Impervious surfaces have been identified as an indicator of the impacts of urbanization on water resources. The design of stormwater control measures is often performed using the total impervious area (TIA) in a watershed. Recent studies have shown that a better parameter for these designs is the " effective"� impervious area (EIA), or the portion of total impervious area that is hydraulically connected to the storm sewer system. Methods to improve estimates of EIA are not highly researched, and need further investigation. The overall goal of this project is to develop a method to estimate EIA in urban watersheds with data that is readily available. First, the existing rainfall-runoff method was improved by reducing the uncertainty associated with EIA estimates and applying it to 40 gauged urban watersheds with different sizes and hydrologic conditions, mostly in the Twin Cities metro area of MN and Austin, TX. The results are then utilized to develop a new method based on the integration of GIS and Curve Number (CN). The GIS-CN method is applicable to ungauged watersheds and is able to estimate EIA fraction based on TIA and hydrologic soil group (HSG). The results are used to evaluate the potential and the limitations of the GIS-CN method. The outcome and applications of this study improves the rainfall-runoff modelling in urban watersheds and will eventually lead to the design of a more sustainable urban stormwater infrastructure.