Fakhredin Azad shahraki - Rural water and waste water of Qazvin province Qazvin, iran
Zinolabdin Bajelvand - Rural water and waste water of Qazvin province Qazvin, iran
Somayeh Zolghadr - Rural water and waste water of Qazvin province Qazvin, iran
Mostafa Panahi - Rural water and waste water of Qazvin province Qazvin, iran

Because of the importance of drought and its effects on all aspects of life and different sections of society, specially thechanges of natural environment, In this paper for the city rivers we have chosen multi variables models, time series of first grade Auto Regressive model and then according to that, a population of hundred series of five hundred years is established. Inconsidering this population and also the 20, 50,100, 200, 500 periods for artificial population made the drought periods arespecifiedcriterion is the Yevjevich model which infact it is the same as classical analysis of shortages. But in second methodFerick .et .al which is the same as the first one but it has only one additional condition for the end point of drought periods. Itmeans that the average in the final year and also the average which is computed after three years should be more than theaverage computed as a total average. Moreover with the help of this periods and also considering three parameters – the length of shortage period, the volume of shortage and the rate of shortage – and also with the help of drought for thirty five period the results are presented.

Drought period, Auto regressive, shortage, population