Reasonable rainfall threshold index is the key to ensure accuracy of debris flow forewarning. It is significant for the study of the formation mechanism of debris flow

the prediction of the future activities characteristics and the guidance of the prevention and controlling engineering. Most of the mountain areas have few rainfall and hazards data; therefore

both the traditional demonstration method and frequency calculated method can’t satisfied the debris flow forewarning. Through studying the characteristics of the forewarning regions

including the rainfall characteristics

hydrological characteristics

and underlying conditions

a method to calculate the rainfall threshold is put forward based on the initiation mechanism of hydraulic debris flow. Firstly

the critical water depth for the initiation of debris flows is calculated according to the topography conditions and physical characteristics of the loose solid materials. Then

the rainfall threshold is calculated combined with the mechanism of runoff yield and confluence

the change laws of rainfall with altitude

and the rainfall pattern. The results have been used in Shijia Gully in Ningnan city

Sichuan province; and it is shown that this method is reasonable and feasible. This method resolves the problem of debris flow forewarning in the data lack areas and provides a new thread for the debris flow forewarning in the mountain areas.