Frequently Asked Questions

2016 IFDs (new)

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Key Questions

How were the new IFDs estimated?

The new IFDs were estimated using a database comprising rainfall data from the Bureau's rain gauge network and data from rainfall recording networks operated by other organisations across Australia. This combined database was homogenised using extensive quality control procedures.

The quality controlled rainfall data was analysed using statistically rigorous tools and techniques, such as: the Generalised Extreme Value distribution, which has been fitted using the technique of L-moments for the rainfall frequency analysis; Bayesian Generalised Least Squares Regression for deriving sub-daily rainfall statistics from daily rainfall values; GIS-based methods for gridding data; and an 'index rainfall procedure' for regionalisation of point data.

Find out more this process.

How do the new IFDs compare to the ARR87 IFDs?

The new IFDs have been estimated using:

  • a more extensive dataset, with nearly 30 years' additional rainfall data and data from 2300 extra rainfall stations;
  • more accurate estimates, combining contemporary statistical analyses and techniques with an expanded rainfall database.

The differences in methods between the new IFDs and the ARR87 IFDs are summarised in the table below:

Method New IFDs ARR87 IFDs
Number of rainfall stations Daily read - 8074
Continuous - 2280		 Daily read - 7500
Continuous - 600
Period of record All available records up to 2012 All available records to up ~ 1983
Length of record used in analyses Daily read >= 30 years
Continuous > 8 years		 Daily read >= 30 years
Continuous > 6 years
Source of data Bureau of Meteorology & other organisations collecting rainfall data Primarily Bureau of Meteorology
Extreme value series Annual Maximum Series (AMS) Annual Maximum Series (AMS)
Frequency analysis Generalised Extreme Value (GEV) distribution fitted using L-moments Log-Pearson Type III (LPIII) distribution fitted using method of moments
Extension of sub-daily rainfall statistics to daily read stations Bayesian Generalised Least Squares Regression (BGLSR) Principal Component Analysis
Gridding Regionalised at-site distribution parameters gridded using ANUSPLIN Maps hand-drawn to at-site distribution parameters, digitised and gridded using an early version of ANUSPLIN

The probability range for the design rainfalls available through the Bureau of Meteorology website has been expanded to cover a wider range of applications.

Design rainfall class Frequency of Occurrence Probability range
Very Frequent design rainfalls Very Frequent 12 EY* to 1 EY
Intensity Frequency Duration (IFD) Frequent 1 EY to 10% AEP#
Intensity Frequency Duration (IFD) Infrequent 10% to 1% AEP
Rare design rainfalls Rare 1 in 100 AEP to 1 in 2000 AEP

* EY = exceedences per year
# AEP = annual exceedence probability

As is to be expected, the differences between the data and methods adopted have resulted in differences between the new IFDs and the ARR87 IFDs. These differences vary not only across Australia but across durations and probabilities. It is emphasised that the new IFDs are only one input to design flood estimation. The full impact of the new IFDs can only be assessed by considered all inputs into the design process (including design temporal patterns and losses).

Information sheets summarising the nature and extent of the differences for each capital city are provided below.

Comparison of 2016 IFDs with ARR87 IFDs

How do I incorporate climate change into the new IFDs?

The new design rainfalls do not include the effects of future climate change. Advice on how to take climate change into consideration when using the new design rainfalls is provided in Book 1; Chapter 6 Climate Change Consideration of ARR2016 http://arr.ga.gov.au/arr-guideline.

Where can I find out more about the new IFDs?

An overview of the methods used for the new IFD estimates can be found in Green et al (2012) and Green et al (2015) as well as in Book 2; Chapter 3 Design Rainfall of ARR2016 The Bureau of Meteorology is also preparing a comprehensive report of the derivation of the new IFDs and will publish on this website when available.

New ARR 2016 probability terminology

The probability terminology used for the 2016 design rainfalls is consistent with the probability terminology for the new edition of Australian Rainfall and Runoff (ARR2016). Further details on the new probability terminology can be found in Book 1; Chapter 2; Section 2.2 Terminology of ARR2016 http://arr.ga.gov.au/arr-guideline.

The main terms used to describe design rainfalls are:

  • Exceedances per year (EY): the number of times an event is likely to occur or be exceeded within any given year.
  • Annual exceedance probability (AEP): the probability or likelihood of an event occurring or being exceeded within any given year, usually expressed as a percentage.

The table below lists the probability terminology used for the 2016 design rainfalls and shows in bold the standard EY and AEP values for which design rainfalls are available. Generally, EY terminology is used for Very Frequent design rainfalls, AEP (%) terminology is used for Frequent and Infrequent design rainfalls, and AEP (1 in x) terminology is used for Rare design rainfalls.

