Comparison and Calibration of Nine Mass Transfer- Based Reference Crop Evapotranspiration Methods at Urmia Lake Basin

Abstract
The aim of this study is to compare and calibrate nine different mass transfer-based reference crop evapotranspiration ( ) estimation methods in monthly time scale at Urmia Lake basin. The selected methods were Meyer (M), Dalton (D), Rohwer (R), Penman (P), Brockamp and Wenner (BW), Mahringer (Ma), Trabert (T), WMO and Albrecht (A) methods. For this purpose the information of ten synoptic weather stations in the period 1986-2010 were used. Results of the mentioned methods compared with the output of the FAO-56 Penman-Monteith (PM56) method. Calibration of methods performed in the case of every station and months in the mentioned time period. Performance of methods evaluated using the R2, RMSE, MBE, MAE and PE statistics. Results showed that before calibration larger biases existed for the selected methods comparing the PM56. Calibration of methods considerably improved their performances. The M method was recognized as a best method after calibration at the study watershed. The median of R2 value of this method was 0.8945. In the mentioned method (after calibration) the median of the RMSE, MBE, MAE and PE was found to be equal to 21.8, -8.7, 17.3 and 21.5 mm per month. The Dalton method selected as the second best one. Methods namely, R, P, BW, Ma, T, WMO and A ordered as the third till ninth rank methods.
Key Word: Evapotranspiration, Mass Transfer- Based methods, Penman-Monteith, Urmia Lake Basin