assessment of the palmer drought severity index in arid and semi arid rangeland: (case study: qom province, iran)

drought is a normal, recurring feature of climate phenomena; it may occur virtually in all climatic regions. the effects of drought accumulate slowly and its impacts are spread over a larger geographical area than other natural hazards. drought is a natural hazard originating from a deficiency of precipitation that result in a water shortage for some activities or some groups and is often associated with other climatic factors such as high temperatures, high winds and low relative humidity that can aggravate the severity of the event. iran is frequently affected by recurring droughts. in this research the qom province with an area 11500 km2 that located in an arid and semi arid region of iran was selected. eighth site of rangeland were chosen that represented rangeland situation in the study area. in these sites relative factors such as plant cover , density, yield, regeneration ,land cover were measured in 60 plots with 2 m2 area along the 4 transect with 400 m along. rangeland yield was measured with the cutting and weighting method in a quarter of plots, and the total yield in each site was obtained from the regression models between plant cover and species yield. data series were cumulated for 9 years (1998-2006). palmer drought severity index (pdsi) carried out in this study. the objective of this study is to performance of the pdsi to assessment of drought in the rangeland of the qom. the analysis of results indicated that each site growth season is different from another. the rangeland yield is not related to the annual rainfall, but the best relationship was between rainfall in growth season and range yield. also the study showed that the highest frequency of significant models is related to march-july period.