Potential-Evapotranspiration in the Appropriate Schedule of Farm Operations to Ensure Sustainable Food Security in South Western Nigeria

Agriculture in Nigeria is mostly rainfed, 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. Again, the knowledge of farmers in understanding the climatic behaviour of their enviroment have since been used to determine their harvest, all of which have contributed to large crop failure over years due to the growing effects of weather and climate change caused by ENSO, Global warming etc.

In this work, sunshine hours, wind speed , vapour pressure and other meteorological parameters for estimating potential evapotranspiration (PET) for the period 1975-1994 were collected over the study area. Both Penman and Thornthwaite methods were used to estimate PET for each station. The PET values were used on the Rainfall-PETmodel.

Results showed that; (a) for the temporal pattern, PET estimation by Penman increases northwards while that of Thornthwaite 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 north-westwards while that of Thornthwaite decreases north-eastwards showing that Penman overestimates PET north-eastwards while Thornthwaite overestimates PET south-westwards of the study area.

Again, the result further showed that the begining of growing period over Ilorin, Shaki, Ikeja, Abeokuta, Oshogbo and Akure falls into various dates in April while that over Ibadan, Ijebu-Ode, Benin-City and Warri 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 where both methods proved reliable.