Analysis of drought transitions using log-linear models in Iran

Abstract

In this study the frequencies of drought categories are evaluated by SPI index in monthly time scale using data of seven stations in Iran. SPI index data of total period (60 years) sets were divided into three periods of 20 years and a log-linear model approach has been used to investigate differences relative to drought class transitions among these three periods. The drought class transitions were measured for three periods to form a three-dimensional contingency table. According to the results in Mashhad, Tehran, Tabriz, Shiraz, Esfahan and Kerman stations had no significant changes at 95% level to number of drought classes, in Zahedan station the drought severity had increasing trend. Results show that the drought behaviour in second period is similar to the third. If just the second and third periods were compared it could be concluded that droughts were aggravating and this behaviour could be attributed to climate change.