Estimation and Assessment of Temporal Stability of Periodicities of Droughts in Iran

The main objective of the present study was to identify the periodic behavior of monthly droughts in Iran. For this purpose, monthly precipitation data obtained from 41 synoptic stations for a period of 22 years (1992–2013) were used. To identify the frequency of different severities of drought, effective drought index (EDI) was calculated at monthly scale and then spectral analysis of time series of EDI was conducted for each station. Temporal stability of these periods was also estimated using the wavelet transform. The results of spectral analysis of monthly droughts in Iran showed that periodicities in time series of Iran’s droughts are very diverse and involve periods of 2 to 22 years. Assessment of temporal stability of periodicities using the wavelet transform also indicated that there is a dominant periodic interval (significant at a level of α = 0.1) only in 8 stations of Ardebil, Iranshahr, Zahedan, Bandar Lengeh, Shahroud, Khorramabad, Oroomieh, and Shahre Kord. Studying the variability of time series of EDI in 41 stations showed that the severity of wet years is declining and the severity of droughts is increasing. It was also observed that periodic intervals were shorter at the beginning but longer at the end of time series. This suggests that the probability of incidence of drought in Iran is increasing. Additionally, a reduction can be observed in interval between incidences of droughts in Iran. This means that the interval between occurrence of droughts were longer at the beginning and then shorter at the end of times series.