Modeling

Environmental computer models are mathematical representations of the real world that estimate environmental events and conditions. Models are used to simulate ecosystems that are too large or complex for real-world monitoring.

Chesapeake Bay model simulations, called scenarios, project pollution loads and flow and simulate how various changes or pollution-reduction actions could affect the Bay ecosystem, especially water quality, wildlife and aquatic life.

Why modeling is important

Because the Chesapeake Bay and its watershed are so large and complex, scientists and restoration managers rely on computer models for critical information about the ecosystem’s characteristics and the impact of various environmental actions to reduce pollution. Models are vital tools that help guide decision-making for reducing pollution and meeting water quality standards.

Although model simulations are an important part of the Chesapeake Bay restoration effort, they are not considered to be perfect forecasts. Rather, model 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.

Chesapeake Bay models

Bay Program 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, and from the land to the air.

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 current version of the Watershed Model, Phase 5.3.

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 land uses and pollution loads are determined

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.

Developing and improving the Chesapeake Bay models

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.

Over time, the Bay models have improved significantly in precision, scope, complexity and accuracy. For example, in the current version of the Watershed Model, Phase 5.3:

• The number of linked monitoring stations expanded from 20 in the previous version to 296
• The number of segments in the model grew from 94 to more than 2,000, providing information at the sub-watershed, county and conservation district levels
• River segments in the model expanded twenty-fold, from 94 to more than 2,000
• The types of land uses that can feed into the model increased from 9 to 25
• The simulation is now run over a 20-year period, rather than 10 years

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.

For more information about modeling, download the Chesapeake Bay Environmental Modeling backgrounder.