Note:

  • The 50% AEP IFD does not corresponds to the 2 year Average Recurrence Interval (ARI) IFD. Rather it corresponds to the 1.44 ARI.
  • The 20% AEP IFD does not corresponds to the 5 year Average Recurrence Interval (ARI) IFD. Rather it corresponds to the 4.48 ARI.
Australian Rainfall and Runoff terminology
Frequency Descriptor EY AEP (%) AEP (1 in x) ARI Uses in Engineering Design
Very frequent 12
6 99.75 1.002 0.17 Water sensitive urban design
4 98.17 1.02 0.25
3 95.02 1.05 0.33
2 86.47 1.16 0.50
1 63.2 1.58 1.00
Frequent Stormwater/pit and pipe design
0.69 50.00 2 1.44
0.5 39.35 2.54 2.00
0.22 20.00 5 4.48
0.2 18.13 5.52 5.00
0.11 10.00 10.00 9.49
Infrequent Floodplain management and waterway design
0.05 5.00 20 20.0
0.02 2.00 50 50.0
0.01 1.00 100 100
Rare
0.005 0.50 200 200
0.002 0.20 500 500
0.001 0.10 1000 1000
0.0005 0.05 2000 2000
Extremely Rare Design of high–consequence infrastructure (eg major dams)
0.0002 0.02 5000 5000
    ↓  
     
     
     
Extreme     PMP  
Why are there no raw values or coefficients available with the new IFDs?

The Raw Data provided at the bottom of the chart of the ARR87 IFDs represent the values from the six master charts; chart of regional coefficient of skewness; and charts of short duration factors that are contained in Volume 2 of the 1987 edition of Australian Rainfall and Runoff. These Raw Data values are a function of the method adopted for deriving the ARR87 IFDs and therefore specific to the ARR87 IFDs.

A different method was adopted to derive the 2016 IFDs, therefore equivalent values are not available for the 2016 IFDs. The polynomial coefficients are not required for the new IFDs as non-standard durations can be extracted directly from the webpage, removing the need for interpolation.

How can I get the grids of the new design rainfalls?

At present, the Bureau is not making the grids that underpin the new design rainfalls available for the whole of Australia. We can provide the design rainfall grid points for specific areas on request, and the provision of co-ordinates of the area or location of interest. Requests should be sent to ifdrevision@bom.gov.au

General IFD queries
What are IFDs?

IFDs are Intensity–Frequency–Duration design rainfall intensities (mm/h) or design rainfall depths (mm) corresponding to selected standard probabilities, based on the statistical analysis of historical rainfall.

What are the IFD values used for?

IFDs are used in the design of infrastructure including gutters, roofs, culverts, stormwater drains, flood mitigation levees, retarding basins and dams. They can also be used to assess the severity of observed rainfall events.

Using the new design rainfalls
How do I use the new design rainfalls?

Follow the steps as outlined on the new IFD website:

  • Select the location required for analysis (sub-catchment centroid, site coordinates, location coordinates) in decimal degrees, degrees-minutes-seconds, or Eastings and Northings. These coordinates can now be checked visually by clicking 'Map Preview'.
  • Add a location name or description for 'label'. The design rainfall extracted does not depend on the label given, however, it is useful to have a title on your output table and/or chart to remind you of the location.
  • Don't forget to acknowledge and accept the Conditions of Use and Coordinates Caveat.
  • Click 'Submit' once you are happy with the location.
  • Update the probability range related to your project (Very frequent, Frequent & Infrequent, or Rare).
  • Update the standard and non-standard durations to suit your analysis requirements. (Note that this option is not available for Rare design rainfalls)
  • Select your preferred units, either depth (mm) or intensity (mm/h)
  • Download the design rainfall data as a table and/or chart, or print the webpage.
Where should I select the coordinates I use for my analysis?

For hydrological and hydraulic calculations for small catchments, the catchment centroid can be used.

For large catchments or for catchments where there is a steep rainfall gradient, it may be necessary to select multiple locations to represent the spatial variation of rainfall.

For analysis of a particular rainfall event, the coordinates of the rainfall recording site should be used.

How accurately do I have to specify the coordinates when I estimate a new IFD?

The 2016 IFDs are gridded at a resolution of 0.025 degrees of latitude and longitude, which is approximately 2.8 km2 at the equator.

