Evaluation of different empirical equations of the estimation of reference crop evapotranspiration in different conditions lacking measured meteorological parameters in some climatic regions of Iran

Reference crop evapotranspiration (ETo) has important effects on estimation of agricultural water use, ecosystem models, aridity/humidity conditions and rainfall-runoff. Although, the accuracy of FAO Penman-Monteith (FPM) method for estimation of ETo in different regions was confirmed, but because of the requirements of many meteorological parameters, its applications was limited. In this research, by using 6 empirical and 5 semi-empirical equations, monthly ETo values were calculated during 1990-2007 for 12 synoptic stations in 4 climatic regions of Iran. By using FPM as a reference method and calculating monthly ETo values, empirical and semi-empirical equations were calibrated for the period1990-2003. By using combined criteria IPE for the period from 2004 to 2007, the best equation of each station was determined in lack of any meteorological parameters. Calibration by using the best linear equation reduced the amount of errors in most equations and in most stations. Estimation of solar radiation via temperature data caused the precision of semi-empirical equations being lower than empirical equation in both “humid” and “warm and semi-arid” regions, but this criteria was higher in both “arid” and “semi-arid” regions. The effects of solar radiation on ETo was quite relevant in “humid” and “warm and semi-arid” regions., while its effects was inverse in arid and semi-arid regions which confirms the effects of temperature parameters in this regions. Selection of the best equation depends on which criteria of MBE, RMSE, MARE or R2 was considered as judgment. The combine criteria of IPE were weighted by four mentioned criteria and provided the condition of the best selection.