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Drought is the most damaging environmental phenomenon, especially in arid and semiarid regions where there are considerable issues in regard to water resources management. In this study, trend of changes in drought severity based on the application of Reconnaissance Drought Index (RDI) evaluated (from 1980 to 2010 at 16 synoptic stations). For assessment drought severity, using linear regression, Mann-Kendall and Spearman’s Rho tests at the 5 % significance level, the trends in different time series of RDI (1, 3, 6 and 12 monthly time series) were investigated. According to results: based on linear regression, the Most droughts occurred in monthly (March) and annually time scale (at 37.5 % of stations). Based on Mann-Kendall and Spearman’s Rho tests, the most of droughts occurred in monthly time scale (July), in this month respectively 43.7 and 50 % of stations have significant decreasing trend in RDI index. Results indicated the frequent decreasing trends in RDI time series for short term time series more (one monthly time series) than long term ones. Decreasing trend in RDI time series means the increasing trend in drought severity.