Development of TMDLs and Implementation Plans Water Quality Analysis
Final Report 42 August 2003
5.2 Model Calibration
TSS concentration in our analysis is computed as the quotient of sediment yield (mass)
and runoff volume. For this analysis, sediment yield, per Equation 1, can be calibrated
using an empirical model specific for ILOK01. A 24-hour, 1-year event was selected for
calibration of this water quality predictor because the available dataset had a maximum
measured flow of 1,260 cfs, lower than the estimated 3-year event. Return periods of
various storm events, and their corresponding discharges, were presented in Table 5. In
analyzing water quality data, we found a statistically significant correlation between TSS
concentrations and discharge (Q) at OK 01 (R2=0.611, P<0.001). The regression allows
TSS concentration, C, to be estimated (Equation 7). The standard error associated with
this estimate is ± 39.7.
C = 0.247Q + 26.9 Equation (7)
Using Equation 7, TSS concentration during a 24-hr, 1-year storm flow is estimated to be
210 mg/L ± 40 mg/L.
There are two variables in the EPA screening procedures that were used to calibrate the
empirical model to measured data. These two variables were the topographic factor (ls) in
estimating soil loss (Equation 2), and the sediment delivery ratio (Sd) used to estimate
sediment yield (Equation 1). The length slope varies widely, depending on soil type, as
shown in Table 29. The sediment delivery ratio is a simple function of watershed
drainage area. The ls and Sd values were calibrated for the 24-hour 1-year storm flow to
match a TSS concentration at OK 01 of 210 mg/L. Calibrated values for ls and Sd are
tabulated below (Tables 29 and 30). Using these values of ls and Sd, the calibrated model
was then used to estimate TSS concentrations for other storm frequencies and durations.
LENGTH SLOPE (ls) CALIBRATION VARIABLES
Soil Type Soil ID ls Range Weighted ls Calibrated ls
Cisne-Hoyleton-Darmstadt IL006 0.12-2.2 0.35 0.12
Bluford-Ava-Hickory IL038 0.13-12.5 1.43 0.5
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