Chesapeake Assessment Scenario Tool (CAST) is a web-based nitrogen, phosphorus and sediment load estimator tool that streamlines environmental planning. Users specify a geographical area, and then select best management practices (BMPs) to apply on that area. CAST builds the scenario and provides estimates of nitrogen, phosphorus, and sediment load reductions. The cost of a scenario is also provided so that users may select the most cost-effective practices to reduce pollutant loads.

Suggested Citation

Chesapeake Bay Program, 2020. Chesapeake Assessment and Scenario Tool (CAST) Version 2019. Chesapeake Bay Program Office, Last accessed [Month, Year].

What is included in the Chesapeake Bay Program's suite of modeling tools?

The Chesapeake Bay Program uses state-of-the-art science and monitoring data to replicate conditions of the Chesapeake Bay watershed. This information is then used by decision-makers at the federal, state and local levels to determine how best to restore and protect local waterways, and ultimately, the Chesapeake Bay. By combining sophisticated modeling data and real-world monitoring data, we gain a comprehensive view of the Chesapeake ecosystem—from the depths of the Bay to the upper reaches of the watershed. The suite of computer modeling tools developed by the Chesapeake Bay Program divides the 64,000-square mile watershed into thousands of smaller segments, and helps us understand the impact of pollution-reducing policies and practices at the regional and local level. The most significant value of the suite of modeling tools is the ability to predict how the Chesapeake Bay will respond to future conditions such as pollutant loads, land use changes and climate change. A fact sheet on the models is available. Information about the changes in moving to the Phase 6 Watershed Model are available here

Why use CAST?

CAST enables planners in the watershed to develop a plan for meeting a nitrogen, phosphorus, or sediment load allocation using the most cost-effective strategy. CAST can be used to answer questions about the effect of different BMPs on loads, the impact of land use development over time, and to identify the geographical location where BMPs will reduce the most load. CAST provides estimates of load reductions. CAST allows users to understand which BMPs provide the greatest load reduction benefit, the extent to which these BMPs can be implemented, and the cost of these BMPs. Based on the scenario outputs, users can refine BMP choices in their planning.

CAST facilitates an iterative process to determine if Total Maximum Daily Load (TMDL) allocations are met. Scenarios may be compared to each other, TMDL allocations, or the amount of nitrogen, phosphorus, and sediment from the Watershed Implementation Plan (WIP) or a current annual progress scenario. CAST is used to facilitate Chesapeake Bay TMDL milestone and WIP development. 

CAST is the Chesapeake Bay Program’s (CBP) Watershed Model. Other available tools have assumptions that may be different from those used in the Watershed Model for developing the 2010 Chesapeake Bay TMDL. Since the Watershed Model is used to assess jurisdictions’ progress toward meeting the TMDL allocations, consistency with the Watershed Model is critical.

What are CAST’s outputs?

CAST estimates of load reductions for load sources include: agriculture, developed, natural, wastewater, and septic loading to the edge of a small stream (EOS) and loads delivered to the tidal portion of the Chesapeake Bay (EOT). CAST stores the geographic area, cost and implementation level associated with each BMP as well as the load for each sector and land use. With these data tables, CAST also serves as a data management system. Thus, users may quantify the impacts of various management actions while improving local management decisions.

Who benefits from using CAST?

CAST is used by multiple local jurisdictions and states for the Phase II and III WIPs, two-year Milestones and even local TMDLs. Any user may see the source of the data that was used in developing the TMDL and the state’s most recent annual progress scenario, Milestone and WIP. This allows involvement of the counties and other local planners in the Bay TMDL. CAST is easily accessible on-line with no need to install specific databases or software. All who request a login are granted one.

Why was CAST developed?

The first version of CAST was launched in 2011 to provide local jurisdictions, such as counties, with a tool to provide input into the TMDL WIP process. The U.S. Environmental Protection Agency (EPA) issued a TMDL in 2010 for the Chesapeake Bay based on allocations established by the states. The jurisdictions that drain to the Chesapeake Bay include New York, Pennsylvania, West Virginia, Maryland, Delaware, District of Columbia, and Virginia. The states agreed that it would be more efficient for states to allocate responsibility within their respective political boundaries, and for EPA to issue one overall TMDL that reflected each state’s allocation. Since planning happens at a more local scale, such as county, some states downscaled the allocation to the county level.

How is information entered into CAST?

CAST is designed to be useful to people with a general knowledge of BMPs. Knowledge of models or BMP load reduction calculations is not necessary. CAST is available on-line to users with a login and password, which may be requested from the website.

Data are entered into CAST in the following sequence:

  • The user selects a geographic area, such as a county.
  • CAST uses the Chesapeake Bay Program Partnership-approved data to populate the parameters of the scenario based on user selections. The user can build a new scenario or import features of an existing scenario. The user may opt to share the scenario with other users on the system.
  • The user establishes costs of BMPs, or can use the defaults provided.
  • The user adds BMPs to the scenario using separate screens with options for developed, septic, natural, agriculture, animals, and manure transport. The user may edit the BMP selections at any time to modify the scenario.
  • The user selects calculate and the loads and costs are provided on screen and in downloadable tables.
  • The user also may compare scenarios.

Users may choose to enter BMPs as percent of land available or as amounts. The number of available acres changes depending on the other BMPs in the scenario. For example, the forest buffer BMP removes acres from one land use and converts it to forest. This means that there are fewer acres for other BMPs in the original land use. More information on the sequence of BMP application is found in the CAST technical manual file posted in documentation.

Who develops and maintains CAST?

The Chesapeake Assessment Scenario Tool (CAST) was first developed in 2011 by Jessica R. Rigelman of J7 LLC and Olivia H. Devereux of Devereux Consulting, Inc., with funding provided by U.S. EPA. The immediate need was to offer local planners within the Chesapeake Bay Watershed with a tool to provide input into the TMDL Watershed Implementation Plan development process. With continued funding from the U.S. EPA, the functionality of CAST has been expanded to serve multiple needs for environmental planning in the Chesapeake Bay Watershed. For more information, visit the Contact Us page. Future enhancements are developed with input from the states and local jurisdictions, the Chesapeake Bay Program Partnership, and the EPA. CAST will continue to evolve to meet the needs of users.