The 2016 IFD webpage provides an IFD for the nearest grid point to the search location. If the input coordinates are not specified accurately enough, the IFDs could be provided for a grid point that is not the closest to the desired location. In some cases there may not be much difference in IFD analyses for points that are located close together, however some locations in Australia are characterised by high gradients in IFD data, particularly around mountainous regions.

When working on two projects a few kilometres apart you may choose to extract two IFDs or use the same one for both projects. You will need to consider 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.

Note: specifying coordinates at a higher precision than the grid size will not result in greater accuracy as these locations may fall near the same IFD grid point.

What if I have set up a local database of IFD values for specific locations that I work with regularly?

Using IFDs from the website to set up a local database is not recommended as any locally stored values will not necessarily remain current. The IFDs on the Bureau's website may be periodically updated as new information becomes available.

However, for major projects, it may be necessary to store IFD values as part of the documentation to support decisions. If you need to do this, you should clearly label the IFDs with the date they were extracted from the IFD website.

Will the old (ARR87) IFD values still be available?

The ARR87 IFDs remain available for a short period.

Where have the 2013 IFDs gone?

The 2013 interim IFDs have been superseded by the 2016 IFDs and removed from the website after a transition period.

Why are the new IFD curves 'backwards'?

The new IFDs are displayed in units of depth in millimetres (mm) by default. This means the new curves increase with rainfall duration rather than decreasing, so the slope of the curve is reversed.

If you require design rainfall values in intensity, that is millimetres per hour (mm/h), there is an option to change the units to the upper right-hand side of the table or chart.

New IFD website features
What can I do with the new IFD website that I couldn't before?
  • The location of the requested coordinates can be checked on a map.
  • Design rainfall estimates are now available for an expanded range of probabilities from 12 exceedances per year to 1-in-2000 annual exceedance probability.
  • Design rainfall estimates can be extracted for non-standard durations for Very Frequent, Frequent and Infrequent probability ranges.
  • Units can be changed between depth in millimetres and intensity in millimetres per hour.
  • Table and chart can be downloaded and saved
The Time of Concentration for my catchment is 7.5 minutes. Why can't I extract the new IFD values for this duration?

Due to the uncertainty in both the IFDs and the estimated Time of Concentration, times containing fractions of minutes are not permitted. Consider rounding up and down to whole minutes and running the analysis twice to investigate the magnitude of the difference in flow from the two rainfall inputs. Select the worst case for design purposes.

Comparing the new IFDs and the ARR87 IFDs
What is the spacing of grid points in kilometres?

The spacing of the grid points is 0.025 degrees, the same as the grids used in Australian Rainfall and Runoff 1987 (ARR87). This works out to be approximately 2.8 km2 at the equator but decreases with latitude.

I heard a rumour that there was an increase of X% right across Australia; is this true?

No, the best way to describe the differences between the ARR87 IFDs and the new IFDs is 'variable'. In some regions the new IFDs are higher; in other regions they are lower and in some regions they are the same. These changes are the result of additional data and new analysis approaches used in the derivation process.

There are significant differences between the ARR87 IFDs and the new IFDs, how can I be sure that the new ones are right?

The differences between the old and new IFDs vary across Australia. Some of the difference is due to increased data availability in locations that previously had limited data, and some is due to the different methods for statistical analysis and interpolation used for the new IFDs.

Both the old and the new IFDs are estimates, but the new IFDs are the Bureau's best estimate of the design rainfalls for Australia based on the current rainfall database and the latest methods. They provide a clear, consistent point of reference for all hydraulic and hydrologic analysis in Australia.

How confident are you that these new IFDs are more accurate than the old ones?

The 2016 IFDs are based on a greatly expanded rainfall database and use contemporary methods for analysis of the rainfall data. In addition, the length of record available for each station has been maximised through quality control processes and Region of Influence methods. The 2016 IFDs provide a better overall fit to the current rainfall database than the old IFDs.

As with all statistical methods, there is a level of uncertainty in the derived results due to the variability inherent in the data sample. In the 2016 IFDs this uncertainty has been reduced through the increased sample size afforded by the additional years of recorded data and inclusion of significant amounts of rainfall data from water agencies around the country.

The process of developing the new IFDs was guided and reviewed by a panel of experts set up by Engineers Australia.

Area X,Y or Z has experienced significant flooding in recent years, however IFD values have decreased – how can this be?

The 2016 IFDs are derived using the complete available rainfall records up to 2012 – some dating back to 1800. It is important to consider recent events in the context of the overall period of record and the cause of recent flooding. Rainfall events that are significant in recent memory are not necessarily ranked high in terms of the whole length of record for a particular location. Due to the nature of Australian topography, many significant floods are the result of river flooding due to upstream or catchment-wide rainfall, rather than local flooding from local rainfall.

