Abstract. This study analyzed the variability in the number of rainfall events related to debris-flow occurrence in the Chenyulan stream watershed located in central Taiwan. Rainfall data between 1970 and 2009 measured at three meteorological stations nearby/in the watershed were collected and used to determine the corresponding regional average rainfall for the watershed. Data on debris-flow events between 1985 and 2009 were collected and used to study their dependence on regional average rainfall. The maximum 24-h regional rainfall R d was used to analyze the number of rainfall events N r , the number of rainfall events that triggered debris flows N d , and the probability of debris-flows occurrences P . The variation trends in N r , N d and P over recent decades under three rainfall conditions (R d > 20,230, and 580 mm) related to debris-flow occurrence were analyzed. In addition, the influences of the Chi-Chi earthquake on N d and P were presented. The results showed that the rainfall events with R d > 20 mm during the earthquakeaffected period (2000)(2001)(2002)(2003)(2004) strongly responded to the increases in the average number of rainfall events that triggered debris flows and the average probability of debris-flows occurrences. The number of rainfall events with R d > 230 mm (the lower boundary for the rainfall ever triggering debris flow before the Chi-Chi earthquake), and R d > 580 mm (the lower boundary for extreme rainfall ever triggering numerous debris flows) in the Chenyulan stream watershed increased after 2000. The increase in the number of extreme rainfall events with R d > 580 mm augmented the number of rainfall events ever triggering numerous debris flows in the last decade. The increase in both the number of rainfall events that ever triggered debris flows and the probability of debrisflow occurrences was greater in the last decade

This paper reports the variation in rainfall characteristics associated with debris flows in the Chenyulan watershed, central Taiwan, between 1963 and 2009. The maximum hourly rainfall I_m, the maximum 24 h rainfall R_d, and the rainfall index RI (defined as the product R_dI_m) were analysed for each rainfall event that triggered a debris flow within the watershed. The corresponding number of debris flows initiated by each rainfall event (N) was also investigated via image analysis and/or field investigation. The relationship between N and RI was analysed. Higher RI of a rainfall event would trigger a larger number of debris flows. This paper also discusses the effects of the Chi-Chi earthquake (CCE) on this relationship and on debris flow initiation. The results showed that the critical RI for debris flow initiation had significant variations and was significantly lower in the years immediately following the CCE of 1999, but appeared to revert to the pre-earthquake condition about five years later. Under the same extreme rainfall event of RI = 365 cm² h⁻¹, the value of N in the CCE-affected period could be six times larger than that in the non-CCE-affected periods