The intensity-duration (I-D) threshold is considered an effective indicator for landslides triggered by short-term highintensity rainfall and long-term low-intensity rainfall. However, previous studies have not considered the influence of antecedent rainfall. Herein, we analyzed hourly rainfall data for 613 shallow landslides that occurred from 1963 to 2018 in South Korea to obtain rainfall thresholds and investigated the effect of antecedent rainfall conditions on threshold variations. The I-D and I MAP -D, which is normalized by dividing I by mean annual precipitation (MAP), thresholds were determined to be I = 10.40D -0.31 and I MAP = 0.006D -0.26 (4 ≤ D (h) ≤ 84), respectively, at the 2nd percentile level through quantile regression analysis. These thresholds were lower than other local thresholds (i.e., excluding global and regional thresholds), suggesting that the southern region of the Korean Peninsula was more susceptible to rainfall-induced landslides. Although the effective length of antecedent rainfall was not presented herein, the I-D threshold of landslide-triggering rainfall was confirmed to be low for the absolute and/or calibrated antecedent rainfalls greater than event cumulative rainfall at 5, 7, 10, and 20 days prior to the event. Such differences in I can be greater at shorter durations; however, these differences gradually decrease as D increases, suggesting that they lose their effect as soil water content increases. The results of the current study can improve the understanding of the effect of antecedent rainfall conditions on landslide occurrence and should be further tested with respect to the hydrologic response of hillslopes by considering regional climate and local site conditions.

26Large wood (LW) plays an important role in river ecosystems, but LW-laden floods 27 may cause serious damage to human lives and property. The relationship between 28 precipitation patterns and variations in LW distribution and export at the watershed scale is 29 poorly understood. To explore these linkages, we examined differences in LW distribution as 30 a function of channel morphologies in six watersheds located in southern and northern Japan, 31 and analyzed the impacts of different precipitation patterns on the fluvial export of LW from 32 river catchments. In southern Japan, intense rainfalls caused by typhoons or localized 33 torrential downpours initiate landslides and debris flows that introduce massive amounts of 34 LW into channels. Gravel bars formed by frequent flood events are widely prevalent, and the 35 LW temporarily stored on these bars is frequently moved and/or broken into smaller pieces 36 by floods. In these systems fluvial export of LW is supply-limited, with smaller 37 accumulations and shorter residence times than in northern Japan. Conversely, in northern 38 Japan, where typhoons and torrential downpours rarely occur, LW is mostly recruited by 39 bank erosion, tree mortality and windthrow into channels, rather than by landslides and debris 40 flows. Recruited pieces accumulate in log jams on valley floors, particularly on floodplains 41 supporting mature forests, resulting in larger accumulations and longer residence times. In 42 these watersheds fluvial export of LW is transport-limited, and the pieces gradually 43 decompose during long-term storage as log jams. 44 45

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Recent climate change caused by global warming has affected the environment in Korea, resulting in increased precipitation and frequency and magnitudes of typhoons. From 1994-2003, a mean of 2.3 days/year experienced heavy rainfall (≥80 mm), in contrast to a mean of 1.6 days/year from 1954-1963. Of the ten typhoons that have resulted in the maximum daily rainfall over the last ten decades, seven occurred between 1990 and 2007. The maximum instantaneous wind velocity of typhoons has greatly increased from 20 m/sec (in the 1970s) to 40 m/sec (in the 2000s). Furthermore, increases in the occurrence and scale of forest fires and landslides, as well as increased infrastructure and land use, contribute to sediment disasters. Accordingly, environmental changes and human-induced factors have resulted in significant increases in the magnitude and frequency of natural disasters, especially in the Gangwon region on the east coast of Korea. Therefore, in 2004 the Korea Forest Service instituted an integrated and environmentally-friendly system for forest management, which has helped prevent sediment disasters. Multiple prevention strategies are also required in addition to these systemic changes to forest structure management, including control of hill-slope erosion and torrent erosion, debris flow mitigation, water storage and slit dams, grade-stabilization structures and forest improvement, and watershed management.

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