in the General Large Area Model (GLAM) to simulate the crop yields for 11 years (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009), and a series of sensitivity experiments were performed to test and optimize the GLAM parameters over the region. The results show that RegCM3 gives a realistic simulation of the Nigerian climate. The correlation coefficients obtained between the observed and simulated climatic variables are between 0.72 and 0.96 at p < 0.01. However, the model slightly underestimates rainfall and maximum temperature in the wet season (April to October) and overestimates rainfall and maximum temperature in the dry season (November to March). GLAM also gives a realistic simulation of the mean and spatial distribution of crop yields in Nigeria. The root mean square errors of the simulations are generally less than 36% of the observed yields. The performance evaluation of the model varies with ecological zones. The model shows the best performance in simulating maize and the worst performance in simulating cowpea over the Savannas. Sensitivity experiments reveal that simulated crop yield is sensitive to model parameters (harvest index, extinction coefficient, optimum temperature, and transpiration efficiency), with both extinction coefficient and transpiration efficiency showing more significant impact. It has been concluded that the performance of GLAM over the country can be further improved by enhancing the quality of meteorological input data.
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