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Acceptable estimation of reference Evapotranspiration (ET₀) values by the Penman-Monteith FAO (PM FAO) equation requires accurate solar radiation (R_s) data. R_s values could be estimated using the Angstrom’s radiation model. The aim of this study was to determine the a_s and b_s coefficient (as Angstrom’s parameters) for the Ardabil plain as an arid and cold region. Angstrom’s radiation model and PM FAO equation were calibrated for the study area, by optimizing the a_s and b_s parameter using Generalized Reduced Gradient (GRG) method. Measured R_sdata were collected from the Ardabil Synoptic Station and measured ET₀ data were determined using three lysimeters that were installed at the Hangar Research Station. Calibrated results showed that optimized a_s and b_s values were 0.117 and 0.384, respectively. Compared to the original models, errors including RMSE, AE and RE values were decreased and fitted parameters including R² and regression line slope (m) were improved in the calibrated models. The GMER values for the original models showed that Angstrom’s radiation model overestimated the R_s values and PM FAO equation underestimated the ET₀ values. Locally calibrated models estimated R_s and ET₀ values better than the original one. Nash-Sutcliffe efficiency coefficient (NSE) values proved that R_s and ET₀ estimation by the original models were not satisfactory, but were acceptable in the case of the calibrated models. However, calibration of Angstrom’s radiation model and PM FAO equation is necessary for each region.
 

Keywords: Penman-Monteith, Model calibration, Validation, Lysimeter, Ardabil

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