Model Development for Washington County Lake
to the basin or subbasin it represents, which was determined with GIS analyses. For the
Washington County Lake Watershed, the six subbasins modeled in GWLF represent
tributary inputs. Loadings were calculated with the monthly flow and total phosphorus
concentrations obtained from GWLF output. The monthly values were summed over
the water year for input to BATHTUB. To obtain flow in units of volume per time, the
depth of flow was multiplied by the drainage area and divided by one year. To obtain
phosphorus concentrations, the nutrient mass was divided by the volume of flow.
7.4 Model Calibration and Verification
The GWLF model was calibrated prior to BATHTUB calibration. The GWLF model
for the Washington County Lake Watershed was calibrated to flow data, as tributary
phosphorus concentrations were not available. Nutrient concentrations entered into the
GWLF model were calibrated based on response occurring in the BATHTUB model.
Therefore, the nutrient block of the GWLF model and the BATHTUB model were
calibrated together to reach agreement with observed data in Washington County Lake.
7.4.1 GWLF Calibration
The GWLF model must run from April to March to coincide with the soil erosion
cycle. GWLF does not retain erodible sediment between model years, so the model
year must begin after the previous year's sediment has been washed off. The model
assumes that the soil erosion cycle begins with spring runoff events in April and that
erodible soil for the year has been washed off, by the end of winter, for the cycle to
begin again the following April. GWLF generates monthly outputs including
precipitation, flow, runoff and nutrient mass per watershed, and annual outputs
including precipitation, flow, runoff, and nutrient mass per land use. These outputs are
part of the input for the BATHTUB model.
Instream nutrient data was not available for model calibration, so GWLF was only
calibrated to flow. The monthly average flow output from GWLF was compared to the
monthly average streamflow calculated from USGS gage 05595730 with the drainage
area ratio method presented in Section 5.1.3. The model flow was calibrated visually
through the recession constant and seepage constant. Visual calibration is a subjective
approach to model calibration in which the modeler varies inputs to determine the
parameter combination that looks like the best fit to the observed data (Chapra 1997).
According to the GWLF manual, an acceptable range for the recession constant is 0.01
to 0.2. No range suggestions are provided for the seepage constant. Figure 7-4 (at the
end of this section) shows the comparison between the two flows for subbasin 1 of
Washington County Lake. The GWLF model for Washington County Lake was
visually calibrated with a resulting recession constant of 0.1 and a seepage constant of
0.05 in each subbasin. Once calibrated, the model output data could properly be
included as BATHTUB inputs. The GWLF model was not validated as flow was
calibrated by visually comparing 16 years of observed flow. The summary output from
GWLF for each subbasin is included in Appendix C.
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