
Design Rainfalls
Design rainfall service
Hundreds of millions of dollars are spent annually on hydraulic structures in Australia, ranging from drains and culverts to bridges and large dams used for town water supply and irrigation.
Rainfall intensity, frequency and duration data play an integral part in the design of these structures and the Bureau of Meteorology has undertaken a major revision of the information it provides in this domain.
The Bureau provides design rainfall information in the form of:
 Very Frequent design rainfall information, related to a range of watersensitive urban design (WSUD) applications and some stormwater design
 Intensity–Frequency–Duration (IFD) information, used in the design of gutters, culverts and stormwater drains
 Rare design rainfall information, used in the design of bridges and spillway adequacy assessment of existing dams
 Probable Maximum Precipitation (PMP) estimates for the design of highconsequence infrastructure such as large dams
2016 design rainfalls should be used in conjunction with the other design inputs contained in the 2016 edition of Australian Rainfall and Runoff (ARR2016).
For more information on the new design rainfalls see the Frequently Asked Questions.
About the 2016 design rainfalls
The 2016 design rainfalls are based on a more extensive database, with more than 30 years of additional rainfall records and inclusion of data from an additional 2300 collected by organisations across Australia. By combining contemporary statistical analyses and techniques with this expanded rainfall database, the 2016 design rainfalls provide more accurate estimates for Australia. In addition, the 2016 design rainfalls provide better estimates of the 2% and 1% annual exceedance probability (AEP) design rainfalls than the interim 2013 design rainfalls.
Note: The 2016 design rainfalls replace both the ARR87 design rainfalls and the interim 2013 design rainfalls.
New features for this release
In response to user feedback, the following features have been added to the 2016 design rainfalls.
Download of table for full range of probabilities
A table containing design rainfalls for the full range of probabilities can now be downloaded for points in .csv format, allowing easier analysis and importing into software packages.
Grid extraction
Design rainfall grids for standard durations and probabilities can now be downloaded in ascii format for rectangular extents up to 10000 grid cells per request.
Map selection for points and extent
Additional mapping functionality has been included to allow users to select points and extents directly from a map if coordinates are not known.
Comparison with observed rainfall
An envelope of observed storm rainfall at a rainfall station can be plotted on the design rainfall chart for that location and added to the design rainfall table. This will allow an assessment of the probability of the observed rainfall across a range of standard durations.
Subdaily rare design rainfalls
The suite of design rainfalls has been expanded to include rare design rainfalls for durations from 1 minute to 1 day. This means that design rainfalls are available for durations from 1 minute to 7 days and probabilities from 12 exceedances per year (EY) to 1 in 2000 AEP. In addition, guidance is provided on Probable Maximum Precipitation estimates
Expanded standard durations
The standard durations for the design rainfalls have been expanded reflect the durations provided for the ARR temporal patterns as the default. The additional durations will no longer need to be added manually when importing into modelling software.
Polynomial coefficients provided for point locations
The polynomial coefficient values are available for point locations. Guidance on using these values is included in the FAQs, along with the relevant equation to calculate the design rainfall depths.
Multiple point extractions
For those projects in large catchments or areas with high design rainfall gradients, multiple points (up to 50 per request) can now be extracted through the design rainfall system using the coordinate templates provided.
Tick boxes removed
The tick boxes in previous versions of the website have now been removed to improve the efficiency of the system. The information in the disclaimer and coordinate caveat is still available here if you have not had a chance to read it yet.
Seasonality guidance
Winter factors have been estimated for those parts of Australia which experience winter dominated rainfall (May to October). In these areas, rainfall events in winter tend to be small and more frequent while the larger events tend to occur in the summer months (for example extropical storms).

Probable Maximum Precipitation
Probable Maximum Precipitation (PMP) Summary
Estimates of Probable maximum Precipitation (PMP) can be produced for any catchment in Australia. There are three generalised methods used to estimate PMP, appropriate for different locations and storm durations. The diagram below shows the available methods and transition zones for different locations and durations. This can assist to determine the appropriate method to use for a particular project from the list.
The map below shows where these long duration generalised methods should be applied in a spatial sense, including the overlapping zones. The Generalised Short Duration Method (GSDM) map is shown separately, under the GSDM heading. The preparation of PMP estimates in the transition zones shown on the map must be considered carefully so potential discontinuities are not introduced. The latest guidance on this process is described in the GTSMR manual.
A larger view of the map is available here.
For more information on each of the methods, click on the headings below.
 Generalised ShortDuration Method (GSDM)

