Evaluation of FAO-56 methodology for estimating reference evapotranspiration using limited climatic data Application to Tunisia

The Food and Agriculture Organization of the United Nations had improved the version of the Penman-Monteith method (FAO-56 PM) which has recently been proposed as the standard for estimating reference evapotranspiration (ETo). Unfortunately, some weather variables, especially solar radiation, relative humidity and wind speed, are often missing which could impede the estimation of ETo with the FAO-56 PM method. To overcome the problem of the availability of climatic parameters, procedures to estimate ETo with missing climate data are proposed as part of the FAO methodology. Therefore, assessing the accuracy of these procedures for different Tunisian locations is important. The comparison of ETo estimates using limited data to those computed with full data set revealed that the difference between ETo obtained from full and limited data set is small considering the 8 locations studied. Both the Mean Bias Error (MBE) and the Root Mean Square Error (RMSE) of the comparison were less than 0.6 and 0.8 with a minimum of -0.4 and 0.2 mm day-1, respectively, leading to small errors in the ETo estimates. The higher deviations occur when the only available information is minimum and maximum air temperature. These deviations were significantly higher when using the Hargreaves equation to calculate ETo.