Application of effective drought index (EDI) in characterizing drought periods (case study: Tabriz, Bandar-e Anzali and Zahedan stations)

In this study, drought characteristics of three stations namely Tabriz, Bandar-e Anzali and Zahedan are analyzed from 1951 to 2012 using the effective drought index (EDI). For this purpose daily precipitation records of each station are arranged in a distinct column of excel spreadsheet. Long-term daily average of the day (in daily scale) is substituted for missing data. In this study, effective precipitation index (EPI) is calculated, too. This is accomplished once for dummy duration (365 days) and once again for actual duration. Then daily mean effective precipitation (MEP) for each day of a year is calculated. Moreover, differences between effective precipitation (EP) and MEP are calculated and named DEP. Then standardized values of DEP (shown with SEP) are calculated, too. Precipitation needed for returning to normal condition (shown with PRN) is calculated for every station. At last, the EDI for each station in whole time period is obtained. In this study, time series including: (1) daily EPI (for dummy period of 365 days as well as actual duration), (2) MEP, (3) DEP, (4) SEP, (5) PRN and (6) EDI for every station are plotted. Results showed that years 2007–2008, 2007–2008 and 2001–2002 were known as the driest years in the 60 years time period, respectively, for Tabriz, Bandar-e Anzali and Zahedan stations. Based on the number of dry days occurred for each station it can be calculated that Bandar-e Anzali, Tabriz and Zahedan having the 2407, 1517 and 1338 dry days in the whole time period ranked in the first to third orders.