Using new Areal Reduction Factors in RORB

With new IFDs and Areal Reduction Factors available, it’s time to start using them in RORB models; even if just for sensitivity testing.  Here I’ll look at how this can be done using the Thomson River upstream of Thomson Dam as an example. This builds on the discussion in Section 10.3 of the RORB manual and the sample data files provided with RORB.

We need to manually add new IFD information appropriate for the area represented by a RORB model.  If you would rather use the old (1987) IFD data, see the discussion here.

Please review the Thomson example in the RORB manual and then proceed with the following steps.


1. Download the new IFD data (rainfall depths)

The web-dialogue to obtain the revised IFD data is here.   We need the centroid of the catchment upstream of Thomson Dam which I’ve estimated from Google Earth as:

  • 37.675 S
  • 146.263 E

Using these coordinates in the IFD website returns the following estimates (Table 1).  Note, these are rainfall depths (mm).

Example IFD data

Table 1:  Example IFD data

2.  Convert to rainfall intensity

RORB is expecting rainfall data as intensity (mm/hour).  The IFD data (Table 1) can be pasted from the BoM website into Excel to do the required calculations i.e. convert mm to mm/hour by dividing by duration (Table 2).

Rainfall intensity mm/hour

Table 2:  Rainfall intensity mm/hour

3.  Apply Areal Reduction Factors

Areal reduction factors need to be applied to the IFD data before it is input to RORB.  ARFs can be found using the guidelines in the new version of Australian Rainfall and Runoff (see this earlier post).

According to the sample RORB data files for the Thomson Dam example (as specified in Section 10.3 of the RORB manual), the catchment area is 519 km2..  I’ve used this to calculate the Areal Reduction Factors below (Table 3).  The new ARFs are only available for durations longer than 30 mins; I’ve used AEPs from 0.5 to 0.01.  To facilitate these calculations I’ve made a web app available here.  It’s very basic at present; I’ll wait until the advice from ARR before I do any more work to improve this.

Areal Reduction Factors

Table 3: Areal Reduction Factors

Multiplying the Areal Reduction Factors from Table 3 by the rainfall intensities in Table 2 produces the following rainfall intensities for the area of interest (Table 4).

Rainfall intensities scaled by Areal Reduction Factors

Table 4:  Rainfall intensities (mm/hour) after scaling by Areal Reduction Factors

4.  Enter rainfall intensities into RORB

Here I’m following the Thomson Dam example in Section 10.3 of the RORB manual.

4.1.  Set up the Run Specification dialogue to undertake a design run using the file (this file is available with the sample data files that download with RORB) (Figure 1).  Click Ok.

Thomson Dam - Desin run specification

Figure 1:  Thomson Dam – Design run specification

4.2 On the next screen, the Design Rainfall Specification Dialogue, select ‘User defined IFD’ and then ‘Select/Edit IFD file’.

Design Rainfall Specification dialogue

Figure 3:  Design Rainfall Specification dialogue

4.3  Edit the data in the ‘User Defined IFD Data’ dialogue.

First click ‘Save As…’ and create a new file by copying the existing melbourne.ifd file.  For example, save the file as thomson.ifd.

Next, edit the file to include the data from Table 4.  You can cut and paste from Excel.  Once the dialogue looks like Figure 5, click Save.  RORB can now use this IFD data.

User Defined IFD Data dialogue

Figure 4:  User Defined IFD Data dialogue


New IFD data ready to use

Figure 5:  New IFD data ready to use

5.  Develop a rainfall areal pattern for the catchment

The new Australian Rainfall and Runoff (ARR) states that for catchments greater than 20 km2, the minimum requirement is that a single non-uniform spatial pattern should be used to describe the rainfall (see Book 2, Section 4.4.2 of ARR).  This should be based on the variation of design rainfall intensity across the catchment using the IFD data from the Bureau as a guide.

