Effects of Climate Change on Frequency of Adverse Weather Events during Wheat-Growing Season (Case Study: Main Areas of Rainfed Wheat Production in Iran)

Adverse and extreme  agro climatic events will disrupt food production and these  changes are expected to increase in the world. The wheat is Iran's dominant diet, especially in the form of bread. It is important as a food product that has an impact on food security. Climate change can affect wheat production in major areas of rainfed wheat production in Iran, with social and economic consequences. Therefore, it is important for policy makers and scientists  to evaluate the effects of climate change on the agricultural sector and food security. Crop models cannot take into account the effects of severe weather events (such as heavy rainfall, heat stresses) on the final yield of the crop. It could be useful to  utilize  agro climatic indices to provide more comprehensive projections of the impact of climate change on  agro climatic conditions. The purpose of this study was  evaluating the  probability of occurrence of adverse and extreme  agro climatic events at different stages of wheat development using  agro climatic indices.

The focus of this study is on main areas of rainfed wheat production in Iran (Kurdistan, Kermanshah, Golestan, Zanjan, Hamedan, and Ardebil provinces). According to the latest statistics and information from the Ministry of Agricultural Jihad, more than 55% of wheat production  achieve in these areas. The evaluations are based on the outputs of seven CMIP5 models and RCP8.5 and RCO2.6 emission scenarios for the period 2045-2065 and 2080-2100.  The equidistant quintile-based mapping method (EDCDF) was applied to bias correct the outputs of CMIP5 models .The proposed method of Allen et al. (1998) was  utilized to estimate daily crop evapotranspiration, soil moisture and relative reduction in crop yield under soil water shortage to describe the major adverse conditions for wheat production;  the set of 13 indicators was used to cover the major causes of low yields of winter wheat.

 The average temperature during the growing season will be increased by 3.1 °C for the late cultivar and RCP8.5 scenario during the period 2080-2100 compared to the baseline. The appropriate sowing dates will occur later for all scenarios relative to the baseline and shift to late autumn. Due to the increased average temperature during the growth period, anthesis and maturity dates will occur earlier relative to the baseline and subsequently the average growth period for all scenarios is shorter than the baseline. Average total crop evapotranspiration (ETc) during the growing season will be reduced in most stations. The average relative reduction in crop yield (YD) and the average total effective solar radiation will be more favorable than the baseline.  Thus, it can be said that these crop yield indicators are better than the baseline. However, increasing frequency of adverse events will be undesirable and the most unsettling possibility is the increase in the likelihood of occurrence of at least one, two and three adverse events during the growing season that can be extremely unfavorable climatic conditions for the production of wheat. The close connection between the likelihood of adverse events  and the duration of growth period (such as moisture and heat stresses) is obvious so that the longer growth period,  is more likely to be exposed to high temperatures and moisture stresses. An early cultivar will be a more suitable cultivar for sowing compare to late and medium-ripening cultivar which can change future climate conditions in favor of rainfed wheat production in most areas, especially cold regions.

In this study, the probability of occurrence of adverse and extreme  agro climatic events during the growing season of wheat was determined, which is usually not well considered in crop models. However, it is well known that the impacts of such extreme events can be substantial. The results of this study showed that, despite high uncertainty in the climate projections within CMIP5 models, the probability of occurrence of at least one (or more) adverse event during the growth period for each cultivar will increase compared to the baseline for the same cultivar. So that, the longer growth period, the greater likelihood of occurrence of at least one (or more) adverse event.