In this paper an atmometer apparatus were used in the greenhouses for estimating reference evapotranspiration values. Experimental work was conducted in the agriculture research center in the College of Agriculture-University of Baghdad west of the city of Baghdad. One atmometer was used in eggplant greenhouse and in cucumber greenhouse through the winter growing season 2013-2014. FAO Penman-Monteith equation was applied outside the greenhouse and used only 65% from the value of ETo in the greenhouses for estimating the reference evapotranspiration in the greenhouse. Moreover, Penman-Monteith equation was applied in greenhouses for the evaluating the performance of the atmometer. The results show that the error analyses between FAO Penman-Monteith equation in greenhouse and the atmometer readings, the root mean square difference (RMSD), mean bias error (MBE) and relative error (RE) for eggplant and cucumber greenhouses were: 1.163mm/day, 0.933mm/day, 30.19%, and 0.688mm/day, 0.518mm/day and 22.93%, respectively. A fair agreement was found. While the error analyses between 65% from outdoor ETo of FAO Penman-Monteith equation and the atmometer readings, the RMSD, MBE and RE for eggplant and cucumber greenhouses were: 0.930mm/day, 0.743mm/day, 24.27%, and 0.374mm/day, 0.289mm/day and 12.47%, respectively. A good agreement was found. The atmometer apparatus could be used successfully by researchers and farmers in estimation daily or weekly reference evapotranspiration in greenhouses as well as in low technology greenhouses.
