Gholamreza Zehtabian - Professors, Faculty of Natural Resources ,University of Tehran,Karaj, Iran.
Kamran Karimi - M.Sc. Student of Deserts Region Management ,Faculty of Natural ResourcesUniversity of Tehran, Karaj,Iran.
Sara Nakhaee nezhad fard - M.Sc. Student of Deserts Region Management ,Faculty of Natural ResourcesUniversity of Tehran, Karaj,Iran.
Mahsa Mirdashtvan - M.Sc. Student of Watershed Management engineering, Faculty of Natural ResourcesUniversity of Tehran, Karaj, Iran.

Comparability analyses are performed to investigate similarities/differences of the standard precipitation index (SPI) and the reconnaissance drought index (RDI), respectively, utilizing precipitation and ratio of precipitation over potential evapotranspiration (ET0). Data are from six synoptic stations in southKhorasan province in Iran.At the first for calculating SPI, the elevation-precipitation regression of each period was obtained then this relationship was applied on DEM layer using GIS software. The result showed that the most severity droughts occurred in 2008, 2000, 2006 and 2001 respectively. In order toobtain RDI index, we used yearly temperature, precipitation data from the stations and potential evapotranspiration were obtained by Torrentwhite method. The resultshowed that 4 stations classified in extremely and severely classes and there was no medium and small class in 2008. The results of RDIindicate that the frequency of droughts with high severity in this area. The calculation of the standardized RDI seems to be as complicated as the computation of the SPI.

SPI, RDI, ArcGIS9.3, Drought, South Khorasan