---
id: twins/chesapeake-cast
about:
  label: Chesapeake Assessment Scenario Tool (CAST)
  description: >-
    The Chesapeake Bay Program's web-based watershed model of the Chesapeake
    Bay, United States; a planner picks an area and management practices and it
    estimates the resulting nitrogen, phosphorus and sediment loads and their
    cost.
  tags:
    - watershed
    - water quality
    - nutrient loads
    - scenario planning
    - United States
twin:
  place: 'Chesapeake Bay watershed, United States'
  scale: region
  operational: operational
  homepage: 'https://cast.chesapeakebay.net/'
open:
  source: unknown
  data: open
  api: unknown
  participation: partly
  note: >-
    Source data tables, public reports, maps and model documentation are on the
    site; building and sharing scenarios needs a login, which is granted to
    everyone who asks; model changes come from the Bay Program partnership, the
    states and the EPA.
domains:
  - water
  - ecology
provenance:
  origin: agent
  sources:
    - title: CAST - Home Page
      url: 'https://cast.chesapeakebay.net/'
    - title: CAST - About CAST
      url: 'https://cast.chesapeakebay.net/About'
    - title: CAST - Model Documentation
      url: 'https://cast.chesapeakebay.net/Documentation/ModelDocumentation'
  created: '2026-10-18'
  reviewed: '2026-10-18'
  reviewed_by: 'Claude, from the sources listed'
  status: draft
---
CAST is the Chesapeake Bay Program's Watershed Model, reached through a web interface. The program's suite of models divides the 64,000-square-mile watershed into thousands of segments to replicate the Bay's conditions and predict how it responds to pollutant loads, land-use change and changing environmental conditions. CAST is the time-averaged form of the Phase 6 Watershed Model: a user chooses a geographic area such as a county, adds best management practices for developed land, septic systems, natural land, agriculture, animals and manure transport, sets costs or uses defaults, and the tool returns nitrogen, phosphorus and sediment loads at the edge of stream and delivered to the tidal Bay, with costs, on screen and in downloadable tables.

It exists for the Chesapeake Bay Total Maximum Daily Load that the US Environmental Protection Agency issued in 2010 for New York, Pennsylvania, West Virginia, Maryland, Delaware, the District of Columbia and Virginia. States and counties use it to write their Watershed Implementation Plans and two-year milestones, to compare scenarios with their allocations, and to find where practices cut the most load for the least money. An Ecosystem Benefits Browser shows the co-benefits of those practices.

What is open: the site offers source data tables (load sources, agencies, practices, animals, geographic references, delivery factors), practice costs, shapefiles, monitoring maps, public reports and the Phase 6 model documentation, calibration plots and the scientific review. Any user may see the data behind the TMDL and each state's latest progress scenario. Creating scenarios requires a login, and the about page says all who request one are granted it. Nothing on the pages read states a licence for the data or says whether the model code is published.

Status: in operation. The first version was built in 2011 by Jessica R. Rigelman of J7 LLC and Olivia H. Devereux of Devereux Consulting with EPA funding; the pages show Phase 6 version 8.5.0 as the model of record, CAST-23 as the planned last Phase 6 update, and a Phase 7 suite of models in development.