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emains in Category 5 until all pollutants are addressed in a completed/EPA-approved
TMDL, or until the water is ‘de-listed’ via other approved administrative means.
QUANTIFICATION OF PROBLEM
Water bodies identified on the State’s 303(d) list as being impaired require the
completion of a TMDL assessment. Such assessments can be done for individual
impaired stream segments. More typically, however, such assessments are done on
a watershed or sub-watershed basis, with the assessed areas ranging in size from
about 2–200 square kilometres. Development of a TMDL implies development of a
report with specific mandated elements. At a minimum, this report must describe
the water quality problem and identify allowable pollutant loads to the water body
from both point and non-point sources that would prevent a violation of water quality
standards in the future. This latter requirement dictates that some means be used to
quantify pollutant loads in a watershed for which a TMDL is being prepared. Given
the general lack of sufficient in-stream water quality data, many states in the United
States have decided to use computer simulation models to derive the necessary load
estimates. In Pennsylvania, a GIS-based modelling approach has been developed
to support TMDL assessments in watersheds where nutrients and/or sediments have
been determined to be the primary cause(s) of stream impairment. This approach
involves the use of AVGWLF, a calibrated modeling tool that facilitates the use of
the Generalized Watershed Loading Function (GWLF) model (Haith and Shoemaker,
1987) and customized statewide datasets and model parameterization routines
within an ArcView GIS software interface.
The general approach used for non-point source TMDLs in Pennsylvania is to: (1)
derive GWLF input data for use in the impaired watershed; (2) simulate nutrient and/
or sediment loads within the impaired watershed; (3) compare simulated loads within
the impaired watershed against loads simulated for a nearby ‘reference’ watershed
that exhibits similar landscape, development and agricultural patterns, but that also
has been deemed to be unimpaired; and (4) identify and evaluate pollution mitigation
strategies that could be applied in the impaired watershed to achieve pollutant loads
similar to those calculated for the reference watershed.
With AVGWLF, a customized interface developed by Penn State for the ArcView GIS
package is used to parameterize input data for the GWLF model (Evans et al., 2002).
In utilizing this interface, the required GIS files are loaded automatically, and various
tools are provided to create and select watersheds for simulation purposes. Once
the selected watershed is identified (see Figure 2), the user is prompted to specify the
desired file location for model output and to supply other information related to “nonspatial”
model parameters (e.g. beginning and end of the growing season; and the
months during which manure is spread on agricultural land). Critical terrain and other
watershed data are extracted based on the watershed boundary and subsequently
used to automatically derive values for required model input parameters that are
then written to the transport.dat and nutrient.dat input files needed to execute the
GWLF model. Also accessed through the interface is a statewide weather database
that contains 25 years of temperature and precipitation data for 78 weather stations
across Pennsylvania. This database is used to create the necessary weather.dat
input file for a given watershed simulation. Subsequent to input file creation, the
GWLF model is executed directly within the GIS interface. An example of some of
the loading output generated by the model is shown in Figure 3.
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