Spatio-temporal characteristics of the hydrothermal conditions in the growth period and various gro wth stages of maize in China from 1960 to 2018

Hydrothermal conditions affect crop distribution and agricultural development. Maize is an important food crop in China and is primarily used as a fodder and energy crop. However, the growth of maize is sensitive to climate change. Understanding the spatiotemporal characteristics of the hydrothermal conditions, especially during the growth period and at various growth stages of maize, is crucial for food security and for coping with the impacts of climate change. This study aims to explore the spatiotemporal characteristics of the hydrothermal conditions during the maize growth period and its various growth stages in different regions of China from 1960 to 2018. The mean temperature (T_mean), growth degree-day (GDD), killing degree-day (KDD), sunshine hours (SD), effective precipitation (P_e), and irrigation water requirement (IR) served as the hydrothermal condition indicators, incorporating the data from 422 meteorological stations and 292 agricultural stations. The spatial and temporal variations of each indicator were explored using trend and spatial analysis methods. The results showed that T_mean and GDD generally increased in the maize growth period, 0.18~0.36℃·(10a)^-1and 5.73~41.23℃·(10a)^-1, respectively, and T_mean and GDD had risen more rapidly in Northeast China, the Gansu-Xinjiang Region, and the Huang-Huai-Hai Plain. KDD increased in the summer maize planting areas, including the Huang-Huai-Hai Plain and Gansu-Xinjiang Region. KDD tended to increase nationally except in the southwestern part of the Huang-Huai-Hai Plain, where there was an insignificant decline. In the various growth stages, KDD mainly increased from the seedling stage to the flowering and kernel stages. SD in the maize growth period showed a significant downward trend, and there was a significant downward trend in each maize growth stage in the Huang-Huai-Hai Plain and the Loess Plateau. SD declined the most in the flowering and kernel stages. Changes in P_e and IR during the maize growth period were generally non-significant, but the changes varied in different regions. P_e in different growth stages generally decreased, except in the Gansu-Xinjiang Region and Inner Mongolia, where, in past years, the P_e values increased non-significantly. IR in the maize growth period tended to decrease in southern Xinjiang and the southwestern part of the Huang-Huai-Hai Plain, whereas IR increased non-significantly in parts of Northeast China. In the various growth stages, P_e decreased and IR increased at the heading and flowering stages in the Huang-Huai-Hai Plain and Northeast China, whereas IR tended to decrease during the sowing and seedling stages in the Huang-Huai-Hai Plain and the western part of the Loess Plateau. Overall, there was an increasing trend of heat resources during the maize growth period in the whole country. Solar radiation tended to decrease, with no significant changes in P_e and IR. However, there were significant differences in the water and heat conditions among growth stages and regions; KDD significantly increased from the seedling stage to flowering stage in the Huang-Huai-Hai Plain and Loess Plateau, whereas SD decreased in the same regions. Thus, high-temperature stress may negatively impact maize growth and affect water consumption in different maize growth stages. This study illustrated that changes in the hydrothermal conditions during different maize growth stages were more important than those experienced during the entire maize growth period. However, knowledge of the possible spatio-temporal changes in the hydrothermal conditions in different maize growth stages is still lacking; particularly, the climate extremes during different maize growth stages in different regions of China. The results of this study highlight the impact of climate change on crop production, although more detailed research is needed.