Attribute Modeling

Attachment 17 – Description of models used to choose attribute levels
EPA uses information from several sources in order to develop an appropriate range of attribute levels for
the baseline and policy scenarios presented in the survey. This information includes different
hydrological, biochemical and ecosystem models and represents a significant multi-agency effort. The
purpose of this survey is to collect data that will enable EPA to value a variety of water quality
improvements relative to a range of baseline scenarios. In order to estimate economic benefits of the
Chesapeake Bay TMDLs the experimental design of the survey must include attribute levels for baseline
predictions and policy scenarios that cover the range relevant to the TMDLs, but need not be limited to
that range. EPA’s choice to value changes in environmental outcomes and a range of attribute levels
relative to multiple baselines provides the flexibility to estimate benefits as expectations of water quality
in the Chesapeake Bay evolve. This Information Collection Request (ICR) and request for public
comment pertain to the survey instrument and stated preference methodology described therein. The data
collected from this survey will be combined with information from numerous other scientific models and
studies to estimate the benefits of the TMDLs. EPA will submit a report of the stated preference study
results for public comment and peer review which will include predictions of conditions under baseline
and policy scenarios with descriptions of our modeling approach.

Summary information about the 3 main models used to generate data for the range of attribute levels in
the baseline conditions and policy scenarios is described below. Briefly, the attribute levels for each
endpoint are generated by the following models:
1.

Water clarity: The Chesapeake Bay Water Quality Models produce water clarity measures for
different baseline and policy scenarios when nutrient and sediment loadings change.

2. Striped bass, Blue crab, and Oysters: Changes in nutrient loadings from the Chesapeake Bay
Water Quality Models are used as inputs in the Chesapeake Bay Fisheries Ecosystem Model
(CBEFM) to forecast changes in striped bass, blue crab and oyster populations under baseline and
policy scenarios.
3. Watershed lakes: Changes in nutrient loadings are used in an application of the SPARROW
model to forecast changes in the eutrophication of freshwater lakes in the Watershed.

Chesapeake Bay Water Quality Models
In order to obtain a range of attribute levels projected for baseline conditions and policy scenarios in the
survey for water clarity EPA uses output from the Chesapeake Bay Water Quality Models, a suite of
models of the Chesapeake Bay Watershed developed by the Chesapeake Bay Program. The Chesapeake

Bay Program is a regional partnership that includes federal and state agencies, local governments, nonprofit organizations, and academic institutions. The Chesapeake Bay Program oversees the largest,
comprehensive modeling effort of the Chesapeake Bay Watershed, to the best of our knowledge. EPA
has worked closely with the Chesapeake Bay Program to obtain scientific information on a range of water
clarity predicted from the models. The Chesapeake Bay Program works with 5 models, several with subcomponent models, to simulate and project pollution loads to the Chesapeake Bay. More on the Water
Quality Model can be found at http://www.chesapeakebay.net/about/programs/modeling. The 5 models
used by the Chesapeake Bay Program are described briefly below.

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. This model divides the Bay into more than 2,000 river segments. Each segment is represented
by several sub-models, including a hydrologic sub-model that calculates runoff and sub-surface flow for
all land uses and a non-point source sub-model that simulates erosion and pollutant loads from land to
rivers. Extensive documentation, including underlying data is available at:
www.chesapeakebay.net/programs/modeling/53.

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.

Chesapeake Bay Fisheries Ecosystem Model (CBFEM)
In order to obtain a range of attribute levels projected for baseline conditions and policy scenarios in the
survey for populations of striped bass, blue crabs, and oysters EPA uses output from the Chesapeake Bay
Fisheries Ecosystem Model (CBFEM). The CBFEM is a fisheries-oriented trophic network model
developed by NOAA for the Bay using a publicly available and widely applied software package,
Ecopath with Ecosim, or EwE, developed by the University of British Columbia Fisheries Centre and
others. Ecopath is a static, mass-balanced snapshot of the system and Ecosim is a time dynamic
simulation module for policy exploration. The major species for the CBFEM application of EwE include
the multi-stanza (or life-stage) representations of commercially important species (striped bass, bluefish,
weakfish, white perch, Atlantic menhaden, blue crab, and oyster) as well as single biomass pool groups of
other commercially important species including American eel, Atlantic croaker, summer flounder, spot,
alewife, American shad, black drum, catfish, and bivalves. CBFEM uses output from the Chesapeake
Bay Water Quality Models, namely changes in nutrient loadings, in order to forecast changes in fish
populations. CBFEM forecasts changes in relative biomass that can be used with current populations to
forecast populations in the future. Documentation for the CBFEM can be found at
http://chesapeakebay.noaa.gov/ecosystem-modeling/chesapeake-bay-fisheries-ecosystem-model.

SPARROW Model
In order to forecast the algae levels in lakes in the Watershed EPA uses information from the U.S.
Geologic Survey’s Spatially Referenced Regressions On Watershed Attributes (SPARROW) nutrient
models. The SPARROW model is a well established hydrological nutrient delivery model that has been
used by government agencies and academic researchers since 1997 to analyze the source and effect of
nutrient loading to water bodies and is available for public review. Moore, et al. (2011) describes the
development of a Northeast US SPARROW model which provides predictions of total phosphorus for
lakes in the region. EPA’s Office of Research and Development (ORD) uses the phosphorus predictions

to convert lakes to trophic states which are associated with different levels of algae. The relationship
between Phosphorus concentrations and the amount of algae are well established in the scientific
literature (see Schindler and Vallentyne (2008) or Wetzel (2010)). Additional documentation for the
SPARROW model and its application to the Chesapeake Bay can be found at
http://water.usgs.gov/nawqa/sparrow/.

References
Moore, R. B., C. M. Johnston, R. A. Smith and B. Milstead (2011). Source and Delivery of Nutrients to
Receiving Waters in the Northeastern and Mid-Atlantic Regions of the United States. JAWRA Journal of
the American Water Resources Association. 47(5): 965-990. Available at:
http://dx.doi.org/10.1111/j.1752-1688.2011.00582.
Schindler, D.W. and J.R. Vallentyne. 2008. The algal bowl: Overfertilization of the World’s
Freshwaters and Estuaries. Edmonton: University of Alberta Press.

Wetzel, R.G. 2010. Limnology: Lake and River Ecosystems. San Diego, CA: Academic Press.