Spatially distributed Unit-Hydrograph model is implemented using a Geographical Information System (GIS) in this study to investigate the effects of how increasing impervious areas may influence surface runoff in urban areas. A distributed velocity grid is developed from a 10m Digital elevation model (DEM) using the kinematic wave approximation and Manning’s equation. By estimating the cumulative travel time across each cell in the catchment along the each flow path a time- area histogram is developed to derive one-hour Unit-Hydrograph. The method is implemented using ArcGIS 9.1 software and used to study the effect of continuously increasing impervious area from 1996 to 2006 for a small Calais Burn catchment in Fife council area. The historical Unit-Hydrographs developed for each of the seven years show that the increase of 11 % in impervious area has increased the peak discharge of the draining area by 35% and has decreased the time to peak by one hour.