Abstract: The simultaneous occurrence of different climatic events (e.g., low precipitation and high temperatures) which means concurrent events, may cause significant impacts on the ecosystem and society. Maize is one of the main food crops in China, but extreme heat and drought concurrent events bring great instability to summer corn production, which has become important factors threatening food security. Based on daily maximum temperature and precipitation data from 27 meteorological stations in Haihe plain from 1960 to 2019, the Standard Precipitation Index (SPI) was used to identify drought and the extreme heat threshold was determined by percentile method. The Cramér-von Mises (CvM) mutation test and Cumulative Distribution Function (CDFs) were used to analysis long-term evolution characteristics and spatial distribution of extreme heat and drought concurrent events during different threshold levels in the whole growth period and main growth periods of summer maize (sowing to jointing, jointing to flowering, flowering to maturity). The results showed that: (1) the summer maize extreme heat and drought concurrent events were no long-term trend at all threshold levels of main growth periods. There was a significant mutation occurred in the 1990s. And the scope of occurrence after mutation was significantly larger than before mutation, especially the flowering to maturity concurrent events had the largest scope and increased the most after mutation. For different threshold levels, the scope of high extreme thresholds concurrent drought events was increased obviously. (2) The frequency of summer maize decreased at the whole periods from northwest to southeast in the study area. The concurrent events occurred at higher frequency in the north of the study area, and the increment was also greatest after mutation. The extreme heat and drought concurrent events during the main periods of summer maize in Haihe plain during nearly 60 years was significantly mutated in the 1990s. After the mutation, the range of concurrent events and the frequency at each stations have increased significantly, flowering to maturity as well as high threshold levels of concurrent events particularly.

Key words: Extreme heat and drought concurrent events, Summer maize, Cramér-von Mises, Haihe plain