Spatiotemporal characteristics of heat and rainfall in the three cotton areas of China under climate warming

Considerable climate change has occurred across the cotton planting area of China in the past few decades. Thus, it is important to study the temporal and spatial changes in water and heat resources during each growth period of the cotton-growing season and to propose corresponding technologies and strategies to stabilize the cotton production of China. Based on observational data from 377 meteorological stations and 50 agometeorological stations, this study analyzed the climatic trend rates of growth degree days (GDD), heat degree days (HDD) and rainfall from 1961 to 2017 in sowing-emergence, emergence-squaring, squaring-flowering, flowering-boll opening, boll opening-harvesting, and the entire growth period of cotton in the Northwest Inland (total cotton output accounts for > 80% of the country, including the northern Xinjiang subregion, southern Xinjiang subregion, eastern Xinjiang subregion, and the Hexi Corridor subregion), Yellow River Basin, and Yangtze River Basin cotton areas from 1961 to 2017. This study also recommended strategies to cope with the climate change-induced variations. The results showed that GDD generally increased over the past 56 years; during the entire growth period of cotton, 94.16% of the sites recorded increased cotton GDD. High GDD values at each growing stage were primarily distributed in the Naomao Lake, Hami, Yiwu, and Hongliuhe areas of the eastern Xinjiang subregion and in Jiuquan, Gaotai, and Zhangye of the Hexi Corridor subregion (the exception being the sowing-emergence and boll opening-harvest stages) followed by the northern Xinjiang subregion. The spatial distribution of HDD during each growth period markedly differed. During the squaring-flowering and flowering-boll opening periods, HDD tended to increase. Among all sites, 85.94% and 76.40% recorded increased cotton HDD, respectively. During the flowering period, cotton HDD increased, and there was a high-temperature risk in the east of lower reaches of Yangtze River subregion, east of the southern and northern Xinjiang subregion, the west of the Hexi Corridor subregion, the northwest of the extra-early maturing subregion of the Yellow River Basin, and in the eastern part of the upper reaches of the Yangtze River subregion. Overall, the lowest risk of high temperature was in the northern Xinjiang subregion. Rainfall during the entire growth period increased in the middle and lower reaches of the Yangtze River in the Yangtze River Basin and in most of the Northwest Inland. In other regions, rainfall tended to decrease. During the entire growth period of cotton, precipitation moved northward, and high precipitation in the Northwest Inland cotton area was primarily distributed in northern Xinjiang. During the sowing-emergence period, the drought risk in most cotton areas increased, except for areas with increased precipitation (the northern Xinjiang subregion). The GDD in the three major cotton areas increased overall, which benefited cotton area expansion and led to high yield and good quality. However, varieties with high yield potential should be selected. During the squaring-boll opening period of cotton in the Northwest Inland cotton area, there was a high-temperature risk in cotton planting in the eastern parts of the southern and northern Xinjiang subregions, the eastern Xinjiang subregion, and the western Hexi Corridor subregion. High-temperature resistant varieties and reasonable water and fertilizer management measures must be employed to cope with increased high-temperature risks. Drought risk is increasing in the cotton areas of the Yellow River Basin and the northern Yangtze River Basin; therefore, appropriate drought-resistant varieties and management measures should be adopted to reduce risks caused by climate change.