Droughts are one of the most Catastrophic climatic extremes that often damages economy, environment and society; etc. Defining droughts based on a single variable may not be suitable for reliable risk assessment and decision-making. Hence, in this study, we propose a multivariate drought index for monitoring Chaharmahal Bakhtiari province drought conditions. The proposed index is called Multivariate Standardized Drought Index (MSDI), which considers both meteorological and agricultural drought using joint distribution between soil moisture and precipitation probabilities based on Copula functions. Copulas are functions that can be used to derive the joint distribution of two or more variables, regardless of their original marginal distributions. Monthly precipitation data from 26 meteorological stations and daily soil moisture data from Europea Space Agency (ESA) site were used to calculated SPI, SSI and MSDI. MSDI combines the Standardized Precipitation Index (SPI) and Standardized Soil Moisture (SSI) for drought characterization and monitoring. In order to assessment of MSDI, we compare MSDI, SPDI, SSI and SPI for five climate regions in Chaharmahal and Bakhtiari province. The results showed that MSDI characterized drought onset time and drought persistence similar to SPI and SSI, respectively and it does not have similar patterns to Standardized Palmer Drought Index (SPDI) for drought determination. Also MSDI shows a more severe drought condition when both soil moisture and precipitation exhibit a moisture d eficit. The cross correlation between MSDI with SSI and SPI showed that higher correlation occurs at lag times between k=0 to k=3 months and then decreases significantly. The MSDI-based drought monitoring for Charmahal Bakhtiari province showed drought conditions in 2000, 2008, 2011 and 2014 when the most extreme drought occurs in 2008. The results also showed that extreme and rare drought conditions have the highest frequency in the southern and central parts specially Ardal and Lordegan stations (3-17 and 6-15 percentage, respectively). In order to depict regional condition of drought across the province we provide the drought frequency/situation maps by two procedures, a) by meteorological station b) by soil moisture cells, the first method has higher accuracy than the second method while it seems that the second method prepares regular polygons. Key words: Drought Assessment, Multivariate Drought Index, Copula, SPI, SSI, SPDI.
