Analysis of Rainfall Trend and Evaluation of Weather Droughts of Iran Using the Herbst Method

Drought monitoring and evaluation in water resources systems, are basic steps in systems analysis as well as the optimal water resources allocation for different demands. Therefore, in this study, whole of Iran country was selected as the study area, and it was attempted to determine the simultaneous droughts which had been occurred in the period of 1981-2010, using monthly rainfall data of 31 synoptic stations. In order to perform the drought monitoring, the Herbst drought index was used and drought intensity and duration were determined for the studied synoptic stations. Additionally, the IDW method was used for interpolation and delineation of drought intensities and durations. The study results showed that in the periods of 1998- 2000 and 2007-2009, the country had faced droughts. Northern and Southern coastal regions and Central region exhibited less susceptibility to drought periods in comparison to the other regions. Also, the East and West of the country, which are the rainfall systems entrances, are more sensitive to rainfall reduction and thus, to drought. According to the results of the spearman's test, the most severity of negative trend belonged to the stations in central, South-east and South-west of the country.