Changing spectral patterns of long‐term drought propensity in Iran through reliability–resilience–vulnerability‐based Drought Management Index

Abstract

AbstractThis study aims to investigate the change in spectral patterns of droughts in Iran based on the concepts of reliability, resilience, and vulnerability (RRV) in the context of time. To achieve this goal, RRV values were first estimated based on monthly gridded soil moisture data for a period of 64 years (1955–2018) for the framework of Iran's political borders. The pairwise comparison of the values of the three variables RRV indicates a positive relationship between reliability–resilience and a negative relationship between reliability–vulnerability and resilience–vulnerability. In order to obtain the Joint Probability Distribution between reliability–vulnerability, Drought Management Index (DMI) for all the gridded points within Iran's political borders were calculated by fitting the Gaussian copula. According to this index, drought is an unfavourable climatic phenomenon that is associated with increased Vulnerability and decreased Reliability. The temporal and spatial changes of this index indicate that DMI values are always high for the south, east and centre of Iran and low for the western and northern half. To study the change in spectral patterns of droughts in Iran, the 64‐year period studied (1955–2018) was divided into two equal periods of 32 years. Then, using spectral analysis through Fourier transform method, the first three periodicities related to each period that had the highest powers were extracted. Finally, their periodicity was compared. The results indicated that the drought periodicities in the north‐west, north‐east, west, south‐west, and south of Iran have become longer, that is, their nature has been changed from shorter periodicities to longer ones. This change of nature in their first three periodicities has been sometimes from 2–10 years to 2–15 years, from 2–10 years to 3–30 years, and from 3–8 years to 8–15 years. However, the periodicities of droughts in the eastern, south‐eastern, and central Iran has changed from longer periodicities to shorter ones. It has been changed from 3–15 years to 2–6 years, from 3–15 years to 2–3 years, and from 3–30 years to 3–15 years.