Elham Forootan - Natural Resources and Environmental Engineering Department, Faculty of Agricultural Sciences, Payame Noor University, Qom, Islamic Republic of Iran

In urban catchment because of developed conditions, the amount of impervious area increases which tends to increase peak discharge rate and runoff volume. Knowing the importance of rainfall volume helps to appropriate design of storm water control facility. In this article analytical model was applied for investigation the importance of small rainfall events in runoff production. Therefore; rainfall events less than 8 mm was considered as small rainfall events. The test catchment was located in upper part of District No. 22 of Tehran. There was no existing rainfall gauge in the test catchment, long term rainfall series was taken from Mehrabad rainfall station and the data from 1959 to 1998 were used. The results of this study revealed that the proportion of rainfall volume less than 8 mm for runoff production was 7.5%.

Storm water , Analytical model , Probability density function