Frequent dry-wet cycles promote debris flow occurrence: Insights from 40 years of data in subtropical monsoon region of Sichuan, China

The increase in extreme weather frequency complicates debris flow prevention and mitigation. Yet, the relationship between debris flow occurrences and extreme dry-wet cycles remains poorly understood, hindering the development of a more accurate debris flow early warning system. Based on the debris flow inventory (1981-2021) in the subtropical monsoon climatic zone of Sichuan, China, a methodology for debris flow susceptibility assessment on a watershed scale was proposed. The study area was divided into 16,195 watersheds where the correlation of 0.49 between debris flow occurrences and dry-wet status was disclosed by calculating the debris flow average recurrence interval and Palmer Drought Severity Index (PDSI). An around 6-year extreme dry-wet cycle period is observed in the study area based on the PDSI time series. Based on underlying surface characteristics, all watersheds were zoned where PDSI time series patterns were further quantified using autocorrelation function and multifractal spectrum parameters. A debris flow susceptibility assessment model was proposed based on gradient boosting machine involving the dry-wet cycle characteristics of debris flow watersheds before 2021, which deciphers that dry-wet cycles contribute to debris flow susceptibility by 44.2-65.7 %. The proposed model considering dry-wet cycle characteristics classified correctly 80 % of the debris flow watersheds in 2021 as high susceptibility, which further demonstrates the effectiveness of the approach. This work reveals the tight correlation between dry-wet cycles and debris flow occurrences, highlighting the necessity of combating global climate change in mitigating geological hazards.