The time of concentration (Tc) is one the most important time parameters to predict the response of a catchment to a given rainfall and plays a key role in the hydrologic design and rainfall-runoff modeling. There are a huge number of empirical/semi-empirical equations for estimation of T c and depending on several parameters such as rainfall attributes, topographic and land cover map scale, DEM resolution and streams delineation threshold causes significant uncertainties in the T c value. How to quantitatively evaluate the uncertainties in model parameters and the resulting uncertainty impacts on model outputs has always been a question which has attracted much attention. In this study, the method based on the First-Order-Analysis (FOA) is used to analyze the uncertainty and the contribution of each parameter on the output of 47 T c formulas in Kasilian and Amameh watersheds. The results show that among the 47 T c equations, equations which are based on watershed's characteristics, rainfall attributes and land cover-related coefficients such as Overton-Meadows, ASCE, Akan, Kinematic-Wave, McCuen et al. and Izzard have relatively high uncertainty and the average CV of these equations is about 45%. In addition, equations that are based on only geomorphological parameters have relatively low uncertainty (the average CV is about 16%). Further analysis of the effects of parameter uncertainties on the T c equations reveals that the uncertainty associated with rainfall attributes and land cover-related coefficients have great impacts on results of the T c equations and the uncertainty caused by these factors in humid regions relative to dry/ semi dry regions is different. Moreover, in the geomorphological-based equations, the uncertainty caused by streams delineation threshold is approximately 3-6 times of scale effects' uncertainty.
Digital elevation model (DEM) resolution and the assigned threshold for river network delineation affect the results of rainfall-runoff models. In this study, the effects of these 2 issues on the extracted geomorphologic parameters of watersheds and the performance of a kinematic wave based model, called KW-GIUH, are investigated. The results show that by decreasing the DEM resolution at fixed thresholds, parameters such as subbasin mean slope and the number of streams decrease and the area of the i th order subbasins and the mean length of the overland flow increase. Moreover, the results indicate that the reduction of the DEM resolution at a fixed threshold causes the peak flow and hydrograph time base to decrease up to the cell size of 100 m and then, after experiencing a jump, again decrease with the increase of the cell size. According to the achieved results, above the threshold of 2%, the difference between the peak flows of different hydrographs at different resolutions is meaningful. The KW-GIUH sensitivity to DEM resolutions and thresholds is sharper in peak flow and then in hydrograph time base and time to peak. At a fixed threshold, the value of time to peak is independent of DEM resolution.
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