Re-examination of the BMN Model for Estimating Evapotranspiration

Abstract
This study re-examined the BMN model making use of the Sigma Plot software (based on the Levenberg – Marquardt algorithm) to generate modified versions of the BMN model that are specific for Ibadan, Kano and Onne and one version which applicable across the country, correcting some perceived shortcomings of the BMN. Records of climatic variables (Solar radiation, Maximum and Minimum Temperature, Maximum and Minimum Relative Humidity and Wind speed) were collected from three International Institute of Tropical Agriculture (IITA) Stations in Ibadan, Kano and Onne. For Ibadan, a 36-year (1973 – 2008) record was obtained, for Kano, a 29-year (1980 - 2008) record was obtained and for Onne, a 31-year (1977 - 2006) record was obtained. Evapotranspiration rates for each of the stations were then estimated using the FAO56-PM model. ETo estimates from the FAO56-PM model were thereafter used to recalibrate the BMN model, generating new model constants for Ibadan, Kano, Onne and a model combining the climatic characteristics of the three stations. The re-calibrated BMN model had higher correlation values of 0.74, 0.79 and 0.75 for Ibadan, Onne and Kano respectively when compared with the FAO-56 model than the Original BMN model when compared with the FAO56-PM model with values of 0.7, 0.77 and 0.75 respectively for Ibadan, Onne and Kano.

Keywords Evapotranspiration, Blaney-Morin-Nigeria, FAO56-PM Model, Levenberg – Marquardt, IITA