Calibration and evaluation of six popular evapotranspiration formula based on the Penman-Monteith model for continental climate in Turkey

Due to the difficulties in measuring and collecting meteorological data, it may be necessary to use methods that allow for the estimation of reference evapotranspiration values by using the few available data. For this reason, it is useful to calibrate empirical formulas locally, which enables obtaining accurate and reliable estimates with a small number of data. The effects of other parameters excluded from the employed data are included in the formula as regional coefficients. In this study, six empirical evapotranspiration formulas among temperature-based and solar radiation-based methods, including Thornthwaite (1948), Makkink (1957), Turc (1961), Jensen and haise (1963), Priestley and Taylor (1972), and Hargreaves and samani (1985) are calibrated, for continental climates of Central Anatolia's. To this end, meteorological data of three synoptic stations located in the west of Turkey in Kutahya province are employed from 2007 through 2019. Moreover, the data of the fourth station are applied for evaluating the accuracy of the estimated results. The main reason for employing these six formulas are behind in the fact that they are globally used, easy-to-use, and need few types of data. Meanwhile, Penman-Monteith 56 equation is applied to calibrate the empirical equations. In total, three methods are followed to estimate the local calibration coefficients. Finally, the evaluation of the results is performed by using the coefficient of determination, percentage error of estimate, mean absolute error, and root-mean-square error. According to the evaluation criteria, among calibrated formulas, Jensen and haise's (1963) formula was obtained to be the best for estimating ETo.