Time distribution of heavy rainfall events in south west of Iran

• Temporal rainfall distribution should be developed based on local storms distribution. However, until now, hydrology studies utilize the Soil Conservation Service (SCS) distribution patterns in Iran due to the lack of sufficient observation data.
• In this research, rainfalls of 35 heavy storms in south west of Iran were simulated with the Weather Research Forecasting (WRF) model. Then, time distributions of rainfalls were computed based on the WRF simulated rainfall data and a three-parametric logistic function.
• Comparison of temporal distribution of rainfall curve in south-west of Iran and SCS demonstrates that the flood designed for large dams by SCS is able to adjust based on the distribution curves of rainfall in South West of Iran.

Accurate knowledge of rainfall time distribution is a fundamental issue in many Meteorological-Hydrological studies such as using the information of the surface runoff in the design of the hydraulic structures, flood control and risk management, and river engineering studies. Since the main largest dams of Iran are in the south-west of the country (i.e. South Zagros), this research investigates the temporal rainfall distribution based on an analytical numerical method to increase the accuracy of hydrological studies in Iran. The United States Soil Conservation Service (SCS) estimated the temporal rainfall distribution in various forms. Hydrology studies usually utilize the same distribution functions in other areas of the world including Iran due to the lack of sufficient observation data. However, we first used Weather Research Forecasting (WRF) model to achieve the simulated rainfall results of the selected storms on south west of Iran in this research. Then, a three-parametric Logistic function was fitted to the rainfall data in order to compute the temporal rainfall distribution. The domain of the WRF model is 30.5N–34N and 47.5E–52.5E with a resolution of 0.08 degree in latitude and longitude. We selected 35 heavy storms based on the observed rainfall data set to simulate with the WRF Model. Storm events were scrutinized independently from each other and the best analytical three-parametric logistic function was fitted for each grid point. The results show that the value of the coefficient aa of the logistic function, which indicates rainfall intensity, varies from the minimum of 0.14 to the maximum of 0.7. Furthermore, the values of the coefficient BBof the logistic function, which indicates rain delay of grid points from start time of rainfall, vary from 1.6 in south-west and east to more than 8 in north and central parts of the studied area. In addition, values of rainfall intensities are lower in south west of IRAN than those of observed or proposed by the SCS values in the US.