Study on the impact of rainfall pattern in small watersheds on rainfall warning index of flash flood event

Rainfall patterns have a potential impact on floods, and the accuracy of peak flow determinations can directly affect the accuracy of rainfall warning index values. Therefore, it is necessary to explore the impact of rainfall pattern on the uncertainty of rainfall warning index for a small watershed. Xiawan, in the small Peihe watershed in Henan Province, China, was used as a case study. Based on an analysis of rainfall characteristics, a fuzzy recognition method was used to identify common rainfall patterns in the study area, following which they were compared with the regional design rainfall pattern. Design rainstorm flood calculation and a water level/flow inversion method were used to analyze the rainfall warning indices for different rainfall patterns and to establish the relationship between rainfall patterns and the values of rainfall warning index. The results show that: (1) rainfall pattern has a major impact on rainfall warning index values, and the rationality of the rainfall pattern requires consideration. (2) Deviations in peak flow between different rainfall patterns were large, and the timing of peak rainfall had a considerable influence on peak flood flow; (3) within the same early warning time interval, the rainfall warning indices where the timing of peak rainfall was at the start and in middle of the event were 1.67 times and 1.39 times that where rainfall peaked at the end, respectively. Further study of the rainfall pattern and its impact on rainfall warning index can provide technical support and an empirical reference value for the analysis and calculation of early warning indices for flash flood events in small watersheds.