In my last blog I wrote about using the new areal reduction factors and IFD parameters in RORB. I’ve had some questions about using the old (1987) IFD data. I outline the procedure here, using the Thomson Dam example which is covered in section 10.3 of the RORB manual. RORB files, including the manual, can be downloaded here.

Review the Thomson example in the RORB manual and then start RORB to follow the steps below.

### 1. Complete the Run Specification dialogue in RORB

Start RORB and complete the Run Specification dialogue as shown in Figure 1 so that RORB loads the Thomson catchment file (tomdes.cat) and generates design storms. The file tomdes.cat is included with the sample data that is supplied with RORB.

### 2. Complete the Design Rainfall Specification

In the Design Rainfall Specification dialogue, select ARR IFD, then use the ‘Select location’ drop-down box to find your location. If it’s not there, click ‘Edit location details’ to add the IFD parameters for your location. We need to add the parameters for Thomson.

### 3. Review the ARR IFD Parameters

Clicking ‘Edit location details’ (Figure 2) brings up the ARR IFD Parameters dialogue (Figure 3). We need to add the required parameters for the centroid of the catchment upstream of Thomson Dam. Ten parameters are required which are labelled in the column headings in Figure 3:

- 2y1h
- 2y12h
- 2y72h
- …
- F50
- Zone

### 3. Determine the ARR parameters for the location of interest

The traditional way of finding the ARR parameters was by reading values from maps in Australian Rainfall and Runoff (1987) Volume 2. An alternative and easier way is to use the BoM website.

Go to the Flash version of the 1987 IFD page here. Go to the ‘Create an IFD’ tab and enter the coordinates of the catchment centroid, which I’ve guesstimated as:

- 37.675 S
- 146.263 E

Enter ‘Thomson Dam’ as the Location, accept the terms and conditions and click Submit. The website will return a design rainfall intensity chart as shown in Figure 4.

Note the numbers highlighted in red in the lower left of the chart. These are 9 of the ten parameters we need. A close up of the parameters is shown in Figure 5.

### 4. Find the IFD Zone

The only additional parameter required is the zone. This can be read from a map in Volume 2 of ARR (1987), which is reproduced in Figure 6. Thomson Dam is south of the Divide in Victoria so is in Zone 1.

### 5. Input parameters into RORB

Go back to the ARR IFD Parameters dialogue (Figure 3) and add the parameters for Thomson (Figure 7). Click Ok. IFD rainfall values for Thomson will now be available for use in RORB.