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 aswell as the optimal water resources allocation for different demands. therefore, in this study, whole of irancountry was selected as the study area, and it was attempted to determine the simultaneous droughts whichhad been occurred in the period of 1981-2010, using monthly rainfall data of 31 synoptic stations. in order toperform the drought monitoring, the herbst drought index was used and drought intensity and duration weredetermined for the studied synoptic stations. additionally, the idw method was used for interpolation anddelineation of drought intensities and durations. the study results showed that in the periods of 1998- 2000and 2007-2009, the country had faced droughts. northern and southern coastal regions and central regionexhibited less susceptibility to drought periods in comparison to the other regions. also, the east and west ofthe country, which are the rainfall systems entrances, are more sensitive to rainfall reduction and thus, todrought. according to the results of the spearman's test, the most severity of negative trend belonged to thestations in central, south-east and south-west of the country.