Calibration of Angstrom-Prescott Equation Coefficients in the Selected Stations of Urmia Lake Watershed

Correct estimation of reference evapotranspiration (ETo) needs to estimate accurate solar radiation values. There are many models to estimate solar radiation so that one of these relationships is Angstrom-Prescott (A-P) equation. The accuracy of Angstrom-Prescott equation in estimating solar radiation value depends on the estimation accuracy of a and b coefficients of the mentioned equation. The purpose of the present research was to investigate the accuracy of models for estimating coefficients in A-P equation and their effects in the solar radiation and reference evapotranspiration values. To do this purpose, the meteorological data of four synoptic stations including Urmia, Takab, Mahabad and Salmas in the Urmia lake watershed in the daily scale from 1998 to 2008 were used. The models which used for the determination of Angstrom-Prescott equation coefficients were included models based on the annual average of relative sunshine hours , elevation and , elevation and latitude and , elevation and latitude, annual average of air temperature, elevation. The obtained coefficients were used to compute the solar radiation values in Angstrom-Prescott equation and then reference evapotranspiration was calculated by FAO-Penman-Monteith equation. To evaluate the models accuracy, statistical indices including RMSE, MBE, MPE and R2 were used. The evaluation results of the models showed that the model based on annual average of relative sunshine hours (Rietveld model) had the best performance in determining the coefficients of AngstromPrescott equation. The average values of RMSE, MBE, MPE and R2 indices for actual reference evapotranspiration and reference evapotranspiration obtained from coefficients of mentioned model were found 0.33 and 0.25 mm/day, 6.27% and 0.9977, respectively. For AngstromPrescott equation coefficients from selected model, a=0.26 and b=0.43 were suggested in the studied stations in the Urmia lake watershed.