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Abstract The Urmia Lake Basin as the world's second largest salt lake has experienced severe drought during recent years. The purpose of this study is to analyse the bivariate characteristics of drought (i.e., duration and severity) using two indices including SPI (standard precipitation index) and SPImod (modified SPI) associated with copula functions. For this purpose, rainfall data of six stations were used for the period of 1971–2017. At first, the characteristics of drought were extracted using the two indices. Then, through coding in the MATLAB software environment, eight families of Archimedean copula functions were applied. The simultaneous return period and conditional and Kendall returns were also investigated. The result showed that the Joe copula function was the best predictor for the analysis of both intensity and duration of drought for the study area. The correlation coefficients of Spearman, linear correlation and Tau Kendall computed for the SPI of stations were >0.65, >0.72 and >0.48, respectively, while all of them were significant. At a given critical probability level, t, the value of the Kendall return period was much greater than the standard return period, so that this difference increased with increasing t value. The results obtained from the time series of indices indicated that at least 40% of the months were dry, and the severity of droughts in the Urmia station was much higher than other stations during the studied period. Moreover, SPImod to a large extent eliminates the disadvantages of conventional SPI and takes into account seasonal variations of precipitation in the calculation of SPI.

Abstract Wind velocity and its direction are important erosive factors affecting sand drift potential and dune morphology in arid and semi‐arid environments. The study assesses sand drift potential using wind meteorological data acquired from eight synoptic stations of Kerman province in southeastern Iran for a statistical period of 2006–2010. Three diagrams including wind rose, storm rose and sand rose were plotted for the selected stations using WR Plot View.8 and Sand Rose Graph 3.0. The analyses were performed at seasonal and mean annual time scales. The unidirectional index and wind energy classification were compared with sand dune morphology obtained from satellite images. The seasonal analysis indicated intermediate directional variability and high energy winds, with the most erosive winds occurring in winter. The total sand flux in all directions and discharge sand flow in the resultant drift direction ranged from 53.4 to 319.1 and from 13.8 to 247.6 t·m−1·year−1, respectively. The transverse dune was the main type of sand dune detected in the region with an intermediate directional variability, implying positive sand supplies. Dune morphology coincided reasonably well with the resultant drift direction. An appropriate match was found between the wind data analyses and the morphology of dunes observed in the satellite images.