Estimating pollution contributions from non-point sources is always difficult. Calculating pollution due to runoff into urban creeks and waterways is no exception. In an effort to model non-point source pollution in Austin, a Geographic Information System (GIS) grid based hydrology model was developed for the City to aid in the development of a citywide Water Quality Master Plan. There are three primary objectives for the model: 1) compute current pollutant loadings for seventeen constituents at Environmental Integrity Index (EII) sites; 2) estimate future loadings for the year 2040 for the same constituents; and 3) model the influence of Best Management Practices (BMPs) on reducing pollution loads. Initial work completed in 1997 by researchers at the Center for Research in Water Resources 9CRWR) was a substantive first step; however, many limitations and recommendations were also indentified. This paper discusses the next manifestation of the model that was developed at CRWR during 1999-2000. The three main modifications made in the second phase concern increasing both model accuracy and accessibility. First, significant improvements were made to improve datasets used as input to the model. Second, corrections for both flow and load calculations were made on a cell-by-cell basis within the GIS environment instead of corrected separately in a spreadsheet. Third, future impervious cover projections, the basis for flow calculations, were tied more closely to undeveloped land parcels. Lastly, to make the model more accessible to a variety of policy makers, reliance on ArcInfo has been minimized; ArcView is now the platform for the model. In addition to these changes, new City assumptions were incorporated, especially with regards to base flow and storm flow separation. With these modifications in place, City objectives were met, and improvements in accuracy and accessibility were realized.