Weather variables such as maximum and minimum temperatures and precipitation influence crop pro-duction especially under rainfed conditions. The goal of this study was to determine the optimum plantingdate of rainfed wheat based on crop evapotranspiration under water stress conditions (ETc adj), usingmodeled precipitation uncertainty. This analysis was conducted for 5 locations in the Khorasan province(Mashhad, Sabzevar, Birjand, Bojnourd and Torbat-heydarye) in Iran, using five planting dates at 15 dayinterval (23 Sep to 23 Nov). The climate data for each location ranged from 29 years to 44 years contain-ing daily values of maximum temperature, minimum temperature, and precipitation. Evapotranspirationwas calculated using the FAO (Food and Agricultural Organization of the United Nations) modified formof the Penman–Monteith equation (FAO 56). Cumulative values of ETc adjfor all locations except Sabzevarshowed the highest values for the final planting date with a sharp increase at the end of the growth period.However, there was no difference among the different planting dates of rainfed wheat for Sabzevar. Rain-fed wheat experienced extreme and medium drought conditions based on the calculated Dry Days SinceLast Rain (DDSLR) index which represents the number of drought days (DDN) during the growing season.Rainfed wheat planted at the final planting date was exposed to the lowest drought intensity duringthe growth period for all study locations. Planting a rainfed crop based on the occurrence of the firstprecipitation is not always a suitable strategy for drought avoidance. The approach introduced here willimprove the appropriate selection of representative planting dates that will produce highest potentialyield under rainfed conditions.

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