Model Documentation
Suggested Citation
Chesapeake Bay Program, 2024. Chesapeake Assessment and Scenario Tool (CAST) Version 2023, Phase 6-[7.XX.X See bottom of any page for specific version number]. Chesapeake Bay Program Office, Last accessed [Month, Year].
Preview of Phase 7
The land data team developed a methodology for the land use for years prior to 2013. In 2013, 2017, and 2021, there are land cover/use data that is at the 1-meter scale. Prior to those years, the data available are much less detailed. This document describes the methodology for developing the back-cast land use for 1985 through 2012.
Land Use Back Cast Methodology
CAST-23 version
CAST-23 is planned to be the last model update to the phase 6 suite of models. This follows the decision to update the CAST schedule, which was approved by the Water Quality Goal Implementation Team, Management Board, and Principals' Staff Committee. The decision can be found in the September 26, 2023 PSC meeting minutes. CAST-23 contains all the planned updates for CAST-21 plus:
- Updated BMP history (as of February 2024)
- 1985-2016 updated AAPFCO fertilizer for the agricultural and urban sector
- Base conditions are newly available through 2030.
- State-supplied fertilizer methodology and data change
- Urban fertilizer methodology and data change
- Oyster BMPs
- Animal mortality BMPs
- No expiration for wetland BMPs
- Resource Improvement Forest Buffer BMPs (9 and 10) added
- Changes to nutrient application eligibility
- Correction to regulated/unregulated Virginia developed acreage in CAST
Additional CAST-23 related resources are linked below.
- Response to comments
- Comparison of Loads and Inputs with prior version--Data Visualization Tool
- 2023 Planning Targets Exchanges Climate CWIP UALs EUs
CAST-21 version (never released)
BMPs in progress scenarios are pulled from the National Environmental Information Exchange Network (NEIEN), and used to update that year's progress data. This means that new inspections, new cumulative BMPs in any year, and new annual BMPs are used for that year's progress. The prior years' progress scenarios are not updated. However, when changing to a new version of CAST, all years' progress scenarios are updated to include new BMP history.
- Fact Sheet
- Understanding Chesapeake Bay Modeling Tools
- Comparison of Loads and Inputs Between CAST-19 and CAST-21--Data Visualization Tool
- Technical Documentation of the Change Between CAST-19 and CAST-21
- Response to Comments
CAST-19 version
CAST-17d is updated to CAST-19 with changes to data and BMPs used in the Phase 6 model for the milestone period. This follows the Principals' Staff Committee decision of July 9, 2018 that changes are made only in advance of the two-year milestone period. The decision can be found in the July 9, 2018 PSC meeting minutes. These changes were agreed to by the WQGIT and its workgroups. The changes are limited in scope so that they do not: 1) impact modeled runoff during the 1993-1995 critical period; or 2) alter the base conditions (land uses, septics, animals, etc.) from 1984 through 2013. Preservation of these estimates enables a consistent assessment of how new management actions and changes in base conditions have influenced loads over time.
- Fact Sheet
- Understanding Chesapeake Bay Modeling Tools
- Comparison of Loads and Inputs Between CAST-17d and CAST-19--Data Visualization Tool
- Technical Documentation of the Change Between CAST-17d and CAST-19
- Comments from the Jurisdictions and the Chesapeake Bay Program Responses
Climate Change
Results of updated methods, techniques, and studies that are used to develop an estimate of pollutant load changes (nitrogen, phosphorus, and sediment) due to 2025 climate change conditions. Planning targets for the TMDL, climate change, and Local Area Planning Goals are available.
Phase 6 Dynamic Watershed Model and CAST-17 documentation
The documentation is for the dynamic and time-averaged Watershed Model. CAST is the same as the time-averaged Phase 6 Model. Creating and running scenarios in CAST is simply using an on-line interface to the time-averaged Model. Due to the length of the documentation, it is divided into sections. Click on the links below to read through the different portions of the documentation.
- Overview
- Average Loads
Appendix 2A: Agricultural Loading Rates - Terrestrial Inputs
Appendices ABCDG: Terrestrial Inputs
Appendix 3E: Swine Characterization Study Final Report
Appendix 3F: Turkey Litter Nutrients
Appendix 3H: Atmospheric Deposition - Sensitivity
Appendix 4A: Sensitivity analysis of the HSPF AgChem Model
Appendix 4B: Sensitivity analysis for all land uses - DRAFT Land Use
DRAFT Land Use Appendix - Best Management Practices
Appendix 6A: BMP Expert Panel Protocol
Appendix 6B: Order of Load Source Change Credit - Land to Water
- Direct Loads
- Stream to River
Appendix 9A: Alternate Stream to River Methods
Appendix 9B: Excluded Reservoir Catchments - River to Bay and Temporal Simulation
Appendix 10A: Ftables and Stations
Appendix 10B: Calibration Stations
Appendix 10C: Nutrients and Sediment Calibration Targets
Appendix 10D: HSPF River Water Quality Parameters
Appendix 10E: Estuarine Model Linkage - Physical Setting
Appendix 11A: List of Segments - Applications
- Reviews
- References
- Errata
Phase 6 Model Performance
More information on the Watershed Model can be found at the Modeling Workgroup page.
Phase 6 STAC Review
- STAC Chesapeake Bay Watershed Model Phase 6 Review
- Letter from STAC Watershed Model Review
- Response to STAC Watershed Model Review Report
- Detailed Response to STAC Watershed Model Review Report
Non-Tidal Water Quality Dashboard
Comparisons between the modeled and monitored data can be found on the non-tidal water quality dashboard. These visual representations show both the loads estimated from the U.S. Geologic Survey's Weighted Regressions on Time, Discharge and Season (WRTDS) and loads estimated from the time-variable Phase 6 Watershed Model.
Non-Tidal Water Quality Dashboard
The Chesapeake Bay Nontidal Monitoring Network Loads and Trends page shares the short-term water year nutrient and suspended-sediment load and trend results and background information for the Chesapeake Bay Program's nontidal network (NTN). It provides primary findings for nitrogen, phosphorus, and sediment trends and examines results for river basins for individual sites.
Chesapeake Bay Nontidal Monitoring Network Loads and Trends
Graphical Interface to the Phase 6 Watershed Model Inputs
Graphs and maps of the inputs to the Phase 6 Watershed Model for all versions, along with the option to download the source data, is provided to facilitate transparency and offer a common, public platform for reviewing data.
Due to the size of the datasets required to present this information, the Graphs and Maps have been divided into the following categories.
- Animal Data
- Atmospheric Deposition Data
- Soils and Plant Uptake Data
- Nutrient Applications and Land Use
- Septic Data
The calibration land use data is available on the Nutrient Applications link above. It is shown in maps with the ability to filter by sector, land use, and agency. The tabular land use for years between 1984 and 2013 is available upon request.
Not all the inputs are applied to each of the load sources. For example, manure nitrogen and phosphorus are not attributed to developed lands. Only leguminous plants fix nitrogen, so the fixed nitrogen input only applies to load sources that may contain crops or plants that are leguminous. The table of Loads by Load Source provides a ready reference of the inputs for each load source.