Watershed Modeling

There are many factors to consider when selecting a modeling software or methodology for a targeting schema. As this section provides details on some of these considerations, the overarching theme is not to let the model define the project, but to choose your model based on the needs of the project. Additionally, in some cases user groups may find that, while a more high resolution would be applicable and useful within a given project, the questions it answers may be more easily and efficiently answered through a lower-tech option.

The application of the SPARROW Model in establishing priority watersheds at the HUC 8 level for the reduction of phosphorus and nitrogen across the entire Mississippi River Basin is a good example of both the utility and the importance of modeling to understand the spatial distribution of pollutants. While the concentration of pollution is seen within the Gulf of Mexico hypoxic zone, the SPARROW model output proves the source, and therefore the area that needs to be prioritized, to be much more diffuse. Models like these can help ensure that dollars invested to improve conditions in the Gulf are being channeled effectively at both the state and federal level.

The output of the MIssissippi River Basin SPARROW model. More information can be accessed through USGS at

The output of the MIssissippi River Basin SPARROW model. More information can be accessed through USGS.

What will the model tell me?

In the EPA’s Handbook for Developing Watershed Plans, they outline the questions that modeling can help answer. These include:

  • Will the management actions result in meeting water quality standards?
  • Which sources are the main contributors to the pollutant load targeted for reduction?
  • What are the loads associated with the individual sources?
  • Which combination of management actions will most effectively meet the identified loading targets?
  • When does the impairment occur?
  • Will the loading or impairment get worse under future land use conditions?
  •  How can future growth be managed to minimize adverse impacts?

How will I decide which model to use?

  • Relevance – Is the approach appropriate to your specific situation, answering the questions you need to develop a watershed plan [or, as the case may be, a targeting schema]?
  • Credibility – Has the modeling system been shown to give valid results?
  • Usability – Is the model easy enough to learn and use that you are likely to succeed in obtaining results? [If not, do you have the funding or resources to bring in experts?] Are data available to support the model?
  • Utility – Is the model able to predict water quality changes based on the changes planned for your watershed management?

Where do I get data to use in my model?

  • There are many resources for downloadable and web-accessible data on this site in the Data Acquisition section.
  • Many of the Models discussed on this page have a web presence, and links will be provided.
  • To explore additional watershed models, the EPA’s Handbook for Developing Watershed Plans is available online. Chapter 8 has a complete list of available models.