T​MDL Stormwater Toolkit


The following instructions present MDE's recommended method for estimating the geographical extents of a stormwater WLA and disaggregating an aggregate allocation.

It is important to note that the NPDES Regulated Stormwater boundaries and the 2006 USGS-CBP Land-Cover are considered by MDE to be the most accurate and readily available estimates of storm sewer ownership and land cover, and while these may be used for TMDL implementation, they are not absolute delineations.


Step 1 – Determine what stormwater WLAs apply to a specific permittee 

Navigate to the WLA Search page to search for WLAs by stormwater permittee.  In the database window, choose the county and permit type of the stormwater discharger. The database will output a list of WLAs that have been assigned to that permitted entity.

Even if the database does not list a TMDL allocations for a specific permittee, that permittee may have received an allocation under the Permit Type "Other" heading. Therefore, to obtain all of the relevant allocations for a given permit, a user must query both the specific permittee and the “Other” field, in order to see if an allocation has been provided for that permit under the broad category of “other regulated stormwater" within any applicable TMDLs. General construction permit allocations are always included as part of the “Other” allocation. For any questions about the "Other regulated Stormwater" category please reference the TMDL Report.

For each WLA, a pollutant, an allocation, a time period and a watershed will be identified in the query output. To obtain a map of the watershed, follow the link under the heading “TMDL Map”. This link will open a new window showing all of the watersheds in Maryland with TMDLs for that given pollutant. Use the GIS ID provided in the database for the allocation to find the specific watershed associated with the WLA. Use the link directly below the map to download a copy of the shapefile. Links to all of the pollutant-specific TMDL maps are provided here

Click here to search for applicable WLAs in the WLA Search database.

Step 2 – Download the relevant spatial datasets

(a) Download a copy of the NPDES-Regulated Stormwater Area shapefile (info | download)
(b) Download a copy of the USGS-CBP Modified 2006 Chesapeake Bay Land-Cover Dataset raster grid (info | download)

Step 3 – Delineate the urban footprint associated with the permittee

Using GIS software, perform an overlay of the applicable shapefiles and raster files available for download above and identify the common areas (i.e., the areas of intersect) between the following three spatial layers:

  1. Shapefile:   TMDL Pollutant Shapefile  (from Step 1)
    Extraction: TMDL Watershed (from WLA database)
  2. Shapefile:  NPDES-Regulated Stormwater Area Shapefile  (from Step 2a)
    Extraction: Area associated with permittee's storm sewer ownership
  3. Shapefile:   2006 USGS-CBP Land-Cover shapefile  (from Step 2b)
    Extraction: Urban Area

This map will correspond to the watershed, pollutant, time period and allocation from Step 1. For an individual allocation, the process is complete.

Step 4 – For aggregate allocations, find the footprint of the aggregate WLA coverage

If the allocation for the permittee in question is provided as an aggregate, determine what other permittees are included in the aggregate. This information is given in the TMDL report. Perform an overlay similar to Step 3, except include all of the stormwater entities from the aggregate allocation. This overlay will provide a delineation of the urban footprint associated with aggregate WLA (i.e., the urban footprint associated with all of the regulated stormwater entities included within the aggregate allocation). Then, calculate the area from Step 3 and divide the area of the permittee’s WLA footprint by the area of the WLA aggregate footprint. The quotient represents an estimate of the fraction of the aggregate allocation that is attributable to the permittee. Thus, to calculate the actual WLA value the permittee is responsible for, apply (multiply) the proportion to the aggregate WLA.

Step 5 – Developing an implementation plan

Guidance documents to assist in developing stormwater implementation plans are provided on the TMDL Stormwater Implementation Resources page.

Additional Information

Please direct questions or comments concerning Maryland's TMDL Program to mde.tmdlcoordinator@maryland.gov at (410) 537-3818.​​