Abstract: Jiangsu Province is a key region for wheat planting in southern China, where droughts occur frequently during the growing season of winter wheat under the background of climate warming. Understanding the regularity of drought occurrence, especially the characteristics of drought in different growth stages of winter wheat, is crucial to avoid the risks to winter wheat yield. Based on data of weather, soil moisture and winter wheat phenology observations, drought indices were established by considering the effects of soil moisture and water demand at each growth stage of winter wheat. Furthermore, based on the relationships between drought index and soil relative moisture in the 0-10 cm layer, drought levels in different growth stages of winter wheat were determined. The spatial and temporal distributions of winter wheat drought in different growth stages in different regions of Jiangsu Province during 1981-2020 were then analyzed using regression and empirical orthogonal function (EOF) methods. The results showed that there were obvious temporal and spatial variations of winter wheat droughts in different growth stages. Spatially, drought intensity was stronger in north region and weaker in south region, and the frequency was higher in north region and lower in south region. Moderate and severe droughts mostly occurred in north and central Jiangsu, while mild drought mainly occurred in south Jiangsu. Temporally, the most severe and frequent droughts occurred in the emergence-overwintering periods. Compared with 1981-2000, the occurrence period with frequent droughts during 2001-2020 gradually shifted from emergence-overwintering stage to heading-maturity stage. In practice, special attention should be paid on the regions with severe drought and the periods with frequent occurrence, in order to reduce the potential losses caused by drought in a scientific and comprehensive way and to transfer the risk reasonably.

Key words: winter wheat, growth stage, drought index, change characteristics, Jiangsu.