Section 6
Methodologies and Models to Complete TMDLs for the Beaucoup Creek Watershed
6-8 v
FINAL REPORT
accurately depict water quality conditions is directly related to the accuracy of input
data and the level of expertise required to operate the model. It is also largely
dependent on the amount of data available. Calibration involves minimization of
deviation between measured field conditions and model output by adjusting parameters
of the model. Data required for this step are a set of known input values along with
corresponding field observation results. Validation involves the use of a second set of
independent information to check the model calibration. The data used for validation
should consist of field measurements of the same type as the data output from the
model. Specific features such as mean values, variability, extreme values, or all
predicted values may be of interest to the modeler and require testing. Models are
tested based on the levels of their predictions, whether descriptive or predictive. More
accuracy is required of a model designed for absolute versus relative predictions. If the
model is calibrated properly, the model predictions will be acceptably close to the field
predictions.
The GWLF and BATHTUB models were calibrated based on existing data. As will be
outlined in Section 7, the GWLF model was calibrated based on historical flow
records. The calibration factors taken into account for the GWLF model were the
recession constant and seepage constant. Water quality data on the tributaries to
Washington County Lake were not available so the GWLF model could not be
calibrated to tributary nutrient loads. Nutrient loads were based on literature values for
Southern Illinois. GWLF model validation was not conducted as the hydrology was
calibrated based on 16 years of observed flow. Data collection activities needed to
calibrate nutrient loads are outlined in Section 10 Implementation Plan. The calibration
process for the BATHTUB model is also outlined in Section 7. For Washington
County Lake, loads from a normal, wet, and dry precipitation year were taken from
GWLF and entered into the BATHTUB model, which predicted average in-lake
concentrations that were in turn compared to observed lake concentrations as the basis
for calibration.
6.2.6 Seasonal Variation
Consideration of seasonal variation, such that water quality standards for the allocated
pollutant will be met during all seasons of the year, is a requirement of a TMDL
submittal. TMDLs must maintain or attain water quality standards throughout the year
and consider variations in the water body's assimilative capacity caused by seasonal
changes in temperature and flow (USEPA 1999). Seasonal variation for the Beaucoup
Creek Watershed is discussed in Section 9.
6.2.7 Allocation
Establishing a TMDL requires the determination of the LC of each stream segment.
The models or methodologies were used to establish what the LC is for each segment
for each pollutant. The next step was to determine the appropriate MOS for each
segment. After setting the MOS, WLA of point sources and LA from the nonpoint
sources were set.
