A comparison of 4 empirical methods of calculating potential evapo-transpiration (PE) using climatic data for the estimation of length of growing season from rainfall – potential evapo-transpiration model was carried out using 15 years of estimates recorded in Shaki, Southwestern Nigeria. The length of the growing season estimated from the 4 empirical methods of PE were related to the moisture requirements of some selected tropical crops. Data were analyzed by one – way ANOVA taking each yearly mean as the average of the 15 years (1991 - 2005) for the station. Results show that the methods varied greatly in their ability to define the magnitude and variability of the length of growing season. The original Penman and the modified Penman of McCulloch models showed no significant difference in their use for the determination of length of growing season. However, the Thornthwaite, the Priestly and Taylor showed differences in the estimates of length of growing season. Priestly and Taylor gave the least estimate of length of growing season. The relative reliability of each method in terms of accuracy of both measured and extrapolated meteorological data utilized was also discussed.
Key words: Potential evapo-transpiration, length of growing season, moisture requirement, wet and dry climate.
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