efficiency of some meteorological drought indices in different time scales (case study: tajan basin, iran)

drought is a climatic phenomenon that causes much detrimental influence on ecological environments. this research investigates the performance of meteorological drought indices in different time scales in thetajan basin in iran. indices undergone in this study include: percent normal precipitation index (pnpi), z-score index (zsi), standard precipitation index (spi), china z index (czi), modified czi (mzci), and decile precipitation index (dpi). in this study, we used data of annual and monthly precipitation from meteorological stations in the basin. drought indices were determined at time scales of 1, 3, 6, 9, 12, 18, 24 year of minimum precipitation with very intense drought and the method of correlation coefficients between drought indices values and monthly precipitation. the results showed that dpi was the best index at annual time scale, while at the time scale of 1,6,12 and 24-months, pnpi was the best index. mczi was the best index for time scales of 9 and 48-months, and the zci had the maximum efficiency for a three-month scale.