Comparison of Empirical Copula-Based Joint Deficit Index (JDI) and Multivariate Standardized Precipitation Index (MSPI) for Drought Monitoring in Iran

Multi-index drought indices are based on a combination of single drought indices and able to monitor the overall status of drought in a certain location. The aim of the present paper is to compare two recently developed multi-index drought indices, i.e., Joint Deficit Index (JDI) and Multivariate Standardized Precipitation Index (MSPI) under different climatic conditions of Iran. Although these drought indices are calculated using different statistical methods, they need twelve modified Standardized Precipitation Index (SPImod) corresponding to 12 time windows from 1 to 12 months as the input variables. In this study, these indices were calculated using the monthly precipitation data of 42 weather stations representing diverse climates of Iran during the period of 1966–2010. JDI and MSPI were compared from different viewpoints including temporal behavior, the relationship between drought severity and duration and frequencies of drought classes. The results showed that both indices have a similar trend of drought severity at all stations. Unlike the MSPI, the JDI faced two substantial problems in all stations of interest: 1) The lowest value of the JDI time series was frequently repeated in different parts of its time series, 2) The JDI was not able to identify the peak of drought severity during the critical drought periods. While the JDI could not identify the extreme (at about all stations) and very severe (at one third of stations) drought classes, the observed frequencies of the MSPI’s drought classes successfully matched the corresponding expected frequencies. The obtained results clearly indicate the superiority of the MSPI for monitoring overall status of droughts in different climatic conditions of Iran in comparison to the JDI.