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A field experiment was conducted during 2016-2017 to investigate the effect of irrigation scheduling on three barley cultivars (viz Nosrat, Roudasht and Behrokh) under water deficit stress conditions. Irrigation treatment was applied based on the maximum allowable depletion of available soil water. Treatment levels comprised of irrigation scheduled at: (I1) 30%; (I2) 60%; and (I3) 90% depletion of available soil water which was applied after flowering stage. Studied traits included: plant height, shoot fresh weight, shoot dry weight, the number of spikes per plant, the number of grains per spike, 1000-grain weight, awn length, harvest index, biological yield, grain yield, and total protein content of grain. Results showed that all traits were significantly reduced with an increase in the severity of stress due to a reduction in the availability of water. However, no significant reduction was observed for cultivar Nosrat regarding grain yield, biological yield and the number of spikes per plant between I2 and I3 treatment levels. Estimation of production functions with respect to yield contributing traits revealed that, at a severe drought stress, the order of the importance of yield-contributing traits was as: harvest index>biomass>spikes per plant>grains per spike. Based on the results, to achieve the highest grain yield and grain total protein content, irrigation should be scheduled at a maximum allowable depletion equal to 30% of available soil water. Nevertheless, an economic production can be achieved with a delayed irrigation. [ABSTRACT FROM AUTHOR]