Rainfall is one of the main causes of flash floods in mountainous watersheds. The critical rainfall, calculated by the rainfall patterns which represent typical rainfall processes, is an important index for the early warning of flash floods. However, due to the randomness and diversity of the rainfall process, the traditional single rainfall pattern is inconsistent with the actual diversified rainfall process, which brings great challenges to the early warning of flash floods. This paper proposes three characteristic parameters to describe the temporal distribution characteristics of typical rainfall processes. On this basis, the appropriate cumulative distribution functions (CDFs) are chosen to fit the cumulative rainfall-duration curves corresponding to typical rainfall processes, and the probability density functions (PDFs) can be used to represent the characteristic rainfall patterns. The HEC-HMS hydrological model is then used to simulate the rainfall-runoff process, and the critical rainfall corresponding to different characteristic rainfall patterns is calculated with a trial algorithm. The results demonstrate that: (1) the critical rainfall calculated by the designed characteristic rainfall patterns is more accurate than that calculated by the traditional rainfall pattern (TRP), proving that the design method of characteristic rainfall patterns proposed in this paper could increase the accuracy of early warning of flash floods in small watersheds. (2) The effects of the antecedent soil moisture condition (ASMC) and rainfall pattern on the critical rainfall are analyzed, which provides a valuable reference for the analysis and calculation of the critical rainfall of flash floods in small watersheds.

降雨是山区流域山洪暴发的主要原因之一。临界降雨量由代表典型降雨过程的降雨模式计算得出，是山洪暴发预警的重要指标。然而，由于降雨过程的随机性和多样性，传统的单一降雨模式与实际的多样化降雨过程不一致，给山洪暴发预警带来了巨大挑战。本文提出了三个特征参数来描述典型降雨过程的时间分布特征。在此基础上，选择合适的累积分布函数（CDFs）拟合典型降雨过程对应的累积降雨-历时曲线，概率密度函数 (PDF) 可用于表示特征降雨模式。然后利用HEC-HMS水文模型模拟降雨-径流过程，通过试验算法计算不同特征降雨模式对应的临界降雨。结果表明：(1)设计的特征降雨型式计算的临界降雨量比传统降雨型式(TRP)计算的临界降雨量更准确，证明本文提出的特征型降雨型式设计方法可以提高准确度。小流域山洪暴发预警。(2) 分析了前期土壤水分条件(ASMC)和降雨模式对临界降雨的影响，