Reconnaissance Drought Index (RDI) is based on ratio of precipitation to evapotranspiration (ETo) for any period of time from 1 month to the entire year. In this index value of ETo were calculated based on mean temperature by Thornthwaite (Th) method. In order to evaluate the effects of evapotranspiration estimation method on drought frequency according to RDI(Th) index, some meteorological parameters of 6 Synoptic Stations in arid and semi arid regions of Iran during 1958- 2007, have been conducted. After some statistical tests, the values of RDI for each station during the mentioned time were calculated. Then, from 11 empirical and semi empirical equations of ETo estimations, the best equation for each situations of lack of variables in each stations were selected. Subsequently, time series of ETo from the best equation were prepared. After that the new RDI values so called RDI(Select) were established. Finally the frequency of drought classes according to RDI and RDI(Select) were compared. The results showed that the Th equation, may underestimated ETo values about 610 mm.year-1 comparing to the actual in selected stations. This underestimation leads to changing the frequency and/or displacement of drought classes. Displacement accrued in all drought classes at Tehran station and the frequency of moderate dry and normal classes were changed from 7 and 33 to 5 and 36 events respectively during 50 years. Kay words: Drought, Reconnaissance Drought Index (RDI), Evapotranspiration, empirical equations, Iran.

Reconnaissance Drought Index (RDI) is based on ratio of precipitation to evapotranspiration (ETo) for any period of time from 1 month to the entire year. In this index value of ETo were calculated based on mean temperature by Thornthwaite (Th) method. In order to evaluate the effects of evapotran...