Abstract: According to runoff data from the Zhimenda hydrological station from 1957 to 2021, we analyzed the abrupt alteration between drought and flood in the Changjiang River source area by using the Long Runoff Drought-Flood Area Index (LRDFAI) and the Short Runoff Drought-Flood Amplitude Index (SRDFAI). We also investigated the periodical change of the intensity of the alteration events by the Hilbert-Huang Transform method. Results reveal that short-term alteration events in the study area mainly occur from March to September with comparable frequencies of “drought to flood” and “flood to drought” events. The frequencies of these events fluctuate considerably in different years and exhibit an overall trend of more-less-more-medium-more-more. The intensity of short-term “drought-to-flood” events displays a weak increasing trend, while the intensity of “flood-to-drought” events shows a weak decreasing trend. Both types of events exhibit main cycles of 7.1 years and 12.3 years. The area witnessed 20 long-term runoff alteration events with 8 and 12 occurrences of “drought to flood” and “flood to drought” events, respectively, and cycles of 8.3 and 14.2 years.

Key words: source region of the Changjiang River, drought-flood abrupt alternation index, runoff, periodicity analysis