TERRA: Two-Dimensional Exposure and Rainfall-Runoff Analysis

Spatially Distributed Watershed Model To Assess Chemical Exposures




   Extreme Storms (pdf)

   Metals Transport (pdf)

User Manual (pdf)

Source Code


   Hydrology (zip)

   Sediment Transport (zip)

   Chemical Transport (zip)

   Rainfall Options (zip)

TREX Tools






NOTICE: This page provides information about a peer review that was conducted by the U.S. Environmental Protection Agency (USEPA).  For that review, the TREX watershed model was renamed to avoid potential confusion with other models used by the Agency. This information was provided courtesy of Colorado State University to facilitate the review process. However, the information on this web page was not subject to Agency review and USEPA is not responsible for its content.  

USEPA reviewed the Two-dimensional Exposure and Rainfall-Runoff Analysis (TERRA) watershed model during a 2-day consultation meeting of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) Scientific Advisory Panel (SAP) that occurred on October 25-26, 2011 [EPA-HQ-OPP-2011-0582; FRL-8881-9].  The purpose of that review was to discuss scientific issues and model use in the FIFRA Ecological Risk Assessment for Antimicrobial Uses of Copper. SAP reviewers and other interested parties should be aware that the name of the computer model used to perform antimicrobial copper transport simulations was changed from Two-dimensional Runoff, Erosion, and Export (TREX) to TERRA. This name change was made to avoid confusion with the Terrestrial Residue Exposure (T-REX) model that the USEPA Office of Pesticide Programs (OPP) uses to estimate pesticide concentrations for avian and mammalian food items. 

Using the links to the left or below, you can download: (1) an overview of model development; (2) detailed information regarding model applications; (3) a user manual; (4) model source code; and (5) example files.

A peer-reviewed journal article detailing model development and application for antimicrobial copper exposure assessment was published following SAP review: 

    Velleux, M., Redman, A., Paquin, P., Santore, R., England, J., and Julien P. 2012. Exposure Assessment Framework for Antimicrobial Copper Use in Urbanized Areas. Environmental Science and Technology, 46(12):6723-6732.

Peer-reviewed journal articles detailing model development and application are also available: 

    Velleux, M., England, J., and Julien P. 2008. TREX: Spatially Distributed Model to Assess Watershed Contaminant Transport and Fate. Science of the Total Environment, 404(1):113-128.

    England, J., Velleux, M., and Julien, P. 2007. Two-dimensional simulations of extreme floods on a large watershed. Journal of Hydrology, 347(1):229-241.

    Velleux, M., Julien P., Rojas-Sanchez, R., Clements, W., and England, J. 2006. Simulation of Metals Transport and Toxicity at a Mine-Impacted Watershed: California Gulch, Colorado. Environmental Science and Technology, 40(22): 6996-7004.


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Department of Civil and Environmental Engineering - Colorado State University
Fort Collins, Colorado 80523-2028  USA   (970) 491-5048

Last update: 07 August 2018