Sensitivity evaluation of the flash flood warning system introduced to ungauged small mountainous basins in Korea

This study evaluates the uncertainty involved in the determination of the flash flood guidance (FFG) of the flash flood warning system (FFWS) for a small mountainous region (FFWS_MR) in Korea. The sensitivity is evaluated both at each step to determine the FFG and for all steps together. The results show that the relative difference of the FFG is about 50% of the current system, most of which involves the channel width. Especially, the use of some specific empirical equations to estimate the major parameters results in a considerable amount of the relative difference of the FFG. In addition, though only four basins were field-surveyed, it is found that the indirect estimation of the major parameters always introduces some amount of additional uncertainty. In conclusion, accurate estimation of the major parameters must be the most important procedure to derive an accurate FFG, among which the channel cross-section at the exit of the basin lies at the center.