Abstract. Complemental interactive effects of the Digital Elevation Model (DEM) and Land Cover (LC) resolutions on the estimated runoff by using Soil and Water Assessment Tool (SWAT), which is of critical importance for water resource management, was investigated in this paper. Also, to specify the optimal DEM and LC resolutions for maximizing accuracy of the estimated runoff for Dokan, Adhaim, and Duhok watersheds located in Iraq. Twenty daily time step based SWAT models of each watershed were implemented using five DEMs in conjunction with five LCs. Assessment of models results shows that the watershed delineation significantly affected by DEM resolution, especially in flat regions. However, there is no clearly discernible trend of this effect on the determination of watershed boundary, stream network, number of sub-basins and total area. Furthermore, the number of Hydrologic Response Units (HRUs) and the maximum altitudes are directly related to the DEM whereas the minimum altitudes have an inverse relationship with the DEM. Also, the number of HRUs increases with the increase in LC resolution until it reaches a maximum value and then starts to gradually decrease. While there is no significant trend between the accuracy of the estimated runoff and the increase in the DEM and LC resolutions. The most accurate estimated runoffs of Dokan, Adhaim and Duhok Watersheds were obtained by using DEM 90 m and LC 1000 m, DEM 250 m and LC 1000 m, and DEM 30 m and LC 30 m with Nash and Sutcliffe Efficiency of 0.59, 0.68 and 0.69 respectively.