The influence of land use and rainfall on shallow landslides in tanralili sub-watershed, Indonesia

Land cover is an important factor affecting the occurrence of shallow landslides triggered by high intensity rainfall. The objectives of this study are the analyses of the influences of land use and rainfall on shallow landslides using the normalized difference vegetation index (NDV1) and antecedent rainfall threshold. This research was conducted in Tanralili sub-watershed, South Sulawesi Province, Indonesia. The analysis of NDVI using Landsat time series data of 2000,2003,2006,2009, 2011 and 2013. The result indicated that in the course of 14 years high vegetation density land cover (81% to 100%) has lost around 24.21% or 6219 ha. The vegetation density decrease is caused mainly by land use change, especially the conversion of shrubs and forests into mixed dryland farming. 76.67% of shallow landslides occurred are concentrated in mixed dryland farming on the low vegetation density (0 to 40%). The analysis of the absolute and calibrated antecedent rainfall associated with each major shallow landslide event showed that shallow landslides occurred in short duration (1 day) with a high intensity 225 mm/day and longer duration (1 month) with a lower intensity 13 mm/day. The rainfall threshold for shallow landslides in the study area are ruled by the function I = 25.5D"'10, where I is the average rainfall intensity in mm/day and D is duration of rainfall in days.