ABSTRACT:Lately, computerized hydrologic models have become an essential tool not only for a better understanding of the hydrologic cycle but also for a faster problem solving in hydrology, such as the ungauged catchments. Since Covasna river, which is a left tributary of the Black river, is monitored only by two gauging stations, an approach for runoff modelling at the ungauged outlets within the catchment, would be extremely beneficial. Thus, the main purpose of this study is to conduct a test of the ability of such a simulation modeling system for flood prediction within the upstream catchment area of Covasna river. The one chosen for carrying out this study, HEC-HMS, is known to be a reliable model with reasonable approximations. Throughout the research, ArcHydro and HEC-GeoHMS extensions are used to delineate the watershed, and to obtain the hydraulic length, the CN grid and other parameters. This study is also a solution for the problem of long simulation time of the model, by following first, the work flow implemented in ArcGis 10.2 through Model Builder.