Abstract
BACKGROUND: Reference evapotranspiration (ET(o)), which is used as the basic data in many studies within the scope of hydrology, meteorology, irrigation and soil sciences, can be estimated by using the evaporation (E(pan)) measured from the class-A pan evaporimeter. However, this method requires reliable pan coefficients (K(p)). Many empirical models are used to estimate K(p) coefficients. The reliability of these models varies depending on climatic and environmental conditions. Therefore, they need to be tested in the local conditions where they will be used. In this study, conducted in Kahramanmaraş, which has a semi-arid Mediterranean climate in Turkey during the July-October periods of 2020 and 2021, aimed to determine the usability levels of six K(p) models in estimating daily and monthly average ET(o). METHODS: The K(p) coefficients estimated by the models were multiplied with the daily E(pan) values, and the daily average ET(o) values were estimated on the basis of the model. The daily E(pan) values were measured using an ultrasonic sensor sensitive to the water surface placed on the class-A pan evaporimeter. The ultrasonic sensor was managed by a programmable logic controller (PLC). To enable the sensor to be managed by PLC, a software was prepared using the CODESYS programming language and uploaded to the PLC. The daily average ET(o) values determined by the FAO-56 Penman-Monteith equation were accepted as actual values. The ET(o) values estimated by the K(p) models were compared with the actual ET(o) values using the mean absolute error (MAE), mean absolute percentage error (MAPE), root mean square error (RMSE) and determination coefficient (R(2)) statistical approaches. RESULTS: The Wahed & Snyder outperformed the other models in estimating daily (MAE = 0.78 mm day(-1), MAPE = 14.40%, RMSE = 0.97 mm day(-1), R(2) = 0.82) and monthly (MAE = 0.32 mm day(-1), MAPE = 5.88%, RMSE = 0.32 mm day(-1), R(2) = 0.99) average ET(o). FAO-56 showed the nearest performance to Wahed & Snyder. The Snyder model presented the worst performance in estimating daily (MAE = 2.09 mm day(-1), MAPE = 37.53%, RMSE = 2.36 mm day(-1), R(2) = 0.82) and monthly (MAE = 1.83 mm day(-1), MAPE = 31.82%, RMSE = 1.87 mm day(-1), R(2) = 0.99) average ET(o). It has been concluded that none of the six K(p) models can be used to estimate the daily ET(o) in Kahramanmaraş located in the Mediterranean-Southeastern Anatolian transitional zone, and only Wahed & Snyder and FAO-56 can be used to estimate the monthly ET(o) without calibration.