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Model calibration consists of adjusting selected input parameters so that the
computed output matches the observed monitoring results at key points in the
system. Upon completion, the calibrated model should provide a good match
between predicted and observed results when applied to storm events not used in
the calibration process. When this criterion is met, the model is considered
validated or verified.
Model Validation Criteria
In general, model validation is concerned with comparison of the computed and
measured flow hydrographs. It is recommended to establish some sort of
validation criteria to measure the accuracy of the model. These criteria can include:
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Peak discharge values
Flow volumes
Graphical shape
Time of peak
Accumulated flow differences
In order to validate the computed discharge time series, it is recommended that the
data be exported into Microsoft Excel spreadsheet (or some other software) for
further processing and comparison with measured time series.
Flow Monitoring and Measurements
Model calibration requires corresponding rainfall and sewer network flow
measurements for a number of storm events. Rainfall data should be obtained from
one or more gages located in or close to the drainage area. Flow monitoring should
be conducted at representative locations in the sewershed collection system.
Monitoring of overflows at the largest flow points in the system is advisable. Even
the best model predictions are only approximations of reality, and emphasis on
the largest flows will help to secure the best accuracy for the most significant
discharges and the best overall estimate of flow volumes and wastewater loads for
the collection system. In addition to overflows, flow monitoring stations should be
located at one or more points in the collection system upstream of any flow
regulators. This is necessary to calibrate the model parameters defining the
contributing drainage area that control the conversion of rainfall into RDII into
the collection system.
The amount of measured discharge and precipitation data, along with its
resolution has an impact in the accuracy and credibility of the obtained RDII
parameter values. An adequate number of storm events (usually 5 to 10) should be
monitored and used in the calibration. This is important because of possible
malfunctions in different flow sensors at different times, and also to compensate
for variations in rainfall distribution over the drainage area. The point rainfall
measurement at the rain gage provides only an approximation of the storm
distribution over the entire drainage area, so variations in individual events should
be expected.
The availability of sufficient number of different storm events will also facilitate
and improve the model calibration. Many users prefer to base the calibration on
the model's ability to reproduce the overall results from a number of different
events, rather than attempt to match instantaneous flow measurements.
Ideally, long time series of measured discharge data with daily values is required
for calibration of RDII parameters. Several months of higher resolution time series
data (i.e., minutes or hours) is required to calibrate the surface runoff model.
RDII Unit Hydrographs | 437