Drought characteristics based on Reconnaissance Drought Index and its variations in different time periods and regions of Iran

Background and objectives: Drought is a climatic phenomenon that occurs in all climatic zones. Drought has three characteristics of duration, severity and magnitude. Reconnaissance Drought Index (RDI) is a indicate for drought. Climate change and subsequent changes in the characteristics of the phenomenon of drought and its impact on terrestrial ecosystems caused concern among researchers in human societies. َAthough drought occurred in all climate zones, but its features may vary by region. Characteristic’s drought may changes at time during due to word warming (Climate change). Therefore, propose of this study is investigating drought characteristics (including: severity, intensity, magnitude, time interval and frequency of drought) and their statistical Characteristics based on RDI in 8 synoptic station at time period of 60 years.
Materials and methods: In order to monitor droughts for 8 selected synoptic stations of Esfehan, Ahwaz, Babolsar, Tabriz, Tehran, Khoramabad, Zahedan and Mashhad from different weather's conditions. Run Theory is a common method in analysis of drought. In addition to the three above characteristics of drought (severity, intensity, magnitude), in this study, other features of droughts include time interval and number of droughts is introduced and studied. While at the same time of drought monitoring, drought characteristics and possible changes in these properties during the studied period, were investigated. For this purpose, first, the values of Reconnaissance Drought Index (RDI) for the entire period of 1951 to 2010 in three scales of annual, seasonal and monthly were calculated. Drought characteristics include severity, intensity, magnitude, time interval and frequency for the "first half and the second half" was compared together using the Mann-Whitney test at 95% level. Cut point was determined by Kolmogorov-Smirnov test. Accordingly, the whole period of 60 years was divided into two periods (the period before the cut point and the period after it) and characterization of drought in these two periods were compared with each other.
Results: In all stations, cut point year in the period of 60 years is different from the middle year (1981). On an annual scale, in any of the studied stations, the characteristics of duration and drought number had no significant difference at95%. Other features of drought in annual scale have significant difference only in Tabriz and Tehran. In the seasonal scale by comparing the two periods of 1951-1980 (first half) with 1981-2010 (second half) in the city of Zahedan, in the second half period, the severity, magnitude and frequency of drought were increased while, the interval time between the droughts was reduced. Other regions show no significant change in the properties of drought. At the seasonal scale, in general it can be concluded that in spring droughts are more severe comparing with the first period. While drought in the winter did not noticeably change in most stations. For monthly scales, In general, the highest reduction of drought events was happened in November, while in many cities the number or severity of drought events were increased in May or April in the second half than the first half. This could indicate reducing of rainfall and increasing of evapotranspiration in the spring months.
Conclusion: In general, time series of drought indices values in all stations have a cut point at 95% level. This reflects the heterogeneity and changes in drought conditions. In addition, some changes were occurred in the drought characteristics or in occurrence time in some stations. This changes in drought characteristics is more pronounced, especially in the spring months. Although these changes were not significant in many cases, at 95% yet. The main reason for this can be seen in relation to the issue of global warming and climate change.