Quarterly Report 4 (April–June 2002)
West Branch DuPage River Watershed
This is the fourth quarterly report for the West Branch DuPage River Watershed Total
Maximum Daily Load (TMDL) project. This report briefly describes project progress during the
quarter and the proposed TMDL endpoints for each impairment.
Recent Project Activities
Work performed during the fourth quarter of the project included finalizing the HSPF
hydrologic calibration and developing calibration and verification information for the
chloride TMDL. Also during the quarter, the IEPA eliminated TMDL development for all
previous parameters other than chloride. The remainder of this project is expected to focus
upon a chloride TMDL development for the West Branch DuPage River.
Hydrologic calibration and validation involves dividing the watershed into subbasins and
then using USGS flow data, point source flow data, land use data, precipitation data, and
other information as necessary to simulate flow in the river. The hydrologic calibration task
was begun during the previous quarter and completed this quarter.
For both streamflow and meteorologic data, long term records of relatively good quality
exist, starting in 1990. The water quality data are relatively evenly sampled through out the
1990s. The land use data are from 1990. The period 1990 to 1995 was used for model
calibration, the period 1996 through 1999 for validation.
The initial parameters for the hydrologic calibration were taken from those used in the East
Branch DuPage and Salt Creek studies, analyzed under a separate contract by the
CH2M HILL team. Since both watersheds are immediately east of the West Branch DuPage
River and in a very similar hydrologic regime, it was anticipated that these parameters
would not need much adjusting for the watershed. Throughout the hydrologic calibration
process, only minor adjustments were made, refining the initial parameter set as is
consistent with most applications.
Most of the calibration statistics computed by HSPEXP, an expert system for hydrologic
calibration, indicate very good calibration. Exceptions are related to high flows and storm
events at the upper gauge. This smaller drainage area is likely to be more susceptible to
variations following thunderstorms since it is some distance from the precipitation gauges.
However, even these differences are within the ranges deemed acceptable according to the
criteria defined in HSPEXP.
Figure 1 is an example plot of the hydrologic calibration data analyzed as part of the project.
The flow duration graph is a tool that provides a comparison between the predicted flows
and the measured flows.
To validate the results of the hydrology calibration, HSPF was run for the West Branch
DuPage watershed for the period January 1996 through December 1999. The same types of