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Modeling the Chesapeake Bay

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
7management actions such as improved sewage treatment, reduced fertilizer or manure application on
agricultural land or controlling urban sprawl.

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.

Models are one of the principal tools crucial to the 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%
of 1985 levels by the year 2000. From this goal, the Bay models
were used to develop tributary nutrient allocations, or
reductions, 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

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Chesapeake Bay Program

A Watershed Partnership

The Chesapeake Bay Estuarine model examines the
effects of the loads generated by the Watershed
Model on Bay water quality. The model divides the
Bay into almostl 3,000 computational cells with an
average surface area of about 1.5 square miles. The
cells are stacked up to 17 layers deep in the deepest
part of the Bay.

Visit the Chesapeake Bay Program website:
http://www.chesapeakebay.net

The Chesapeake Bay Program is restoring the Bay through a partnership among the U.S. Environmental Protection Agency representing the federal government, the State of Maryland, the
Commonwealth of Pennsylvania, the Commonwealth of Virginia, the District of Columbia, the Chesapeake Bay Commission, and participating citizen advisory groups.


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Modeling the Chesapeake Bay

Page 2

The Watershed Model

The Watershed model divides the 64,000 square mile
Chesapeake Bay drainage basin into 94 model segments. Each
segment contains information generated by a hydrologic
submodel, a nonpoint source submodel and a river submodel.

The hydrologic submodel uses rainfall, evaporation and
meteorological data to calculate runoff and subsurface flow for
all the basin land uses including forest, agricultural and urban
lands. The surface and subsurface flows ultimately drive the
nonpoint source submodel, which simulates soil erosion and the
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.

The Estuary Model

The estuarine 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 almost 13,000 computational cells
which average six miles long, two miles wide and five feet deep.

The cells are stacked up to 17 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 Chesapeake Bay Watershed Model divides the
Bay's 64,000 square mile drainage basin into 94 model
segments.

The Airshed Model

The Chesapeake Bay airshed covers an area 6.5 times larger than its
watershed. The Bay Airshed Model divides this area into 22,000 cells,
each measuring twenty kilometers square.

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 eighty kilometers square.
Stacked up, the cells make fifteen vertical layers
reaching about fifteen kilometers high. The airborne
nutrient loads are transported by the Airshed Model
and linked to the Watershed Model through
deposition to land surfaces and to the Estuary
Model through deposition to the water surfaces of
the tidal Bay.

Visit the Chesapeake Bay Program website:
http://wvw.chesapeakebay.net

The Chesapeake Bay Program is restoring the Bay through a partnership among the U.S. Environmental Protection Agency representing the federal government, the State ot Maryland, the
Commonwealth of Pennsylvania, the Commonwealth of Virginia, the District of Columbia, the Chesapeake Bay Commission, and participating citizen advisory groups.


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