The differences between the old and the new IFDs are estimation differences. They do not imply trends over time. It is more correct to consider the 2016 IFD estimates as being greater or lesser than the old IFD estimates rather than increasing or decreasing since the old IFDs were estimated.

Comparing the new IFDs to at-site frequency analyses
Why don't the values from my observed rainfall event plot along one of the lines from the new IFDs?

The 2016 IFDs are based on discrete statistical distributions derived from the Annual Maximum Series (AMS) of rainfall records which are then regionalised and gridded. They are not based on plots of individual rainfall events. The rainfall durations of the AMS used to derive the new IFDs range from one minute to seven days, whereas the frequency of an individual rainfall event will vary with the duration of the bursts within the storm. Therefore analysis of an individual rainfall event will not follow a single frequency line in the 2016 IFDs.

The IFD lines connect rainfall depths of equal probability of exceedance across a range of discrete durations. This results in a relationship in the vertical direction, based on the rainfall probability at each duration, rather than a horizontal relationship across multiple durations that would be representative of an observed rainfall event.

Around the country, the significant rainfall totals recorded across this wide range of durations are often the result of different meteorological conditions. Although a single rainfall event might produce annual maximum values across more than one duration for a particular year at one location, statistically it is unlikely that it will cover the full range of durations.

Why doesn't the at-site frequency analysis that I did for a specific rain gauge match up with the IFDs extracted for that location?

Although at-site frequency analysis of the Annual Maximum Series (AMS) of observed rainfall was an integral part of the method adopted for the 2016 IFDs, it was only one of many steps used to produce the new gridded, regional IFDs.

A regionalisation method was applied to give more weight to longer record stations within each region. This improved the estimates of rare (less frequent) events. A spline interpolation method was then applied to the regionalised rainfall data from across Australia to estimate gridded values for the whole country. Factors including latitude, longitude, elevation and consistency with neighbouring sites were used, in addition to rainfall characteristics at recording sites, thus allowing more reliable interpolation of rainfall depths in data sparse areas.

Rainfall values from a Generalised Extreme Value (GEV) distribution fitted to the AMS at a specific duration for a particular site will vary from the point values extracted from the grid of IFD values. Although each event in the AMS is a record of the actual rainfall at a site, these measured rainfall values are effectively point samples of the rainfall distribution across Australia. Each point sample has its own uncertainty and does not represent completely the underlying population of rainfall values. The extracted grid values, created from the regionalised rainfall inputs, will generally fall within the 95% confidence limits of the GEV distribution for the specific duration at each location.

The length and period of record at a site makes a significant difference in the level of uncertainty of any at-site comparisons. Regionalisation was applied to the measured rainfall data to effectively smooth out the effects of sampling uncertainty.

Integrating the new IFDs with hydrologic and hydraulic design methods
Can I keep using my hydrologic and hydraulic design spreadsheet for projects in the future?

The 2016 IFDs will not change the spreadsheet model only the design rainfall input to the model. However, some changes to the spreadsheet will be required to allow for the new format, particularly durations and depths. It is recommended that you check regularly for updates to the IFDs; it is probably best to do this at the start of each project or design cycle. In addition, the spreadsheet method may also need to be revised at a later date as revisions of Australian Rainfall and Runoff are released.

Will the IFD values in current design software be updated?

Several software companies have been notified of the IFD Revision project. For further information, contact your supplier directly.

Can I use the Probabilistic Rational Method with the 2016 IFDs to estimate peak flow rates?

No, the Probabilistic Rational Method was calibrated using the ARR87 IFDs not the new IFDs. The Probabilistic Rational Method and other flood estimation techniques have also being revised as part of the current Australian Rainfall and Runoff Revision project. Please refer to the ARR website for updates on design guidelines.

My hydraulic calculation sheet uses rainfall intensity rather than rainfall depth. How do I convert the rainfall depths to intensities so that I can use the revised values?

Rainfall depth and rainfall intensity are related as follows:

  • intensity (mm/h) = depth (mm) / duration (hours)
  • depth (mm) = intensity (mm/h) x duration (hours)

The 2016 IFD website also has functionality to change design rainfall outputs between depth and intensity. Consider updating the hydraulic calculation sheet anyway as the standard durations have changed.

Estimating the probability / severity of an observed rainfall event
Why can't I estimate the Exceedance per Year (EY)/Annual Exceedance Probability (AEP) of an observed rainfall event from the new webpage?

