Abstract: Confusion arises in practical application due to multiple definitions of rainfall threshold for flash flood warning. In view of this, we analyzed and summarized the existing definitions, and redefined the rainfall threshold for early warning into two types planned rainfall threshold and emergency rainfall threshold. In line with the mechanism of flow yield and confluence, we proposed a simple and feasible method of calculating the emergency rainfall threshold for ungauged areas. Comparison of calculation result between the two types indicates that the variations of planned rainfall threshold and emergency rainfall threshold are featured with different rules. In forming prevention and control measures for flash flood, we should make clear the objectives (for early warning or for engineering design) in determining the indicator type to realize a reasonable prevention and control.

Key words: flash flood disaster, early warning period, planned rainfall threshold for early warning, emergency rainfall threshold, ungauged area