Modeling

Environmental models are mathematical representations of the real world. Models allow scientists and decision-makers to observe patterns, test scenarios and predict how the environment will respond to pollution-reducing practices, land use policies and changing environmental conditions. Although model simulations are an important part of environmental restoration, they are not considered perfect forecasts. Simulations are best estimates based on state-of-the-art, peer-reviewed science, and it is by combining modeling and monitoring data that we gain a comprehensive view of the Chesapeake ecosystem.

Modeling Tools

The Chesapeake Bay Program has developed a suite of modeling tools that work together to determine how much nitrogen, phosphorus and sediment pollution is entering local waterways, where it is coming from, how it is affecting water quality and more. These tools have been developed over decades of collaboration by federal, state, academic and private partners, and have undergone extensive independent peer review. An overview of these tools can be found in our Modeling the Chesapeake Bay Watershed Fact Sheet (PDF).

Airshed and Land Change Models

The Airshed Model estimates the amount and location of nitrogen deposited onto the land and water by vehicles, power plants and other emission sources. The Land Change Model forecasts the effects of population and land use changes on sewer and septic systems. Information from the Airshed and Land Change Models is fed into the Watershed Model.

Watershed Model

The Watershed Model estimates the amount of nitrogen, phosphorus and sediment reaching the Chesapeake Bay. It divides the watershed into more than 2,000 segments delineating political and physical boundaries. Each segment contains information generated by three 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 non-point source sub-model 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 Bay.

The Watershed Model is available as a free, web-based tool called the Chesapeake Assessment Scenario Tool (CAST). CAST helps state and local governments, watershed planners and others develop more cost-effective strategies for reducing nitrogen, phosphorus and sediment loads. It can help users better understand which best management practices (BMPs) could provide the greatest load reduction; the extent to which these BMPs could be implemented based on available resources, available land and the cost of implementation; and how to refine these BMPs to meet planning needs.

Estuary Model

The Estuary Model examines the effects that pollution loads generated by the Watershed Model have on water quality. The Estuary Model 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.

Watch: Monitoring and Modeling the Chesapeake Bay

Model Improvements

The ongoing improvement of our modeling tools is a collaborative process involving partners, stakeholders and experts. Over time, these tools have improved in precision, accuracy, complexity and scope. While past improvements occurred whenever new data and information became available, the program has transitioned to a more predictable update schedule to provide stability while still allowing for the incorporation of the best available science, data and information. 

A history of model updates, governance, policy and procedures can be found in our Understanding Chesapeake Bay Modeling Tools Backgrounder.

Phase 6 Improvements

The newest version of our suite of modeling tools is known as Phase 6. Phase 6 includes expanded and improved data about land use, pollution sources, pollution loads and the efficiencies of best management practices, allowing us to better answer questions about how different land use types and land management decisions can affect nutrient and sediment levels. It can also better predict the impacts of population growth and climate change and better account for sediment build-up behind the Conowingo Dam, helping planners explore options for addressing these factors influencing environmental restoration. More information can be found in our Phase 6 Modeling Tools Fact Sheet (PDF).

Phase 7 Development

Phase 7 modeling tools are currently under development and will be available for use by 2027.