Ali Azareh, Mohammad Reza Rahdari, Elham Rafiei Sardoii and Fatemeh Azariya Moghadam
Drought is a deficiency in precipitation over an extended period, usually a season or more, resulting in a water shortage causing adverse impacts on vegetation, animals and people. It is a normal, recurrent feature of climate that occurs in virtually all climate zones, from very wet to very dry. In this study for assessment and analysis the occurrence of droughts use from Standardized Precipitation Index (SPI) and Standardized Discharge Index (SDI). For this purpose, used from fifteen monthly meteorological stations statistics as well as five stations in Karaj Dam basin and indices were evaluated in six time periods (3, 6, 9, 12, 24, 48 monthly). To study the effect of drought on surface water resources the first were examined relationship between rainfall and discharge and a time lag of rain fall rate was calculated. Then Standardized Precipitation Index and Standardized Discharge Index evaluated on different timescales with using Pearson correlation in the whole area was compared by separation in the sub-basin. The results shows that time relationship occurrence of Meteorological drought and hydrological with together in99% level is significant as well as in the range of 3months was maximum. Occurrence of drought in the area their effect showed on the same month on the same month and one month delay on surface water sources. The highest correlation is related to the following areas of the Sira station which can be attributed to the geographic location of the studied watershed.
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