Rainfall threshold and soil moisture indexes for the initiation of landslide in Banjarmangu sub-district, central Java, Indonesia

Rainfall is an identified trigger of landslides, and researchers have long attempted to ensure the total precipitation that can trigger slope failures under different local climate condition, a problem of scientific interest. In this study, we propose new empirical rainfall thresholds combining with antecedent soil moisture indexes for analysing the initiation of landslides in Banjarmangu districts, located in Central Java. Daily rainfall and landslide information were obtained from Banjarnegara Geophysical Station and Regional Agency for Disaster Management (BPBD); evapotranspiration data were obtained from the United States Geological Survey (USGS). We plotted the mean rainfall intensity (mm per hour) and rainfall duration values in logarithmic function. For the results, we found that the increase of rainfall duration led the decrease of rainfall intensity defined by intensity-duration (ID) threshold I=43.2D -0.87. In addition, we determine a formula for a cumulative rainfall threshold (CT), P-3=120.75-0.475P15, recognized by rainfall amounts (in mm) during the last 3 days and previous 15 days, was constructed from landslides historical data from 2011 to 2017. Both thresholds have to be used with the combination of soil moisture index for failure above 0 mm at positive pore pressure for developing a landslide warning.