Agriculture in Nigeria is mostly rain fed; thus, farmers have always been assessing the size of their future harvest in relation to what they have sown and the prevailing rainfall situation. But rainfall alone cannot adequately determine the wetness or dryness of a place because rainfall is the most variable of all climatic elements. It was established that agricultural production is largely determined by climatic conditions rather than by the prevailing rainfall situations (Kumar, 1987).

In this work, sunshine hours, wind speed, vapor pressure and other meteorological parameters for estimating potential evapotranspiration (PET) were collected for the period 1975-1994 from 18 selected meteorological stations distributed over the tropical rainforest and the guinea savanna in Nigeria. Both Penman (1992) and Thornthwaite (1990) methods were used to estimate PET. The PET values were used on the Rainfall-PET model as given by Cocheme and Franquin (1967).

Results showed that; (a) for the temporal pattern, PET estimation by Penman (1992) increases northwards while that of Thornthwaite (1990) decreases northwards showing that overestimation of PET occurs by Penman in areas of high net radiation and temperature while overestimation of PET occurs in Thornthwaite in areas of low net radiation and temperature. (b) The spatial pattern shows that the estimation of PET by Penman formula increases northwards while that of Thornthwaite decreases northwards showing that Penman overestimates PET northwards while Thornthwaite overestimates PET southwards of the study area. Again, the result further showed that the beginning of growing period over Ilorin, Shaki, Ikeja, Abeokuta, Oshogbo, Akure, Enugu and Ikom falls into various dates in April while that over Ibadan, Ijebu-Ode, Benin City, Warri, PortHarcourt and Calabar falls into various dates in March. Finally, the study showed that Penman method of estimating PET is more reliable over Ikeja, Ibadan, Ijebu-Ode and Abeokuta while Thornthwaite is more reliable over the remaining stations except at Oshogbo and Enugu where both methods proved reliable.