EVALUATING THE METHODS OF ESTIMATING EVAPOTRANSPIRATION POTENTIAL IN HUMID AND SUB HUMID STATIONS OF NIGERIA

Abstract

Potential Evapotranspiration (PET) is essential to soil and water studies and design of hydraulic structures, etc. This study was conducted to evaluate the relative performance of four commonly used methods of estimating potential evapotranspiration. These are: Thornthwaite method (THON), Blaney-Morin Nigeria model (BMN), Penman method (PEN) and Penman-Monteith method (PM). Data used for computation of PET were collected from IITA stations. The PET values were compared on annual, monthly and decadal basis using Pan evaporation values as reference. The PET values from each of the model was regressed with Pan values using a linear regression model. Penman-Monteith closely followed by Blaney-Morin-Nigeria model were highly correlated (on annual, monthly and decadal basis) with the observed values (Epan). PM overpredicted Epan by about 2.1% in the humid zone, by 0.42% in the sub-humid environment while BMN overpredicted Epan by about 20%-33%, PEN by about 32%-34%. For THON the range of overprediction was between 38% and 69% at Onne and Ibadan. The results showed that PM, PEN and BMN gave comparable estimates of PET. However, BMN has the advantage of fewer numbers of input data required. While the Thornthwaite method is most affected by temperature, the BMN is highly sensitive to solar radiation and relative humidity. Simple models relating PET (obtained by each of the method) to Epan and the relative humidity, were developed. The models were tested with independent data sets. The results of this study are useful in the management of irrigation systems, crop simulation models, planning and operation of water resource project