Relation between deforestation and rainfall intensity - duration thresholds for landslide occurrences: A case study of Mt. Ichifusa, Japan

Vegetation cover is an important factor influencing the occurrence of rainfall-induced landslides. This study analyzed the effect of deforestation on landslide occurrence in terms of an empirical rainfall intensity-duration (I-D) threshold at Mt. Ichifusa, Japan, which is characterized by granite rocks. Rainfall-induced landslides have occurred frequently in the northern part of the Mt. Ichifusa since extensive deforestation began around 1967. We interpreted orthorectified aerial photographs and mapped deforestation areas and landslides in 1969, 1976, 1980, 1985, 1990, 1995, 1999, and 2005. We then analyzed the mean rainfall intensity (I, mm/h) and the duration (D, h) of all rainfall events during 1952-2011 using raingauge data obtained around Mt. Ichifusa. Rainfall events were separated by the absence of rainfall for 24 h. Finally, we examined the I-D thresholds in each period of aerial photographs. We used a quantile-regression method and assumed that the threshold curve is a power law I = α × D^β, where α and β are constants, to determine the I-D threshold. Results show that few landslides occurred at the beginning of deforestation, and that landslides occurred frequently after deforestation, such as during periods of 1976-1980, 1980-1985, and 1990-1995. During these periods, many landslides occurred in the deforested area, although few landslides occurred in the non-deforested area. The I-D analysis indicated that I-D thresholds after deforestation declined to one-third of those at the beginning of deforestation and in the non-deforested area. This result is expected to be important for future forest management and for landslide hazard assessments that should be verified in other deforested areas in Japan.