Time interval identification of the occurrences of meteorological and hydrological droughts in Zayandeh-Rud basin

The objective of this study was to analyze the relationship between meteorological and hydrological droughts using related variables in the Zayandeh-Rud basin. Standardized Precipitation Index (SPI) and Surface Water supply Index (SWSI) were usede to evaluate the temporal and spatial variations of meteorological and hydrological droughts were used. Drought monitoring in the basin shows that the phenomenon has occurred in different years during the statistical periods. Andysis of the 12-months time scale of SPI proves the occurrence of the main drought in all of the basin area during the period of 1999 to 2001. The hydrological drought analysis also shows that the most severe drought has occurred between October 1998 to September 2001. Moreover، analysis of the correlation between meteorological and hydrological droughts indicates a difference in start time of droughts diagnosed by using these tow drought indices. However، this lag time is varied on average of 7 months with a variation of 3 to 11 months.