Accuracy Evaluation of Twenty Empirical Models in Estimating Coastal Regions Reference Evapotranspiration in different climate

  Evapotranspiration is a main component of the water cycle and is important in agricultural water management. There are a whole host of factors that affect evapotranspiration, which makes it difficult to estimate accurately. Accordingly, several methods have been proposed for its estimation. Measurement of evapotranspiration using weighing lysimeter is the most accurate method. But due to expensive build of Weighing lysimeter, simplified empirical methods for calculating potential evapotranspiration are widely used and its results were compared to reference evapotranspiration (FAO-Penman-Monteith) method. In this study a cross comparison of FAO-Penman-Monteith method and 20 Empirical Equations for Calculating Potential Evapotranspiration in a period of 10 years from 2007 to 2016, in Rasht, Sari and Gorgan synoptic stations in coastal zone of Caspian Sea, was made. These stations were selected for climatic variation from humid climates to Mediterranean climates. The Root mean squared error (RMSE), Mean Absolute Error (MAE) and Percent error (PE) were used to measure accuracy of these methods. The results showed that daily evapotranspiration values obtained from Ravazzani method were very closer to FAO-Penman-Monteith method than those of the other models. So that results in terms of the measured errors involve RMSE, MAE and PE, which obtained 0.77, 0.54 (mm/day) and 21.6% for Rasht, 0.8, -0.39. (mm/day) and 9.4% for Sari and 0.99, -0.42 (mm/day) and 9.88% for Gorgan. Also, the results showed that the accuracy of Romanenko model was lower than other models. Overall, in all of the studied climates, the Ravazzani et al, Berti et al, Hargreaves- Sammani, Irmak et al and Valiantzas models had better performance than other models in estimating the reference evapotranspiration The results of this study showed decrease in the accuracy of ET models compared to reference model of FAO-Penman-Monteith for moving from humid to Mediterranean climate. The mass transfer based models like Mahringer and Tabari showed relative undesirable efficiency in coastal zone. Therefore, it can be noticed that empirical equation can lead to variable efficiency in different climate. The results of this study showed decrease in the accuracy of ET models compared to reference model of FAO-Penman-Monteith for moving from humid to Mediterranean climate. The mass transfer based models like Mahringer and Tabari showed relative undesirable efficiency in coastal zone. Therefore, it can be noticed that empirical equation can lead to variable efficiency in different climate.
Keywords: Coastal Regions, Empirical methods, FAO-Penman-Monteith, Reference Evapotranspiration