Subwatershed Delineation and Estimating Impervious Cover: Using topographic maps and GIS to delineate watershed (< 100 square miles) and subwatershed boundaries (<10 square miles). Teaching watershed groups the importance of impervious cover and how to estimate current and future land use in subwatersheds.

Mapping and Analysis
Small watershed groups are becoming more sophisticated and because many have access to GIS, these organizations have the ability to delineate subwatershed boundaries and anyalyse subwatershed characteristics such as land use, development trends, and sensitive areas inventory. In particular, organizations in urbanizing watersheds have the ability to estimate current impervious cover, and more importantly, future buildout potential-a valuable indicator in assessing watershed vulnerability based on the Impervious Cover Model (ICM) --without relying on local agencies or other entities to perform the assessment.

Below is an example of catchment delineation done by the Weems Creek Conservancy of the Weems Creek watershed. Breaking watersheds down into smaller, more manageable units (either subwatersheds or catchments) can be important for assessment, planning, and implementation effectiveness. Because the watershed is almost completely built out, only current impervious cover was estimated for each catchment. Catchments were then classified using the ICM as sensitive, impacted, or non-supporting. For management goals for these three classifications click here

Additional Subwatershed Delineation Resources:
For more information on these techniques, the following resources are available from the Center:

• Watershed Planning Do-It-Yourself Kit
• Impervious Cover and Land Use for the Chesapeake Bay Region
• Rapid Watershed Planning Handbook
• Delineating Subwatershed Boundaries slideshow
• "Importance of Imperviousness" article from The Practice of Watershed Protection
• Impervious Cover Model: Stormwater Manager's Resource Center under Monitoring and Assessment