Performance Evaluation of the HEC-HMS Hydrologic Model for Lumped and Semi-distributed Stormflow Simulation (Study Area:Delibajak Basin)

Rainfall-runoff processes in small Delibajak basin(16.3 km 2 ) in kohgilouye and boyerahmad , iran was examined.At first, In this study, Delibajak basin was considered as Lumped and then basin divided into a number of sub- basins where the hydrologic parameters may vary from one sub-basin to another. In such case, lumped models may be labeled as semi-distributed. The hydrologic model HEC-HMS (Hydrologic Engineering Center, Hydrologic Modeling System), used in combination with the Geospatial Hydrologic Modeling Extension, HEC-GeoHMS. The SCS curve number method (Soil conservation Service, 1972) was considered for the Rainfall-runoff modelling and in both cases The model was carefully calibrated and verified in basin using historical observed data. The determination coefficients and coefficients of agreement for all the flood events were above 0.9, and the percent errors in peak flow and volume were all within the acceptable range. Then,a local sensitivity analysis was adopted for evaluating the event model. There are three parameters(curve number,initial abstraction and lag time) of the event model that were subject to the sensitivity analysis.In the Delibajak basin. In both lumped and distributed model,The highest differences between the generated peak hydrographs and the baseline peak hydrograph was caused by initial abstraction,Ia. The results indicated that Semi-distributed model captured the peak runoff discharges and total runoff volume better than Lumped model. However, overall, the performance of both models was quite reasonable.