Ecological drought evolution characteristics under different climatic regions in the Yangtze River basin

The increase in the frequency and intensity of drought has caused serious changes and threats to the ecosystem. There is an urgent need to improve the characterization performance of the ecological drought index to better monitor the ecological drought. Thus, this study constructed a new ecological drought index, the standardized ecological water shortage index (SEWSI). The definition of SEWSI was based on the difference between ecological water requirement and consumption. Utilizing vegetation water requirement models to comprehensively consider environmental conditions, such as climate, soil, and vegetation. And, the twi-difference algorithm method was used to divide the crop planting system. The spatio-temporal evolution characteristics of drought, including drought frequency, drought intensity, drought duration, and affected area in three climatic regions (plateau mountain climate region, north subtropical humid region central subtropical humid region) in the Yangtze River Basin (YRB), were analyzed by the run theory method. Besides, the EOF model was used to determine the spatio-temporal distribution of ecological drought. The results showed that the ecological drought in the YRB presents a catabatic trend. An average of 31 ecological drought events occurred, and the average duration of the drought was about 3.18 months from 1980 to 2020. The frequency of light drought was more than 10 %, accounting for 55.82 % of climate region I. About 26.11 % and 26.91 % of climate region II and climate region III, where the frequency of severe drought was more than 5 %. The average duration of ecological drought in climate region I was the longest and the average drought intensity was the largest. The first four modes of EOF can explain 73.20 % of the temporal and spatial changes of ecological drought. Although various climatic regions generally exhibit a trend of humidification, there is also a trend of drought in some regions, such as drylands in Climate II, and Sichuan and Chongqing regions in Climate III.