A semi-distributed hydrological model has been developed in this study for determining runoff in an ungauged peri-urban catchment of north east India. In the absence of discharge data, the study has focused on how to improve the accuracy of runoff prediction based on imperviousness determination by spatial method. The imperviousness is defined in terms of total impervious area (TIA) and effective impervious area (EIA). The study highlights misclassification as one of the factors causing error in imperviousness determination. It is recommended that integrating GIS data with high resolution satellite imagery can be an effective solution to minimise misclassification. The predicted peak runoff was 33.4% larger when GIS based imperviousness is employed in the model. The peak runoff was 72.2% more in 2006 as compared to 1980 due to an increase in imperviousness, which is attributed to urbanisation.