In this study reservoir inflow volume is used to investigate drought conditions by various methods so that by comparison with drought conditions at meteorological and hydrometric stations, the used drought indices could be evaluated. Also, relationships between cumulative reservoir inflow volumes at critical dates and those of the previous months were investigated, in order to identify effective variables for future utilization of stored water and implementation of optimal management decisions for reservoir operation. At first, the SPI was used in two different manners, by applying the method by Mckee and also by applying the method by Nalbantis and Tsakiris, in order to investigate meteorological droughts in four rainfall stations at Kor watershed (upstream of Doroodzan Dam). Also, hydrological drought conditions were investigated based on data on inflow into the Doroodzan reservoir and Chamriz hydrometric station, which were modeled utilizing the SDI, which according to the results the SPI and the SDI from the hydrometric station data, would not be sufficient for drought monitoring and utilization of the SDI on inflow into the reservoir as a good representative of the entire watershed would be more appropriate. In order to establish relationships between cumulative reservoir inflows at critical dates with cumulative reservoir inflow of previous months, the first days of Farvardin, Ordibehesht, Khordad and Tir were considered as critical dates. According to final equations, inflow into the reservoir on the first days of Ordibehesht, Khordad and Tir are directly related to cumulative inflows of the previous month. This would indicate that as the non-rainy season approaches, inflow volume stabilizes and is less effected by rainfalls or in other words by climatological factors. Whereby, for the first day of Farvardin, month of Mehr along with Dey and Bahman as effective variables indicates that inflow volume on the first day of Farvardin is dependent on the rainy season, which would highlight the importance of drought indices and also climatological factors on reservoir inflow during the rainy season.