A comparative study of Meteorological (SPI) and Hydrological Drought Index (SSI) based on the best cumulative distribution function in Tehran province

The permanence of rainfall shortage compared with the long-term average of meteorological droughts, reduces river flow, groundwater level and hydrological droughts. To quantify the drought, several indexes has been defined. In this study, two drought indexes, including standardized precipitation index (SPI) and standardized streamflow index (SSI), were used for meteorological and hydrological drought monitoring. The aim of this study was the monitoring the drought in 5 stations in Tehran province during the period of (1985-2016), determining the most appropriate cumulative distribution function for calculating the SSI drought index and also a comparative study of meteorological and hydrological drought indicators in this province. The results show that the most suitable cumulative distribution function at the  Darake station is Gen-Loglogestic, at the Firuzkuh and Abali station is Gamma, at the Namrud station is Fatigue life and at the Sharifabad station is Beta. Among the stations with proper functions, Derek with the highest P-value (0.996) and the lowest RMSE (0.916) had the best fit with the Gen-Loglogestic distribution  function. Also, the similarity in the process of drought resulting from two different indicators, indicates that the meteorological and hydrological indicators are completely same, in other words, surface water resources in Tehran province are completely influenced by rainfall.