Study of reference crop evapotranspiration in I.R. of Iran

Accurate estimation of regional evapotranspiration (ET) is essential for many agricultural water related studies. The data from 81 weather stations, with at least 30 years of data during the period of 1956–2000, were used for estimation of reference crop ET (ET0) in Iran. Monthly ET0 values were computed based on corrected mean air temperature for non-ideal conditions. This study focused mainly on the prediction of ET0 of 7 months of active crop growth season (April–October). Two main objectives of this study were: (i) prediction of mean monthly and annually ET0 in Iran using the suitable method from the three selected candidates, which are Hargreaves adjusted in 1985, adjusted Thornthwaite and Linacre methods, and (ii) study of spatial variation of annual ET0. Results showed that long-term mean annual ET0 vary between 830 mm to over 3627 mm across the country. The lowest monthly and yearly ET0 belonged to the Caspian Sea shoreline but highest ET0 belonged to the central and southeast parts of Iran. The mean annual ET0 in the southeast parts of Iran was about 33 times greater than that of its mean annual precipitation.