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Drought is a complex natural phenomenon that can occur in any climate. Hydrologic drought in the river flow of arid and semi-arid areas causes serious shortages, threatens the quality of life, and impacts on the economy. Understanding this feature is then essential for the management of water resources. Hydrologic drought in the sense of deficient river flow is defined as the periods that river flow does not meet the needs of planned programs for system management. In the present study, changes in the monthly discharge of 14 hydrometric stations throughout the Gorganroud watershed over 30-year period (1980-2010) were studied. Then the deficit flow was determined based on threshold level method, and the results were analyzed. It was revealed that periods of severe shortages have happened in the very humid and semi-arid climates and the downstream of the study area, while longer periods (28 months) of low flows have occurred in the arid climate. The trend of severity and persistence in the central stations of the watershed was increasing. Also shortages occurred with greater frequency at the end of the study period, and river flow shortage during the years 1998-99, 2007-2008, 2008-2009 and 2009-2010 has occurred in most of the stations. So in these years, flow deficit has happened in 50, 85.9, 64.3 and 92.8 %of the stations, respectively.

Keywords: Flow deficit, Threshold level, Trend, Zonation, Gorganroud

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