Longterm Drought Monitoring of Urmia Using Joint Deficit Index (JDI)

Monitoring and prediction of droughts، especially accurate determination of its emersion time and duration، are very important in water resources management and developing drought mitigation strategies. In this study، meteorological drought conditions in Urmia station during 1951-2011 were evaluated by means of the modified Standardized Precipitation Index (SPImod) and the Joint Deficit Index (JDI). Results showed that use of the modified SPI covered some flaws of the traditional SPI، and took into account the seasonal variation of precipitation. Nevertheless، the modified SPI same as the traditional SPI is sensitive to the time scale and it can lead to confusion because inconsistent results may emerge under different time scales. In order to resolve this issue، the JDI was used. The JDI was calculated through constructing the joint distributions of multiple modified SPIs with time scales 1 to 12 months using copula functions. The results indicated that the Urmia station had experienced a prolonged and sever (JDI<-1) drought during 1996-2011، as 101 out of 192 months were dry (JDI<0). Also، the dry periods had more severity than the wet periods. These drought events led to serious environmental consequences such as Urmia Lake drying up.