300 - Critical Sources Areas for Stormwater Runoff in Urban Watersheds

The growth of cities has notable consequences including, but not limited to, increasing stormwater runoff volume and non-point source pollution loads carried to water bodies. To efficiently design urban watershed management activities, it is crucial to identify critical source areas (CSAs) or the areas that contribute disproportionately higher pollution loads compared to other areas. While there is substantial literature on CSAs in agricultural and natural watersheds, knowledge gaps remain in urban watersheds. This study developed a CSA determination procedure for nutrient (Phosphorous and Nitrogen) loads in urban areas using the Soil and Water Assessment Tool (SWAT). Three small urban watersheds with a drainage area of 452 to 1007 ha and diverse land uses in St. Paul, Minnesota, all with runoff quantity and quality monitoring programs by the Capitol Region Watershed District, were used as case studies. The models were constructed considering all hydrological response units (HRUs) under three scenarios with varying input data (land cover and digital elevation model) resolutions and watershed delineations. From the sensitivity analysis, it was found that soil-related parameters had minimal impact on the results comparing with parameters such as slope and curve number. This was addressed by the pre-delineation process in the third scenario with finer resolution input data. The results highlight the importance of input data resolution and preprocessing for accurate CSA determination in urban watersheds. This research provides valuable insights for enhancing urban watershed management practices, including the targeted placement of stormwater control measures.