Ala'a Homaidan1, Ayman Suleiman1, and Gerrit Hoogenboom2. (1) Land, Water and Environment Department, University of Jordan, Faculty of Agriculture, Amman, Jordan, (2) Department of Biological and Agricultural Engineering, University of Georgia, Griffin, GA 30223-1797
Evapotranspiration (ET) rate from a grass cultivated land is called the grass reference evapotranspiration (ETο). The FAO-56 Penman Monteith model (PM) has been documented to be the most accurate procedure for ETο calculation under various climatic conditions worldwide. The objective of this study was to evaluate the FAO-56 daily ETο in Amman, Jordan and in Camilla, Georgia, USA, representing semi-arid and humid climates, respectively. Actual ETο was obtained as a residual from the water balance using daily measured soil moisture content data. The root mean square error (RMSE) between FAO-56 and actual daily ETο values were 2.98 and 2.22 mm d-1 for Amman and Georgia, respectively. The relatively high RMSE in both locations was partially due to the fact that enough fetch did not exist for the vapor gradient to equilibrate especially at the semi-arid site. The cold season months (October through March) had lower RMSE values than the warm season months (April through September) for both sites. The RMSE for the cold season months was 2.78 and 1.77 mm d-1, while for the warm season months it was 3.04 and 2.48 mm d-1 for Jordan and Georgia, respectively. Higher RMSE in warm season months than the cold season months is typical because of the higher evaporative demand in warm season. These results showed that the RMSE between FAO-56 and actual daily ETο was lower in the humid climate than in the semi-arid climate throughout the year. These results demonstrated that the FAO-56 PM was relatively more accurate for the humid climate than for the semi-arid climate. Accurate estimation of daily ETο can help with efficient irrigation scheduling, precise water management, and sensible water resources planning. Future studies will examine the accuracy of using the FAO-56 procedure in estimating crop ET.
