Determination of effective antecedent rainfall for debris flow forecast based on soil moisture content observation in Jiangjia Gully, China

Abstract

Effective antecedent rainfall (EAR), one of important factors for debris flow forecast, has different ways of influencing debris flow initiation for different types of debris flows. For soil-mechanics-typed debris flow, EAR impacts its initiation by changing the soil moisture content, and for hydromechanics-typed debris flow, EAR impacts its initiation by increasing the surface runoff. For this reason, the methods of determining EAR for different types of debris flows are different. The method for the former type of debris flow is discussed in this paper. Under the assumptions that soil moisture content due to antecedent rainfall decays in the same way as EAR, and that the decays for different events are mutually independent, the variation of EAR can be carried out through analyzing the variation of soil moisture content. The EAR for debris flow forecast can then be determined. The decaying of soil moisture content had been analyzed according to the field observation of rainfall and soil moisture content in Jiangjia Gully in Yunan Province, where debris flows are caused by soil mechanical changing triggered by rainfall. Based on the field observation and data analysis, the formula of calculating EAR for Jiangjia Gully is made out with least-squares procedure.