The functionality to estimate the probability of an observed rainfall event will be included at a later stage. Until then it is possible to determine the approximate the probability of an observed rainfall event from the 2016 IFD chart for the location at which the rainfall was observed using the approach outlined below.

  • 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 you can contact the National Climate Centre or your nearest Climate Services Centre. Note the latitude/longitude coordinates of the rain gauge.
  • Next, obtain an IFD estimate from the 2016 IFD web page for the coordinates of the rain gauge for which you have rainfall data.
  • Plot the data you obtained from the rain gauge on the 2016 IFD chart. To do this you need the rainfall depth in millimetres for specific durations of the rainfall event. Find the point on the IFD chart that corresponds with your rainfall depth/duration combination.
  • If the observed rainfall data is above or below the standard IFD range, then the Very Frequent or Rare design rainfalls may be required to determine the probability.
  • Find the probability for the rainfall by interpolating between the curves of on your chart.

More information on the new probability terminology (EY and AEP) can be found on the Frequently Asked Questions page.

Alternatively, the Bureau can provide an estimate if you contact ifdrevision@bom.gov.au

Which IFDs should I use to estimate the probability of an observed rainfall event – the ARR87 IFDs or the 2016 IFDs?

The 2016 IFDs should be used to estimate the probability of an observed rainfall event as they represent the best estimate of the probability that should be assigned to an observed rainfall event. However, as the AEP assigned to an observed rainfall event using the 2016 IFDs may differ from the AEP assigned to the same event using the ARR87 IFDs, you should specify which IFD version you are using.

Rainfall data used for the 2016 IFDs
Was the rainfall event that flooded my backyard, stormwater system, treatment plant, local shopping centre, in XXXX year included?

In the development of the 2016 IFDs, rainfall data from Bureau of Meteorology sites as well as other organisations was used. In order to ensure reliability of the statistics from the data, minimum record lengths for the rainfall sites included were set. If one of the sites used was located within the area of rainfall event and it was recording at the time of the event and the recorded rainfall was the highest recorded for that year, then yes.

For your own interest, you could try extracting the IFDs for your specific location and then plot up the rainfall from the storm event recorded at the nearby gauge to check the significance of the event. Daily values will need to be converted to unrestricted values prior to any comparison. Remember flooding from rainfall events can be exacerbated by blockages of infrastructure or obstacles in the usual overland flow paths.

My neighbour recorded xx mm in his own rain gauge which was much larger than the rainfall recorded at the Bureau gauge during the same rainfall event; was my neighbour's record included in the data base for the 2016 IFDs?

The Bureau has made every effort to include data recorded for all large rainfall events in order to ensure the 2016 IFDs are based on as complete a data base as possible. However, the collection of rainfall data is a complex process which needs to be undertaken by trained observers from rain gauges that meet quality control requirements specified by the Bureau in terms of instrument type, location, etc., and all rainfall stations must meet minimum record length criteria to be used in the derivation of IFD estimates. Therefore, records from private rain gauges were not included in the 2016 IFDs.

Climate change and the new IFDs
Do the 2016 IFDs incorporate/accommodate climate change models?

The 2016 design rainfalls do not include the effects of future climate change. Advice on how to take climate change into consideration when using the new design rainfalls is provided in Book 1; Chapter 6 Climate Change Consideration of ARR2016 http://arr.ga.gov.au/arr-guideline.

The climate change science community is saying that severe weather (including intense rainfall) is more likely under climate change. Your analysis is the most detailed analysis of rainfall ever undertaken in Australia. However, you have assumed a-priori, and validated with stationarity tests, that there is no trend in rainfall data for particular regions, nor for particular durations. Doesn't this prove that Climate Change doesn't exist?

The time series of extreme rainfall was examined for evidence of trends (non-stationarity) to determine whether the full rainfall record should be used in deriving the new IFDs.

Although the analyses found that the full record at some stations shows significant changes over time, there were no clear indications of trends or non-stationarity in rainfall bursts across regions or durations.

These analyses do not contradict the observed trends in temperature that indicate that the climate is changing because:

  • Rainfall in Australia is highly variable in time and space, so tracking rainfall changes is not as straight-forward as tracking temperature changes
  • Climate change proposes more frequent extreme rainfall bursts (not necessarily bigger extreme bursts) but the frequency of an extreme burst cannot be determined without looking at a long time series
  • Any climate change-related changes in extreme bursts would be swamped in a long and highly variable series.

For these reasons, we did not expect to see a clear climate change trend in the series of extreme rainfalls but this does not mean that a trend won't become apparent in the future.


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