Re-examination of the BMN Model for Estimating Evapotranspiration

Abstract

This study re-examined the BM N model making use of the Sig ma 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 climat ic variables (So lar radiat ion, Maximu m and Min imu m Temperature, Maximu m and M inimu m Relat ive Hu mid ity and Wind speed) were collected fro m 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 fro m the FAO56-PM model were thereafter used to recalibrate the BM N model, generating new model constants for Ibadan, Kano, Onne and a model co mb ining the climatic characteristics of the three stations. The re-calibrated BM N model had h igher correlat ion 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 BM N model when compared with the FAO56-PM model with values of 0.7, 0.77 and 0.75 respectively for Ibadan, Onne and Kano.