Assessment of Hydrological Drought Using Constant Threshold Level Method (Case Study: karaj Dam Basin)

Drought is an insidious environmental phenomenon. This phenomenon causes water shortage for some activities, communities or environment. Drought should be compared with average climate and hydrological Conditions for Long period of time. Hydrological drought is detected by reduced water storage in lakes, underground water levels drawdown and reduced river flow and often will affect a large area. The most common definition of a hydrological drought is to define a threshold limit, such that if the river is less than that, a hydrologic drought is considered. In this study, the fixed threshold limit of Q70 was used in Karaj drainage Basin. Results showed that Q90 of year 1998 was the driest year. The results show that 1377 year was one of the driest years in this area and in Karaj station 284 days and for Gachsar station 255 days were dry. Also, the results show a total lack of water in Gachsar station Q70 threshold equal to 68/11 million cubic meters, Kalavan stations equivalent to 75.2 million cubic meters and for Karaj station equal to 44/42 million cubic. The general extreme value (GEV) best fit the parameters of the drought. The results can be used in water resources management and planning necessary to prevent or mitigate the effects of drought in the area.