Spatial characteristics and temporal trends of meteorological and hydrological droughts in northwestern Iran

In recent years, droughts have become more intense and frequent in arid and semi-arid regions like Iran on the one hand, and water demand has been rising on the other hand and, as a result, their impacts are being aggravated. Therefore, the meteorological and hydrological droughts are receiving much more attention. This paper focused on the meteorological and hydrological drought characteristics for the overlapping periods of 3, 6, 9, and 12 months in northwestern Iran over the period of 1981-1982 to 2010-2011. The results showed that the majority of drought events over the reference periods were in the last 15 years from 1995-1996 to 2010-2011. Furthermore, the driest year based on the meteorological drought index was 2007-2008, while it was detected to be 2010-2011 based on the hydrological drought index. The Spearman's rho and Kendall's tau tests were used for the temporal trends analysis of the meteorological and hydrological droughts. The decreasing time series trends were more evident for the streamflow droughts index than for the standardized precipitation index series. In general, the results of the meteorological and hydrological drought trends showed that the study area suffered from the hydrological drought more than meteorological droughts. Moreover, the results revealed that the study area has become drier during the last three decades. Finally, the Spearman correlation analysis was applied to explore the relationships between meteorological and hydrological droughts which indicated a strong correlation between May-Jul-SPI series and the Jun-Aug-SDI series with a value of 0.65.