Toward analyzing meteorological droughts in western Iran: a multivariate approach based on vine copulas

Although two-dimensional analyses of drought have been frequently performed in recent years, considering multiple drought variables simultaneously can provide more accurate information. In this study, drought characteristics (i.e., severity, duration, peak SPEI, and inter-arrival time) were modeled using the D-vine structure in western Iran in a 30-year period (1990 to 2019). First, SPEI was calculated for all stations using monthly precipitation values and evapotranspiration estimated by Penman–Monteith FAO method. Then, three different scenarios for creating the D-vine structures were investigated. The results showed that 865 drought events in western Iran during the studied period. For instance, the average drought duration, severity, peak, and inter-arrival time were 2.67 months, 3 months, 1.07 months, and 7.32 months, respectively, at the Dehloran station. In each scenario, the best copula family for each paired variable was selected based on various criteria. In all scenarios, the most appropriate D-vine structure was identified at 13 investigated stations. The Frank copula had the best fitness in the first and second scenarios, while the Gaussian copula had the best fitness in the third scenario. To evaluate the accuracy of the D-vine model simulation, the simulated and observed variables were compared at all stations that indicated good agreement between the simulated and the observed variables. Finally, the spatial distribution of the bivariate drought return period for the study area was presented. The results showed that the joint return period of severity and peak SPEI is about 201 years. The joint return period of droughts in the central, eastern, and western regions is also short. Zoning maps have shown that most parts of the study area are moderately susceptible to drought, as the southern and western parts of the studied area are more susceptible to severe drought. Therefore, it is necessary to consider the required management strategies for the western part of Iran to cope with drought.