Assessment and Comparison of SPI and RDI Meteorological Drought Indices in Selected Synoptic Stations of Iran

Abstract

Drought is one of the natural disasters that in comparison with the other disasters are of tremendous importance due to intensity, duration, areal extent, economic damages and long term effects. The aim of a drought index is simple and quantitative determination of fundamental characteristics of drought events that include intensity, duration, frequency and spatial extent. This research was performed on forty one synoptic stations placed in different parts of Iran, with more than thirty years of data. For drought monitoring and the associated characteristics analysis in the given stations, meteorological drought indices of SPI and RDI were used. During this stage, precipitation data and parameters of potential evapotranspiration, that needed by the Penman-Monteith method were implemented for the period of 1967-2005. In this study first, values of SPI and RDI indices for 3 and 6 months and annual time series for all months of years in the specified stations were calculated and then the highest values of these indices and their times of occurrence were computed. Also, the duration of dry periods identified by values less than -1 were computed. In addition, the number of dry periods, highest duration and the time of events for both indices in different time series were also calculated. The results showed that usually in comparison with the SPI, a lower RDI value is calculated (-2) for classification in similar months. Also, the number of months placed in the extremely dry classification for the RDI is higher than the SPI, while occurrence frequency of months with values of less than -0.5 for the RDI is lower than the SPI. Of course, the occurrence frequency of months for values of less than -1 in some stations for the RDI had higher values than the SPI. The results of this study showed that the RDI is more sensitive than the SPI to climatic conditions and so we can’t neglect the role of evapotranspiration in drought assessments. The results also showed that both indices behave in the same manner, but RDI due to use of potential evapotranspiration in similar climatic conditions is more sensitive. Based on the findings of this research, it is recommended that the RDI index should be used as the proper drought index in drought monitoring systems for water resources planning and management in Iran.