This study is on the purpose of leading Geomorphoclimatic Instantaneous Unit Hydrograph(GcIUH) by
using GIS Techniques, and estimating trigger rainfall for predicting flash flood in Seolmacheon catchment, mountain
river watershed. This study leads GcIUH by using GIS techniques, calculates NRCS-CN values for effective rainfall
rate, and analyzes 2011 main rainfall events using estimated GcIUH. According to the results, the case of Memorial
bridge does not exceed the amount of threshold runoff, however, the case of Sabang bridge shows that simulated
peak flow, approximately 149.4㎥/s, exceeds the threshold runoff. To estimate trigger rainfall, this study determines
the depth of 50 year-frequency designed flood amount as a threshold water depth, and estimates trigger rainfall of
flash flood in consideration of duration. Hereafter, this study will analyze various flood events, estimate the
appropriateness of trigger rainfall as well as threshold runoff through this analysis, and develop prototype of Flash
Flood Prediction System which is considered the characteristics of mountain river watershed on the basis of this
estimation.