comparative study of four meteorological drought index based on relative yield of rain fed wheat in hamedan province

quantitative measurement of meteorological drought, z score(z), rainfall anomaly (rai), standard precipitation (spi) indices and percent of normal precipitation (pnpi) based upon seasonal and annual rainfall data of the 15 meteorological stations of meteorological in hamedan province were used. also, in studying influences of drought on agriculture relative yield index of wheat was used. the results have shown that z, rai and spi indices in seasonal and annual scales and pnpi in seasonal scale are good estimator of drought intensity and drought duration, irrespective of indices kind mostly was medium and lower than medium. z, rai and spi indices were same with agronomic index of relative wheat yield, but spi index based upon intensity of droughts had more similarity with agronomic index of relative wheat yield. the results show that hamedan region have suffered from one drought which lasts from a two years lasting drought period in each decade. it has been also experienced a very severe drought during the last 30 years. occurennce of more droughts in the region might be resulted from shortening the drought return periods, and increase of low intensity drought events in the region.