A novel multi-scale standardized index analyzing monthly to sub-seasonal drought-flood abrupt alternation events in the Yangtze River basin

Drought-flood abrupt alternation (DFAA) events are often cause severe disasters that affect economic growth and ecological security. Since the current monthly to sub-seasonal drought-flood abrupt alternation index (DFAI) has the limitations of misjudgments, omissions and neglecting alternation speed, this study proposes a novel multi-scale standardized DFAI index (MSDFAI) based on the definition of the misjudgment and omission rates as well as alternation speed. A comparative test and spatial–temporal characterization analysis in the Yangtze River Basin (YRB) are conducted. The results show that the misjudgment rate of DFAI in the YRB varies between 3 %–181 %, and its omission rate varies between 0.5 %–45 %, which is unable to find a suitable point that will satisfy both misjudgment and omission at the same time. The alternation speed has a significant impact on the level of DFAA events, and there are 30 underestimations of DFAA events in the YRB during 1962–2022 when only considering the alternation intensity. The MSDFAI index makes a conditional judgment on whether a drought or flood event occurs based on the standardized precipitation index (SPI) value, which can effectively avoid these misjudgments and omissions. Meanwhile, the MSDFAI index describes the DFAA events in a more comprehensive manner by introducing the speed factor. The DFAA has a spatial pattern with relatively low frequency in the Poyang Lake basin and the middle YRB, and high frequency in the upper YRB, the Han River, and the lower YRB. The inter decadal variability of DFAA events in the YRB is characterized by a turning point in the 1990 s, when DFAA events are increased after the 1990 s. The proposed MSDFAI index can monitor DFAA events much comprehensively and effectively in the YRB, and provide technique support for identification and prevention of DFAA in the changing environment.