Evaluation of Four Meteorological Drought Indices in Hamedan Province

This research evaluates the meteorological drought spells by multi-dimensional comparison of fourmeteorological drought indices supported with 35 years rainfall data (1967-2001) series records for 22 rain gauge stations in a cold semi-arid region located in west of Iran. Drought indices include: Percent of Normal Precipitation Index (PNPI), Z Standard Index (Z), Rainfall Anomaly Index (RAI) and Standard Precipitation Index (SPI). After calculation of drought indices for the desire time scale, they were evaluated and compared using correlation relations. Using Cluster Analysis technique, the drought status in the region was classified into different categories. The criteria were the statistical ones based on the succession of dry, normal and wet years. It was found that both Z and RAI indices were statistically similar and gave almost similar results in drought evaluations. By correlation, all indices of PNPI-Z, PNPI-RAI and Z-RAI in all stations indicated high reciprocal correlations in addition to Z-RAI which showed similar results explaining drought conditions. Finally, from the correlation relationships and the output den do-grams, RAI and Z were respectively identified as the best indices for meteorological drought evaluations in the region, due to more relevance to the reality than the rest. These indices showed that four droughts have occurred during years of 1967-1968, 1976-77, 1983-84, 1988-89, 1996-97 and 1998-99. Despite the common use of SPI in monitoring the meteorological drought in Iran, this index is not recommended for cold semi-arid climate conditions.