In this study, the rainfall-runoff response of the Ammameh watershed located in Tehran, Iran, was studied using TOPMODEL which is a semi-distributed and geomorphologic model that simulates runoff at the watershed’s outlet on the basis of saturation excess runoff and subsurface flow concepts. Topographic index as an indicator of the spatial distribution of excess runoff generation in the catchment was calculated using the flow direction, output from two different methods, i.e. Dinf and D8. The analysis was done at three time scales: event, daily, and monthly. The modeling performance of TOPMODEL in simulating runoff process for each of three types of time series was analyzed and compared visually and statistically. Also, the effects of D8 and Dinf methods on rainfall-runoff modeling were compared and it was realized that modeling result of Dinf algorithm, especially in event-based rainfall-runoff modeling was more accurate than the D8 algorithm; whereas, the difference between the results were not notable in the daily modeling. Although the obtained results demonstrate the capability of the TOPMODEL in both event-based and daily simulations, the model could lead to the more reliable results in daily modeling because of considering the watershed soil moisture conditions.
