The impervious surfaces in the city of Cluj-Napoca have experienced a continuous growth over time. These surfaces reduce the infiltration of the water into the soil, they alter the natural direction of the flow paths they lead to the increase in runoff volume and to the reduction of water quality. In the present paper we aimed to evaluate the hydrologic response to the spatial and temporal variability of impervious surfaces, for a subwatershed in the city of Cluj-Napoca. The variability of impervious surfaces was identified for the period 1986-2014 on the basis of remote sensing, and their impact on surface runoff was assessed by means of a hydrological modelling. The results have shown an increase in the peak flow along with the increase in the impervious surfaces during the period analysed. These results point out that the impervious surfaces directly influence the runoff volume, being an important variable in managing storm water.