m HEYDARI - phd student, Department of Watershed Management, Faculty of Natural Resources and GeoSciences, Shahrekord University
s.j SADATINEJAD - associate professor, Department of Watershed Management, Faculty of Natural Resources and GeoSciences, Shahrekord University
a HONARBAKHSH - assistant professor, Department of Watershed Management, Faculty of Natural Resources and GeoSciences, Shahrekord University

The use of hydrological modeling systems for water resources planning and management is becoming increasingly popular. Since these hydrological models mostly deal with land phase of hydrological cycle, therefore, data related to topography and physical parameters of watershed are a necessary pre-requisite for these models. In this article a new physical parameter is presented for use in hydrological modeling in rangeland regions. This parameter, the equivalent diameter, De, is the diameter of the circle whose perimeter equals that of the watershed. To investigate its application, De, was used in modeling the time of concentration and lag time in a field study carried out in north Karoon river basin, west Iran. The result of the sensitivity analysis carried out on 15 physical parameters demonstrated that De, was the third most important parameter, having the determination coefficient of 0.995, 0.992 for time of concentration and lag time respectively. It should also be noted that calculation of De was much easier compared to other parameters. To evaluate further the importance of this parameter, it is recommended to expand this study to other watershed basins with different characteristics

hydrological modeling, physical parameter, equivalent diameter