Evaluation of 13 Empirical Reference Potential Evapotranspiration Equations on the Island of Crete in Southern Greece

Knowledge of reference potential evapotranspiration (PETref) conditions is important for a number of vegetation- and hydrological-related applications. Direct estimations of PETref are difficult and require sophisticated instrumentation. The Food and Agriculture Organization (FAO), therefore, proposed a method for the estimation of PETref using only meteorological data. This equation has been widely accepted as the standard method for the estimation of PETref because of its good fit with measured values. Nevertheless, it requires several meteorological variables (e.g., wind speed), which are rarely available. Where such data are not available, its application is hampered. To overcome this problem, a number of simpler, empirical equations requiring only a fraction of the meteorological input variables required by the FAO PETref have been developed. Before using these equations, it is important to evaluate their performance and choose the equation that will have the lowest possible bias in the estimation of PETref. Using daily meteorological observations obtained from seven meteorological stations on the island of Crete (southern Greece), the performance of 13 empirical equations (radiation- and temperature-based) for the estimation of PETref has been evaluated against the estimations of PETref using the FAO equation. Performance was evaluated on a daily and a monthly basis, and five different measures of goodness of fit were used. The results showed that when the use of the FAO equation is not possible because of the unavailability of data, some empirical methods can serve as appropriate alternatives. The radiation-based equations generally performed better than those that included only temperature-related input variables. The equations proposed by Hansen and Turc were the most useful because they had an average monthly absolute error ranging from 5.7 to 17.7 mm and 5.5 to 19.2 mm, respectively.