Determining optimum planting dates for rainfed wheat using the precipitation uncertainty model and adjusted crop evapotranspiration

Weather variables such as maximum and minimum temperatures and precipitation influence crop production especially under rainfed conditions. The goal of this study was to determine the optimum planting date of rainfed wheat based on crop evapotranspiration under water stress conditions (ETc adj), using modeled 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 day interval (23 Sep to 23 Nov). The climate data for each location ranged from 29 years to 44 years containing daily values of maximum temperature, minimum temperature, and precipitation. Evapotranspiration was calculated using the FAO (Food and Agricultural Organization of the United Nations) modified form of the Penman–Monteith equation (FAO 56). Cumulative values of ETc adj for all locations except Sabzevar showed 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. Rainfed wheat experienced extreme and medium drought conditions based on the calculated Dry Days Since Last 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 during the growth period for all study locations. Planting a rainfed crop based on the occurrence of the first precipitation is not always a suitable strategy for drought avoidance. The approach introduced here will improve the appropriate selection of representative planting dates that will produce highest potential yield under rainfed conditions.