IFD Frequently Asked Questions
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- What is IFD?
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The abbreviation IFD stands for Intensity-Frequency-Duration; it refers to statistics on rainfall.
In order to explain what it is, we must first define some terms about rainfall. We usually think of rainfall as being of a certain DEPTH (measured in mm). But for completeness, we also need to specify the length of time over which the rainfall occurred: one year - in the case of annual rainfall; one month (for many climate purposes); or so many days, hours or minutes. This period of time over which the rain is measured is called the DURATION.
So rain is well described in terms of a depth of rainfall over a certain duration. To compare the severity of different rainfall events, we might compare several rainfalls measured over one hour. For instance, we might be interested in the highest one-hour rainfall during a calendar year at a certain location. And going one step further, a civil engineer designing a drain intended to fail no more frequently than once every ten years (on average) might ask the question, "What is the rainfall depth over one hour exceeded, on average, once in ten years?". This "once in ten years" is a FREQUENCY.
Before going further, we will explain that for AR&R 87 IFDs the Bureau of Meteorology does not use depth of rainfall in this particular context but prefers to use rainfall rate (in mm per hour), known as INTENSITY. It is calculated by dividing the depth by the duration and is simply a measure of the 'heaviness' of the rain.
We now have our three terms: "Intensity", "Duration" and "Frequency" used in the abbreviation "IFD".
Just as the engineer might ask about the 1-hour rainfall exceeded on average once in 10 years, so another might ask about the 72-hour rainfall exceeded on average once every 100 years. In fact the number of possible combinations of duration and frequency is many - take any one of 13 standard durations from 5 minutes to 72 hours, and any one of 7 frequencies from 1 per year to 1 per 100 years, and you have 91 combinations. For this reason, the results are best represented as a table, or as a graph of intensity versus duration for 7 different frequencies (see below). [For convenience, frequencies (e.g. 1 per 100 years) are replaced by "Average Recurrence Intervals" or "Return Periods" (e.g. 100 years).]
As implied above, the main use of these data is in engineering design.
- Water Storage Tanks.
How do I calculate the volume of water (in litres) that runs off my roof when a certain depth of rain (in mm) falls on it? -
One millimetre (mm) of rain falling on one square metre produces one litre of water. Knowing this, you just multiply the depth of rain in mm by the horizontal area in square metres covered by the roof and your answer is in litres.
For those who want to know more, the well-known formula is:
Volume = Depth x Area.
But the units of depth and area must be 'consistent', such as depth in metres, area in square metres and volume in cubic metres. Rainfall is usually measured in millimetres and one millimetre is 1/1000th of a metre. So for a rainfall of one millimetre on a horizontal area of one square metre:
Volume = Depth x Area
Volume = 1/1000 x 1 (answer in cubic metres)
Volume = 1/1000th of a cubic metre.
But 1/1000th of a cubic metre is exactly one litre so, converting to litres....
Volume = 1 litre - Your AR&R 87 IFD charts and tables provide rainfall intensity rather than depth; how do I convert rainfall intensity to rainfall depth?
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Rainfall intensity is another name for rainfall rate; it is measured in millimetres per hour. Because rainfall intensity usually varies over time, the intensity we use is actually the average over a given length of time (Duration). The calculation for depth can be made obvious by an analogy: if we drive at an average speed of sixty km per hour for three hours we will have travelled 180 km (60 x 3). Similarly, if rain falls at an average intensity of sixty millimetres per hour for three hours then the rainfall depth will be 180 millimetres. The formula is:
Depth (in mm) = Intensity (in mm per hour) x Duration (in hours)
and vice versa:
Intensity(in mm per hour) = Depth (in mm) / Duration (in hours) - How do I find the location (Latitude/Longitude or Easting/Northing) for which I want IFD information?
