A spectral analysis of Iran's droughts

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Daneshmand Hojjat; MAHMOUDI PEYMAN

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Journal: IRANIAN JOURNAL OF GEOPHYSICS Year:2016 | Volume:10 | Issue:4 Page(s): 28-47

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Title

A spectral analysis of Iran's droughts

Author(s)

Daneshmand Hojjat | MAHMOUDI PEYMAN | Issue Writer Certificate

Keywords

droughtQ3

effective drought indexQ3

spectral analysisQ1

cycleQ2

IranQ2

Abstract

Identification of periodic droughts can provide a scientific tool for predicting the occurrence of this environmental risk. One of the most important methods that can be used for detection of periodic droughts is the spectral analysis or frequency-domain analysis. The purpose of this study is to detect the periodic behavior of Iran’ s monthly droughts. To this aim, use has been made of the monthly rainfall statistics of 41 synoptic stations for a period of 31 years (1983– 2013) obtained from the IR of Iran Meteorological Organization (IRIMO). First, to identify considerable differences in droughts, the “ effective drought index” (EDI) was employed. Assigning a numerical value to each rainfall event on a monthly time scale is the main objective of the EDI in order to compare the areas with different climates based on it. Then the spectral analysis by Fourier transform was used to identify the dominant available periods in the stations’ drought time series. In this analysis, Radix-2 Fast Fourier Transform was used. Since Fourier transform leads to difficulties in the spectral analysis of non-stationary series, first the stationary condition of the EDI monthly time series was investigated for all of the 41 stations used in this study. In those stations with a non-stationary condition, the Box– Cox procedure was applied to make them stationary. The results were classified into five areas. The results showed that the dominant periods in the time series of Iran’ s droughts vary greatly from periods of 2 to 30 years. In addition to the dominant short-term periods, droughts in the northwest of Iran have two dominant long-term periods of 10 and 30 years. Droughts in the southern coast of the Caspian Sea have shorter periodicities. So with the exception of the two stations of Ramsar and Ardebil with the periodicities of 30 and 15 years, the other stations have periods of less than 10 years. The dominant periods in most stations in the northern districts of central Iran are short-term ones, and the longest periodicity in this area is 15 years. The southern districts of the central Iran have a more consistent spectral behavior. In this area, the dominant period with the highest power is the period of 30 years. All of the stations in this area with the exception of Mashhad station have a 30-year period in their first three components. The 10-year period, as the second dominant period, can also be seen in half of the stations in this area. The southeast area does not exhibit any specific behavior for the dominant periods. The longest periodicity in Chabahar station is a 10-year period. The two stations in Zabol and Iranshahr have shown the longest period of 30 years and the long-term period seen in Zahedan is a 15-year one. In addition to these long-term periods, a period of 2. 5 to 3 years can play the role of the dominant short-term period in this area. In general, for the long-term behavior, the two periods of 10 and 30 years can be considered as being the dominant periods across Iran. Among the long-term periods, the frequency of occurrence of these two periods is maximum in the first component and reduces in the other two components. In the third component, the minimum frequency of occurrence of these two periods is seen.

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APA: Copy

Daneshmand, Hojjat, & MAHMOUDI, PEYMAN. (2016). A spectral analysis of Iran's droughts. IRANIAN JOURNAL OF GEOPHYSICS, 10(4 ), 28-47. SID. https://sid.ir/paper/133700/en

Vancouver: Copy

Daneshmand Hojjat, MAHMOUDI PEYMAN. A spectral analysis of Iran's droughts. IRANIAN JOURNAL OF GEOPHYSICS[Internet]. 2016;10(4 ):28-47. Available from: https://sid.ir/paper/133700/en

IEEE: Copy

Hojjat Daneshmand, and PEYMAN MAHMOUDI, “A spectral analysis of Iran's droughts,” IRANIAN JOURNAL OF GEOPHYSICS, vol. 10, no. 4 , pp. 28–47, 2016, [Online]. Available: https://sid.ir/paper/133700/en

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