5.1  Complete Areal Pattern details in the Design Rainfall Specification

Go back to the Design Rainfall Specification (step 4.2) and specify a Non-uniform pattern then click ‘Edit Pattern’ (see Figure 6).

Areal Pattern details

Figure 6: Areal Pattern details

The Edit Rainfall Spatial Pattern dialogue will appear (Figure 7).


Figure 7: Edit Rainfall Spatial Pattern dialogue

We will base the rainfall spatial pattern on the variation in IFD across the catchment.

There are 11 sub-catchments (A:K)  used in this RORB model as specified in the file.  The catchment layout is shown in Figure 10-2 of the RORB manual (reproduced below, Figure 8).


Figure 8: Thomson River Catchment showing sub-catchment locations (source: RORB manual)

IFD is a function of location, duration and AEP.  The location is given by the centroids of the sub-catchments.  AEP is a design decision, e.g. we may be interested in the 1% flood.  The duration should be the critical duration for the catchment.  Generally the critical duration can not be determined until the RORB model is running and the RORB model can’t be run until the rainfall spatial pattern is decided.  I suggest doing some preliminary runs with a uniform spatial pattern to get an approximate critical duration.

To proceed in this example, assume the storm of interest has a duration of 24 hours and a AEP of 1%.

5.2 Determine the latitude and longitude of the centroids of all sub-catchments

These can be read from a map or determined using a GIS.  Approximate sub-catchment centroids for the Thomson model are shown in Table 5.

5.3 Determine the rainfall intensity for the storm of interest

Using the IFD page at the Bureau of Meteorology, determine the 24 hour rainfalls for the 1% AEP storm (our assumed critical storm) at each of the sub-catchment centroids.  These are shown in Table 5.

5.4 Calculate the weighted average rainfall and the percentage of this average for each sub-catchment

Weight the rainfall by the sub-catchment area and divide by the total area to determine the weighted average.  The spatial pattern is based on the rainfall for a sub-catchment divided by the weighted average rainfall for the whole catchment.  The pattern is expressed as a percentage for each sub-catchment (Table 5).

As an example, consider the pattern percentage for Sub-catchment A.  The design rainfall of interest is 176.3 mm, the weighted average rainfall is 172.3 mm so the required percentage is 176.3/172.3 = 102%.

There are some possible refinements.  I’ve ignored the influence of Areal Reduction Factors but given that all the sub-catchments have a similar area this effect will be small.


Table 5: Calculating the spatial pattern of rainfall

5.5 Input the spatial pattern data into RORB

Go back to the Edit Rainfall Spatial Pattern dialogue and input the pattern information (Figure 9).  Click Ok and RORB will use this spatial pattern to specify rainfall across the catchment next time you run the model.


Figure 5:  Input spatial pattern

5.6 Adjust the design IFD data in RORB

In steps 1 through 4, we calculated and then input design IFD data for the catchment centroid.  We have now calculated the weighted average IFD rainfall depth for the whole catchment (172.3 mm) for a 24 hour, 1% storm.  We can now check that the IFD data at the centroid is, in fact, representative of the whole catchment.

I’ve reproduced Table 1 below.  Note that the 24 hour 1% rainfall was 169.4 mm while the weighted average was 172.3 mm.  Therefore, all the IFD data specified in Figure 5, needs to be scaled up by 172.3/169.4 = 1.017.  This is probably not worth doing in this case as the scale factor is close to one.

Thomson-RainfallIFD copy

Table 6: Same as Table 1 with the critical values highlighted

6.  Run RORB using the new IFD data

The Design Rainfall Specification now has the new IFD data and a spatial pattern available for use in modelling.

New IFD data is now available within RORB to use for modelling

Figure 9:  New IFD data is now available within RORB to use for modelling

2 thoughts on “Using new Areal Reduction Factors in RORB

  1. Pingback: Using the old (1987) IFD parameters in RORB | tonyladson

  2. Pingback: Using new IFD data in RORB | tonyladson

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