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IFD information is held at a resolution of about 2.5 km. It is therefore important to provide us with accurate location data and not just the name, or central location, of a large city such as Melbourne or Sydney (which would contain many grid points). Professionals such as engineers and architects may be expected to have a topographic map or a GPS (Global Positioning System) receiver. An alternative is to use the Geoscience Australia website:
http://www.ga.gov.au/map/names/
where a general place-name and state may be entered and then the latitude and longitude read from the appropriate entry within the comprehensive list displayed. - What is the most current Intensity Frequency Duration analysis procedures based on?
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The current Intensity Frequency Duration analysis is the 2013 IFDs.
- How do I find out how significant a past rainfall event was?
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In other words, you have a rainfall event in mind that has occurred sometime in the past, and you want to know how 'big' the event was.
You should use the 2013 IFDs. You may use the AR&R 87 IFDs for comparison if you wish. The procedure of finding the significance of a past rainfall event is similar for both AR&R 87 IFDs and 2013 IFDs.
- Obtain rainfall data detailing what depth of rain fell over a particular duration at a particular location. This could be from a private rain gauge or pluviometer, or you could obtain rainfall data from The National Climate Centre (Data Services) webpage at http://www.bom.gov.au/climate/data-services/. Note the latitude/longitude co-ordinates of the rain gauge or pluviometer.
- Next, obtain a 2013 IFDs analysis or an AR&R 87 IFD for the same co-ordinates as the rain gauge or pluviometer for which you have rainfall data.
- Plot the data you obtained from the rain gauge or pluviometer on the IFD chart. To do this you need the rainfall depth in millimetres if you are using 2013 IFDs or rainfall intensity in millimetres per hour if you are using AR&R 87 IFD for a particular duration of the rainfall event. (You may need to convert rainfall depth to rainfall intensity). Find the point on the IFD chart that corresponds with your rainfall depth (or intensity)/duration combination.
- You find the Annual Exceedance Probability for 2013 IFDs or Average Recurrence Interval (ARI) for AR&R 87 IFDs by interpolating between the curves of AEP/ARI on your chart. You could say, for example, that for that particular event, at that location the AEP/ARI was between 20 and 10 %/5 and 10 years, or between 10% and 5 % /10 and 20 years, or less than/more than 1 %/100 years.
- How do I find the rainfall intensity which is exceeded 0.01% of time (for telecommunications purposes)?
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We have a suite of programs which carry out frequency analysis of rainfalls. The program which enables telecommunications engineers to determine the rainfall intensity exceeded for a particular percentage of time at a given location according to the International Telecommunications Union standards is ITUWM. A sample output of ITUWM appears at the Rainfall Statistics webpage. The program inputs are:
- A pluviometer station number.
- 6 rainfall intensities for which the percentage of time exceeded are required.
Since the percentage of time exceeded is the program output, the program must be run several times so as to converge towards the percentage required and then suitable interpolations performed by the client himself or herself to determine the final answer. There is a cost for each run. The cost for the first run of ITUWM is listed in Standard Charges.
- How accurately do I have to specify the coordinates when I request an IFD?
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The IFD data has a resolution of 0.025 degrees of latitude and longitude which is approximately 2.7 km. The program which produces the IFD analysis gives an IFD analysis for the nearest grid point to the co-ordinates specified. If the input co-ordinates are not specified accurately enough, the IFD analysis produced could be for a grid point that is not the closest to the desired location. Whilst in some cases there may not be much difference in IFD analyses for points that are located close together, there are some locations in Australia which are characterised by high gradients in IFD data, particularly in mountainous regions. Since it is often not known whether a particular location lies in an area of high gradient or not, it is the responsibility of the agency or person who provides the co-ordinates to ensure that they are correct. The Bureau of Meteorology does not accept any responsibility for providing data for unrepresentative grid points due to misspecification of latitude/longitude or easting/northing (and Zone) co-ordinates.
- I'm working on two projects a few kilometres apart. Do I need two IFDs or can I use the one for both?
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It depends on what the IFDs are being used for and where the projects are located. For design purposes it is necessary to be as accurate as possible. Refer to How accurately do I have to specify the coordinates when I request an IFD?
