The frequency and severity of high temperature and drought extremes are expected to increase under future climate change (CC) and considerably affect the agricultural sector particularly in water-limited ecosystems. This study was conducted to assess future CC impacts on rainfed wheat yield, water requirement (CWR), water use efficiency (WUE), precipitation use efficiency (PUE), reference crop evapotranspiration (ET0), and agricultural rainfall index (ARI) in northeast of Iran. The outputs of five global climate models (GCMs) under RCP-4.5 and RCP-8.5 during three time periods (i.e., the 2025s, 2055s, and 2085s) downscaled by MarkSimGCM model were applied. CWR was estimated using the CROPWAT 8.0 model. Further, the CSM-CERES-Wheat model was employed to simulate rainfed wheat yield, WUE, PUE, and ET0 responses to CC. The results showed that the mean monthly ET0 and CWR would likely increase under both emission pathways over the studied sites. The mean monthly ARI is also anticipated to decline in the future indicating a drier climatic condition over northeastern Iran by 2100. Furthermore, CC is highly likely to decrease rainfed grain yield, WUE, and PUE during the current century. The largest changes in ET0, ARI, CWR, yield, WUE, and PUE were projected in the late twenty-first century (the 2085s) under RCP-8.5. The CC-induced wheat yield loss will likely endanger food security in the country. Yield reduction can be partially offset by adopting appropriate adaptation measures.

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