Forecasting and Simulation of Floods in Western Boundary Basin Ilam, Iran

Flood is a destructive phenomenon with meteorological and hydrological nature that causes casualties and financial losses every year. Therefore, simulation and prediction of severity and time of occurrence of this event can prevent these losses to a great extent. In order to achieve this goal, simulation of observed flooding in selected events is performed by optimization and calibration of the parameters of two hydrological models including Hydrological Model (HyMod) and a rainfall-runoff model named Asymmetric Laplace Unit Hydrograph (ALUH). More than 90 flood events, having different magnitude and duration; in the period 1991-2015 was selected in western boundary basin in Ilam, Iran (46º25′ E33º38′N). The Ilam province and watershed, located in western end of Iran near the borders of Iraq. In this study, the climatic conditions of temperature and precipitation, potential evapotranspiration, vegetation classification in the area is investigated. In order to identify return periods and the most likely time for occurrence of large-scale flood events, extreme values and harmonics analysis are performed on the time series of observed annual maximum precipitation and runoff. Calibration, verification and evaluation of performance of models according to Nash-Sutcliffe Efficiency (NSE) and root mean square (RMS) error of observations to standard deviation ratio indices are performed in 18 selected events. Then, the empirical relationships between characteristics of rainfall and runoff and sensitivity of ALUH model to each parameter are investigated. The results indicate that regional precipitation is mainly in form of rain and more than 200 mm/day rainfall and 2000 m³/s peak flow is possible with return period of 100 years. Overall, Harmonics of precipitations are in a 15 days advance compared to those of runoff and mostly reach to their peak in November, showing a high possibility of flood in it. The simulation results show that both models had good performance in calibration stage but ALUH model has good performance of in all conditions, including extremely positive to symmetric and unusual negative skewed event. Also a powerful empirical relation found between the observed amounts of volume and peak flow of flood. The sensitivity of NSE index of ALUH to the parameter of time to peak flow and the sensitivity of its simulated peak flow to scale parameter found to be higher than other parameters. Favorite results of study suggest applicability of ALUH for operational use on hydrograph simulation and prediction.