v 7-1
FINAL REPORT
Section 7
Model Development for Washington County
Lake
7.1 Basis for DO TMDL
The relationships between DO, chlorophyll "a," and phosphorus were discussed in
Section 5.1.5.1.1. Figure 7-1 shows the relationship between chlorophyll "a" and DO
for Washington County Lake. As explained in Section 5.1.5.1.1, the figure is expected
to show a decrease with DO as chlorophyll "a" increases; however, Figure 7-1 shows a
general increase of DO with chlorophyll "a." Reasons for poor correlation between DO
and chlorophyll "a" could include diurnal fluctuations of DO and seasonal growth of
algae impacting chlorophyll "a" concentrations. Figure 7-2 shows the relationship
between chlorophyll "a" and total phosphorus. This figure indicates that as total
phosphorus concentrations increase, so do chlorophyll "a" concentrations. The
relationship in Figure 7-2 and the expected relationship in Figure 7-1 suggest that
controlling total phosphorus will decrease chlorophyll "a" concentrations, which will
in turn increase DO concentrations. It is therefore recommended that a TMDL
endpoint of 0.05 mg/L for total phosphorus for Washington County Lake be utilized so
that the DO standard of 6.0 mg/L (16 hours of any 24-hour period) is achieved.
7.2 Model Overview
The models used for the TMDL analysis of Washington County Lake were GWLF and
BATHTUB. These models require input from several sources including online
databases, GIS-compatible data, and hardcopy data from various
agencies. This section describes the existing data reviewed for
model development, model inputs, and model calibration and
verification.
Schematic 1 shows how the GWLF model and BATHTUB
model is utilized in calculating the TMDL. The GWLF model
predicts phosphorus loads from the watershed. These loads are
then inputted in the BATHTUB model to assess resulting
phosphorus concentrations.
The GWLF model outlined
in Schematic 2 shows how
GWLF predicts phosphorus
loads from the watershed.
The transport block of the
GWLF model uses the
Universal Soil Loss
Equation to determine
erosion in the watershed. The transport block also
calculates runoff based on the SCS Curve Number
Schematic 1
Models used for
Washington
County Lake
TMDL calculation.
GWLF
BATHTUB
TMDL
CALCULATIONS
NUTRIENT
BLOCK
TRANSPORT
BLOCK
WEATHER
BLOCK
GWLF
OUTPUT
Schematic 2
GWLF Model.
