Abstract:
Evapotranspiration (ET) rates have been reported to be decreasing in many parts of the world and this scenario had been attributed to the effect of climate change/variation. This study analyzed this phenomenon in the context of Nigeria and also carried out the performance evaluation of four ET models (Penman, Blaney-Morin-Nigeria (BMN), Hargreaves-Samani and Priestly-Taylor models). In addition, the study also 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 applicable across the country and corrected 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 estimated using the FA056-PM model. A non-parametric Mann-Kendal trend analysis was performed on the estimated ET rates and also the Aerodynamic and Radiative parts of these estimates to ascertain whether any or both the parts are major determinants in the trend of ET at the three stations.
The results obtained during the study showed that there was a significant decline in monthly and annual ET rates Ibadan (P < 0.001) over the 36 years considered, a not significant decline in monthly and annual ET rates in Kano over the past 29 years and a significant monthly and annual decline in ET rates for Onne (P < 0.1) over the past 31 years. The percentage annual decline for Ibadan, Kano and Onne are 4.05%, 0.54% and 1.87% respectively. Also BMN model was found out to be the best model that can be applied to estimate ET in each of these stations because it has a high correlation value with the values obtained from FA056-PM model along with favourable statistic values and it requires a considerably less number of variables for its estimation. The re-calibrated BMN model also had a 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 Fh-056-PM model with values of 0.7, 0.77 and 0.75 respectively for Ibadan, Onne and Kano.
