Journal of Geography, Environment and Earth Science International

This study was designed to proposed a multivariate model of monthly mean daily Photo synthetically active radiation (PAR) from the constant ratio of the global solar radiation for six selected tropical ecological zones between 4.40°N and 13.03°N of Nigeria (Port Harcourt, Enugu, Abeokuta, Ilorin, Bauchi and Sokoto). The measured global solar radiation data was obtained from the Archives of the Nigerian Meteorological Agency, Oshodi, Lagos, over a period of thirteen years (2000-2012). The estimation was based on a correlation between clearance index, PAR and extraterrestrial computed using generalized 45% and 40% of monthly mean daily data set of global solar radiation and extraterrestrial solar radiation respectively on horizontal surfaces. The performance of the models developed were tested for validation using mean bias error (MBE), root mean square error (RMSE), mean percentage error (MPE), Nash-Sutcliff equation (NSE), chi squares (χ²) and index of agreement (d). The polynomial regression model developed to estimate PAR judging from the model performance and validation test indicates that the proposed model could be used to estimate PAR in six tropical ecological zones in Nigeria and other stations with similar climatological conditions across the globe. The paper also revealed that there is an apparent increase in the magnitude of PAR and clearness index along tropical belts as a result of the movement of the Hadley cell circulatory system along the equatorial line from the Sahel Savannah to the Mangrove Swamp belt.