Calibration Guidelines
Each Ground Infiltration record contains 10 parameters. Of these, the Porosity of Soil, Porosity of Ground and the Soil Depth have a physical basis and should be estimated or measured. The remaining 7 parameters are calibration coefficients and these should be obtained by hand, or by using an appropriate optimization routine.
The Ground Infiltration data can be viewed and edited on the Ground Infiltration Grid Window of the Subcatchments Grid. Further details about these fields can be found in Ground Infiltration Data Fields.
The recommended procedure for calibrating the infiltration model by hand is to first calibrate the rainfall induced infiltration and then calibrate groundwater infiltration.
Calibrate the rainfall induced infiltration
The first phase of the calibration involves finding the following parameters:
- Percolation Percentage Infiltrating
- Percolation Threshold
- Percolation Coefficient
To find these parameters calibration should be performed on single storms occurring when ground water infiltration is known (or expected) to be zero. Follow these steps to calibrate the rainfall induced infiltration:
- Create a ground infiltration record and infiltration event using initial estimates of Percolation Percentage Infiltrating, Percolation Coefficient, and Percolation Threshold. Ensure the initial groundwater level is substantially lower than the baseflow/infiltration thresholds so no groundwater infiltration enters the network during the simulation and enter an estimate of initial soil saturation.
- Run the simulation.
- Compare measured and modelled flows. To calibrate the infiltration model toggle the three unknowns. Note the following points:
- increasing the Percolation Threshold increases the lag between when the storm starts and when infiltration starts
- increasing the Percolation Coefficient increases the duration of the infiltration flow
- increasing Percolation Percentage Infiltrating increases the volume infiltrating
- Repeat steps 2 to 3 until the measured and modelled curves compare well.
- Repeat steps 2 to 4 for a number of events/locations and calculate average values for the calibration coefficients.
Calibrate groundwater infiltration
If groundwater infiltration is thought to be insignificant then the ground water infiltration component can effectively be switched off by setting the Baseflow Coefficient to a value of order 0.01 and setting the Baseflow Threshold below the Infiltration Threshold. No more calibration is needed.
If groundwater infiltration is a significant inflow the following parameters need to be found:
- Infiltration Coefficient
- Baseflow Coefficient
- Infiltration Threshold Level
- Baseflow Threshold Level
Calibration is performed on a portion of time series rainfall where at least two spikes in infiltration flow (due to groundwater infiltration) are observed in the measured flow profile. Calibration is performed in five steps
- Set all upper reservoir coefficients to those values obtained from Phase 1 of the calibration. Enter initial estimates for Infiltration Coefficient, Baseflow Coefficient, Infiltration Threshold Level, and Baseflow Threshold Level, ensuring that Baseflow Threshold Level is greater than Infiltration Threshold Level. Set initial groundwater level to the Infiltration Threshold Level.
- Start the simulation when groundwater infiltration starts in the measured profile and stop the simulation when groundwater infiltration reoccurs.
- Toggle the calibration parameters. Note the following points:
- increasing the Infiltration Coefficient increases the duration of groundwater infiltration
- increasing both the Baseflow Coefficient and the difference between the Baseflow Threshold Level and the Infiltration Threshold Level, increases the time between groundwater infiltration events
- Repeat steps 2 to 3 until the measured and modelled curves compare well.
- Repeat steps 2 to 4 for a number of events/locations and calculate average values for the calibration coefficients.
