Drought duration is defined from different viewpoints. Estimation methods of drought duration return

period are different in terms of drought duration definition. This paper is aimed to perform six estimation

methods of drought duration return period at Kermanshah station, as a case study. The standardized precipitation

index (SPI) is used to monitor drought based on historical and synthetic series of total precipitation

data in annual and monthly timescales. Probability distribution of drought duration was analyzed for

time series of SPI. Different methods for estimating return period of drought duration on historical and synthetic

time series of SPI were applied. Findings answered two main questions: a) if the length of historical

record was enough to estimate drought return period at the station of interest? b) If the obtained return periods

based on different methods have enough conformity with empirically obtained return period? Results

are discussed in details in this paper.