Environmental computer models are mathematical representations of the real world that estimate environmental conditions, and can be used to simulate ecosystems that are too large or complex for real-world monitoring. Chesapeake Bay model simulations, called scenarios, simulate how various changes or actions could affect the Bay ecosystem, especially water quality, wildlife and aquatic life.

Why is modeling important?

Because the Chesapeake Bay and its watershed are so large and complex, scientists and decision-makers rely on computer models for information about the ecosystem and the impact of efforts to reduce pollution and improve water quality.

Although model simulations are an important part of the Chesapeake Bay restoration effort, they are not considered to be perfect forecasts. Rather, simulations are best estimates based on state-of-the-art, extensively peer-reviewed science. Modeling is part of a broader toolkit that includes research and monitoring to gain the highest possible level of accuracy.

What types of models are used for the Chesapeake Bay?

Our partners and other stakeholders use a suite of computer models that are among the most sophisticated, studied and respected in the world. The models provide a comprehensive view of the Chesapeake ecosystem: from the depths of the Bay to the upper reaches of the watershed, from the land to the air. The Chesapeake Community Modeling Program also links to a variety of Bay-related models, data sources and other resources.

Watershed Model

The Watershed Model incorporates information about land use, fertilizer applications, wastewater plant discharges, septic systems, air deposition, farm animal populations, weather and other variables to estimate the amount of nutrients and sediment reaching the Chesapeake Bay and where these pollutants originate.

The Watershed Model divides the 64,000-square-mile Chesapeake Bay watershed into more than 2,000 segments delineating political and physical boundaries. Each segment contains information generated by several sub-models:

  • The hydrologic sub-model uses rainfall, evaporation and meteorological data to calculate runoff and sub-surface flow for all land uses, including forest, agricultural and urban lands.
  • The surface and sub-surface flows ultimately drive the non-point source sub-model, which simulates soil erosion and pollutant loads from the land to rivers.
  • The river sub-model routes flow and associated pollutant loads from the land through lakes, rivers and reservoirs to the Chesapeake Bay.

Learn more about the updates made to the Watershed Model in Phase 6.

Estuary Model

The Estuary Model examines the effects that pollution loads generated by the Watershed Model have on water quality. In the Estuary Model, the Chesapeake Bay is represented by more than 57,000 computational cells and is built on two sub-models:

  • The hydrodynamic sub-model simulates the mixing of waters in the Bay and its tidal tributaries.
  • The water quality sub-model calculates the Bay’s biological, chemical and physical dynamics.

Scenario Builder

Scenario Builder can generate simulations of the past, present or future state of the Chesapeake Bay watershed to explore potential impacts of management actions and evaluate alternatives.

Scenario Builder produces inputs for the Watershed Model based on factors from a wide range of land uses and management actions. For example, information such as acres of different crops, numbers of animals and extent of conservation practices is used to generate Watershed Model inputs for agricultural land use types.

Airshed Model

The Airshed Model uses information about nitrogen emissions from power plants, vehicles and other sources to estimate the amount of and location where these pollutants are deposited on the Chesapeake Bay and its watershed. That information is fed into the Watershed Model.

Land Change Model

The Land Change Model analyzes and forecasts the effects of urban land use and population on sewer and septic systems in the Chesapeake Bay watershed.

The forecasts are based on:

  • Reported changes from the U.S. Census Bureau in housing, population and migration
  • Land cover trends derived from satellite imagery
  • Sewer service areas
  • County-level population projections
  • Conversion of forests and farmland development is based on a thorough examination of urban development and land conversion trends derived from satellite imagery dating back more than 25 years.

How do models determine land uses and pollution loads?

To accurately simulate the Chesapeake ecosystem, models are built on current and specific uses of land in the watershed, such as forests, farms and development. Land uses are determined using authoritative sources, such as satellite imagery and the USDA Census of Agriculture. Models are further refined by inputting land management features such as cover crops on farm fields and stormwater controls in urban areas.

The types and amounts of pollution that run off a particular land use are based on comprehensive reviews of the latest scientific literature. For example, the pollution loads incorporated into the Watershed Model are based on research from more than 100 academic papers. This comprehensive literature review provides the average pollution loads that various land uses contribute.

Pollution loads are also cross-checked with previous versions of the model and other regional and national models. Pollution loads are further adjusted based on in-stream monitoring data, which increases accuracy for land use and location. Conservation practices, management actions and pollution controls that are implemented in specific places are then entered into the model to simulate reductions from these factors.

How are the Chesapeake Bay models being improved?

The suite of Chesapeake Bay models has been developed during nearly 30 years of collaboration by federal, state, academic and private partners. Developers include the U.S. Environmental Protection Agency, U.S. Geological Survey, USDA Natural Resources Conservation Service, U.S. Army Corps of Engineers, University of Maryland, Virginia Tech, Penn State University and Chesapeake Research Consortium. Advisers include Delaware, Maryland, New York, Pennsylvania, Virginia, West Virginia and the District of Columbia.

The Bay Program continues to improve the quality of the data in the models by involving a wide range of partners, stakeholders and experts. Revised versions of the models are regularly shared with partners throughout the Chesapeake Bay community to allow for review, testing and suggestions. Anyone can participate in improving the Bay models, including working with the Bay Program to have credible data and restoration practices incorporated. The models also undergo extensive independent scientific peer review by federal, state and academic scientists, as well as modeling experts.

Over time, the Bay models have improved significantly in precision, scope, complexity and accuracy. For example, the current version of the Watershed Model is currently in Phase 6.

What was improved in Phase 6?

  • The model can better predict the impacts of population growth and climate change and better account for the sediment build-up behind the Conowingo Dam on the Susquehanna River in Maryland.
  • Ten additional years of water quality monitoring data doubles the amount of real-time data and provides more insight into how pollution loads have changed.
  • High-resolution land cover data allow for a one-by-one meter resolution, providing 900 times the amount of information than was previously available.
  • Additional categories of land use data are included.
  • New inputs from the agricultural community, particularly the poultry industry, and improved and updated information on the application of fertilizer and manure will improve accuracy.
  • Additional best management practices are now credited and incorporated.
  • Improved nutrient input data have been added.
  • A simulation period that runs over 20 years rather than 10 years allows for finer-scale analysis and planning.

Learn more about the Phase 6 Modeling tool‍

Phase 7 Model Development

Phase 7 Modeling Tools are currently in development and will be available for partnership use by 2027.

Learn more about the Phase 7 Model Development project