The Blaney-Morin-Nigeria (BMN) model was developed for the estimation of reference evapotranspiration (ETo) in Nigeria using commonly available meteorological parameters. In the development of the model, there were some perceived shortcomings which are believed to affect the reliability of ETo estimation, particularly in some specific locations in Nigeria, due to varying environmental factors. This study re-examined the model by using the non linear regression model based on Levenberg-Marquardt Algorithm (Excel Solver) to minimize the error space and generate new model constants specific for Enugu. A 25-year (1989 to 2014) monthly record of climatic variables (Solar radiation, Temperature, Relative Humidity and Wind speed) for Enugu were collected from the Nigeria Meteorological Agency (NIMET), Abuja in Nigeria and used. The newly generated model constants, H and m, were 392.2 and 1.19, respectively. Reference evapotranspiration for Enugu [ET 0(nBMN)] was thereafter estimated using the optimized BMN model and compared with that calculated using the FAO56-PM model [ET 0(FA056-PM)]. The optimized model showed a more accurate estimation of ET for Enugu as indicated by the higher correlation coefficient of 0.82 compared to 0.76 for the original BMN, lower mean absolute error (MAE) of 0.133 compared to 0.42 and root mean square (RMSE) of 0.3641 compared to 0.44. The X2 of 0.135 obtained using the optimized model was also low showing that the new model gives a better estimation of ET for Enugu, Nigeria.
Key words: Evapotranspiration, Blaney-Morin-Nigeria model, FAO56-PM model, Levenberg-Marquardt Algorithm, Enugu, Nigeria.
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