In recent years, debris flows occurred almost annually at Tansan-dani stream in Mt. Unzen-Fugen-Dake. In a previous study, the authors proposed a debris-flow trigger model hypothesis. The hypothesis essentially assumed the increase in surface water near the exposure of pre-eruptive ground surface with a low infiltration rate. This study seeks to verify this assumption by evaluating the rainfall-runoff response through on-site hydrological observations. Firstly, images captured by time-lapse

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... ras were examined to identify when surface water appeared. Next, in accordance with the observed timing of the emergence of surface water, critical success index (CSI) was applied to optimize the half-life time of antecedent precipitation index and time-window of accumulative rainfall. Based on these optimized figures, runoff response was examined as follows : (i) At observation site A located on pyroclastic-flow deposits 20 meters upstream of exposed pre-eruptive ground, runoff was observed to respond sensitively to rainfall intensity. (ii) At observation site B located on exposed pre-eruptive ground, runoff with a small flow rate continued for more than a month. In addition, a relatively sensitive response to rainfall intensity was observed with increases in the flow rate.