Element 4 - TMDL and GIS

There are several ways in which GIS applications are used in the TMDL process, with the potential for even greater integration in the future. Currently, waters that have an approved TMDL, including streams and lakes, are mapped as GIS features. These geodatabases are available both internally for department staff, and externally for use by the general public

GIS is used during TMDL development to perform spatial analysis of land cover and land use, soil types and conditions, population estimates, and potential sources of pollutants or impairment. It is also used for creating maps within TMDL documents and for displays and public presentations.

TMDL staff continue to explore ways in which GIS applications may be utilized for TMDL prioritization and to help support TMDL implementation. The use of geospatial techniques to address water quality issues has resulted in a cost-effective and research quality solution to TMDL issues. See Element 3 for more discussion about TMDLs.

GIS layers contain several forms of essential data for inputs to the predictive models used in TMDL development. Here are a few examples:

  • Watershed characteristics: GIS layers include 8, 10, and 12-digit watershed boundary data. National Elevation Dataset and contour files are useful in delineating watersheds that are only partially defined by the watershed boundary dataset, or WBD, layer. From these files, information such as flow direction, slope, and overflow velocity can be calculated.
  • Land cover: Runoff potential toward a stream is highly influenced by land surface characteristics. Impervious surfaces, cropland, grassland and forested areas all have different capacities to absorb precipitation and significantly influence the NPS pollutant load a stream may receive during precipitation events. Modeling may include testing the types and placement of best management practices. GIS land cover files and aerial photography provided by the National Agricultural Imagery Program provide this type of information.
  • Spatial relationships: Locations of point source dischargers and their proximity to water bodies are essential factors in estimating pollutant loads to the stream or lake in question.