Modeling

Environmental models are essential for simulating ecosystems that are either too large or too complex to isolate for experiments in the real world. Models allow scientists to simulate changes in an ecosystem due to changes in population, land use or pollution management. These simulations, called scenarios, allow scientists to predict positive or negative changes within our ecosystem due to management actions such as improved sewage treatment, controlling urban sprawl and reduced fertilizer or manure application on agricultural lands.

Models use mathematical representations of the real world to estimate the effects of complex and varying environmental events and conditions. For example, the Chesapeake Bay Watershed Model estimates the delivery of nutrients and sediments to the Bay by simulating hydrologic and nutrient cycles, using inputs such as atmospheric nutrient deposition, precipitation, fertilizer application, and land cover or land use.

Why are Bay Program models important?

Models are one of the principal tools crucial to Bay Program goals of reducing nutrients and sediments delivered to the Bay. In 1992, Bay Program partners agreed to reduce controllable loads of nitrogen and phosphorous delivered to the Bay by 40 percent of 1985 levels by the year 2000. From this goal, the Bay models were used to develop tributary nutrient allocations for each of the nine major tributaries in the Chesapeake watershed.

In 2000, these tributary allocations became a nutrient cap, not to be exceeded even with future increases in population and growth. With the nutrient cap in place, Bay models are used to track nutrient loads to ensure the cap is not exceeded. Currently, the models are directed toward the examination of the need for further nutrient and sediment reductions to fully restore the water quality required for the Bay's living resources.

Models produce estimates, not perfect forecasts. They reduce, but do not eliminate, uncertainty in environmental decision-making. Used properly, they are a tool that can assist in developing nutrient and sediment reductions that are most protective of the environment, while being equitable, achievable and cost-effective.

What models does the Bay Program have?

The Watershed Model

The Watershed Model divides the 64,000-square-mile Chesapeake Bay watershed into 94 model segments. Each segment contains information generated by a hydrologic submodel, a non-point source submodel and a river submodel.

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

Learn more about Phase 5, the latest version of the Watershed Model. (Note: The current version of the Watershed Model is Phase 4.3)

The Estuary Model

The Estuary Model, commonly referred to as the Water Quality Model, examines the effects of the loads generated by the Watershed Model on Bay water quality. In the Estuary Model, the Bay is represented by more than 10,000 computational cells that average 6 miles long, 2 miles wide and 5 feet deep. The cells are stacked up to 15 layers in the deepest areas of the Bay.

The Estuary Model is built on two submodels: the hydrodynamic submodel and the water quality submodel.

• The hydrodynamic submodel simulates the mixing of estuarine waters with coastal ocean waters and the mixing of water within the Chesapeake.
• The water quality submodel calculates the chemical and physical dynamics of the Chesapeake.

The Airshed Model

The Airshed Model (Regional Acid Deposition Model - RADM) tracks nitrogen emissions from all sources in the airshed. The model is three-dimensional; it simulates movement both vertically and horizontally across a region. The Airshed Model covers the eastern United States from Texas and North Dakota eastward to Maine and Florida with 22,000 cells. Each cell measures 80 kilometers square. Stacked up, the cells make 15 vertical layers reaching about 15 kilometers high.

The airborne nutrient loads are transported by the Airshed Model and linked to the:

• Watershed Model through deposition to land surfaces.
• Estuary Model through deposition to the water surfaces of the tidal Bay.