Application of Joint Deficit Index (JDI) for Analyzing Droughts over the Southern Margin of the Caspian Sea

Drought is a climatic phenomenon that slowly and gradually emerges, and is of a latent nature. It lasts long, damaging the different sectors of agriculture, environment and consequently the society. Monitoring and prediction of droughts, especially accurate determination of their times of emergence, and duration, are very important in water resources management and in planning for drought mitigation strategies. Throughout the present study, drought conditions in three provinces (Golestan, Guilan and Mazandaran) located in the southern margin of the Caspian Sea, were evaluated by means of Joint Deficit Index (JDI). The performance of JDI was compared with two other drought indices, Standardized Precipitation Index (SPI) and Modified Standardized Precipitation Index (SPImod). To follow the purpose, monthly precipitation data from 5 synoptic stations, namely: Babolsar, Bandareanzali, Ramsar, Rasht and Gorgan during the period of 1971 to 2011 were used for calculating the drought indices. Results showed that in recent years, the number of dry months across the study area had increased, significantly, as for all the considered stations (except Babolsar) the percentage of dry months had increased to more than 50% during the recent 10 years of 2002-2011. Based upon the calculated JDI, SPI and SPImod values, it becomes evident that the dry condition (along with deficit in precipitation) increase with an increase in the distance from the Caspian Sea. The results also indicate that JDI provides for a comprehensive assessment of droughts and that it is capable of reflecting both emerging and prolonging of the droughts in an accurate manner, allowing for a month-by-month drought assessment.