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Drought is a random characteristic of natural phenomena, drought about by the irregular deficit or shortage of available water, affects injuriously the plant growth and reduces their yield. Drought is not a physical but a biological phenomenon and should, be defined separately for each plant species and environment .To estimate the intensity and frequency of drought will help to reduce the injurious effect of drought.
In this study we used the water-budget methods and Thornthwaite’s aridity index and its standard deviation for Tehran city of Iran during 1951-2013 to show the frequency and intensity of drought effects in this place.
 The precipitation water evaporates as fast as it comes with the result that there is no accumulation of any moisture in the soil at any time of the year. The capital city of Iran, i.e., Tehran during its study period (1951-2013) experienced a total of 32 different types of droughts.
The result shows during this period this place is faced 32 times different type of drought on the five time disastrous and from the years of 2000 its severity and intensity is increased so far 2011 and 2012 the station is affected disastrous drought.