Dlpak Hamaamin - Department of Geography, Faculty of Literature and Human Sciences, Razi University, Kermanshah, Iran.
Amjad Maleki - Department of Geography, Faculty of Literature and Human Sciences, Razi University, Kermanshah, Iran.
Arash Azari - Department of Water Engineering, Faculty of Agriculture, Razi University, Kermanshah, Iran.
Azzadeen Darwesh - Department of Geography, Faculty of Education, Garmian University, Kurdistan of Iraq, Iraq.
Mohammed Ahmadi - Department of Geomorphology, Faculty of Literature and Human Sciences, Agricultural Education and Natural Resources Center of Kermanshah Province, Kermanshah, Iran.

Flood is inherently an uncertain phenomenon and the certainty and credibility of flood forecasting and warning systems will cause errors regardless of the sources of uncertainty. Extreme rainfall events are one of the most important input data to rainfall-runoff models, which always have uncertainty. Considering this issue the uncertainty of the design flood hydrograph can be investigated for different return periods. In this research first to simulate the flood hydrograph the HEC-HMS model was calibrated and validated based on the hourly flood hydrographs recorded at the basin outlet. Historical data were collected on the ۲۴-hour maximum rainfall of Gharesoo Basin stations with ۳۰-year statistics and the affected basins were identified. Then in each station ۳۰ series of ۳۰ years of artificial data with a maximum ۲۴-hour rainfall were produced. For each of these produced stochastic series the best statistical distribution was fitted and in each series extreme values with a return period of ۲۵ ۵۰ ۱۰۰ and ۱۰۰۰ years were calculated. Finally in each return period by combining ۳۰ different amounts of rainfall obtained from stochastic series, the uncertainty bandwidth of the flood hydrograph was obtained during this return period. The results indicated that the highest predicted peak discharge for different return periods was between ۱.۲ and ۱.۷ times the historically recorded discharge during that return period. Generally the maximum discharge of different return periods was between ۱.۵ and ۳ times the minimum discharge.

Flood Design, HEC-HMS, uncertainty, Gharesoo basin