The Generalised ShortDuration Method is appropriate for durations up to 6 hours, suitable for small catchments up to 1000 km2 such as those of reservoirs and tailings dams anywhere in Australia. The method considers terrain, moisture availability and catchment elevation as well as observed rainfall.
Formerly known as the 'Bulletin 51 Method', it was revised and republished as Bulletin 53 under the title 'The Estimation of Probable Maximum Precipitation in Australia: Generalised ShortDuration Method' (BoM, 1994, amended December 1996 and January 2003).
GSDM PMP estimates
GSDM PMP estimates may now be obtained by following the procedures outlined in the current publication available for download:
Bureau of Meteorology (2003) The Estimation of Probable Maximum Precipitation in Australia: Generalised ShortDuration Method, Bureau of Meteorology, Melbourne, Australia.
GDSM procedure

Selection of duration limits
The first step is to establish the maximum duration for which the method is applicable to the catchment. 
Selection of terrain category
For durations longer than one hour, classify the terrain of the catchment as 'rough' or 'smooth' based on given criteria. 
Adjustment for catchment elevation
Calculate the Elevation Adjustment Factor (EAF) based on the mean elevation of the catchment. High resolution map available here. 
Adjustment for moisture
Read Moisture Adjustment Factor (MAF) for the catchment from the map provided in the GSDM method report (BoM, 2003). High resolution chart available here. 
Calculation of final GDSM PMP estimates
The initial rainfall depths are read from the chart provided in the GSDM method report (BoM, 2003) The PMP estimates for the catchment are then calculated using the equation:PMP Value = (S * D_{S} + R * D_{R}) * MAF * EAF
There is also information on including the spatial and temporal distribution of the short duration PMP for design purposes, as well as some guidance on seasonality.

Selection of duration limits
 Revised Generalised Tropical Storm Method (GTSMR)

The Revised Generalised Tropical Storm Method (GTSMR) is applicable to those parts of Australia affected by tropical storms and divides the region into 3 parts: the coastal zone and the inland zone with an additional winter only zone covering southwest Western Australia. The maximum duration covered by this method is 120 hours in the coastal zone in summer and 96 hours for all other zones and seasons.
The development of the GTSMR is documented in the "Revision of the Generalised Tropical Storm Method of Estimating Probable Maximum Precipitation" (2003), Hydrology Report Series, Report No. 8 (HRS8). A catalogue of the significant rainfall occurrences used as a basis for the GTSMR is available in Hydrology Report Series, Report 9 (HRS9).
GTSMR PMP estimates
GTSMR PMP estimates may now be obtained using the data and procedures available for download.
This .zip files contains the following files: Bureau of Meteorology (2004) Guidebook to the Estimation of Probable Maximum Precipitation: Generalised Tropical Storm Method (revised September 2005).
 GTSMR datasets needed to produce estimates of PMP
 Temporal patterns
GTSMR procedure

Choose the correct zone
The first step is to confirm the appropriate zone/s based on the catchment location and area. 
Obtain raw depths
Select raw PMP depth values for the standard durations using the appropriate table or envelope curves. 
Calculate catchment adjustment factors
 The Moisture Adjustment Factor (MAF) is the ratio of the extreme precipitable water (EPW_{catchment}) to the standard precipitable water (EPW_{standard}), where EPW_{catchment} is obtained by calculating the catchment average based on the national grid.
 The Decay Amplitude Factor (DAF) is the catchment average DAF based on the national grid.
 The Topographic Adjustment Factor (TAF) is determined by the catchment average based on the national grid.

Calculate preliminary GTSMR PMP depths
Multiply the raw depth values for each standard duration by the catchment adjustment factors. Include short duration depths from GSDM if appropriate.Preliminary PMP depth = Raw PMP depth × MAF × DAF × TAF

Calculation of final GTSMR PMP estimates
Combine GTSMR and GSDM depths and smooth inconsistencies using an enveloping curve. Read off final PMP values for all standard durations.
There is also information including the spatial and temporal distribution of the GTSMR PMP for design purposes, as well as some guidance on seasonality.
 Generalised Southeast Australia Method (GSAM)

The Generalised Southeast Australia Method (GSAM) was developed for estimating PMP in those regions of Australia where tropical storms are not the source of the greatest depths of rainfall, and where topographic influences vary markedly. Development commenced in 1985 and was completed in 1992.
The region of GSAM applicability is defined, by default, as that part of Australia outside the region of applicability of the GTSMR, but not including the West Coast of Tasmania or the Southwest region of Western Australia. The GSAM region is further divided into two zones, Coastal and Inland separated by the Great Dividing Range. The geographical boundaries between the two methods and zones are given in the above diagram. The GSAM is appropriate for durations of 12 hours to a maximum of 120 hours depending on the location and catchment area.
The development of the GSAM is documented in the Hydrology Report Series, Report No. 4 (HRS4). A catalogue of significant rainfall occurrences used in the development of the GSAM is included in the Hydrology Report Series, Report No. 3 (HRS3). Design temporal distributions developed for use with large and extreme design rainfall bursts over southeast Australia are included in the Hydrology Series, Report No. 5 (HRS5).
GSAM PMP estimates
GSAM PMP estimates may now be obtained using the data and procedures available for download. This .zip files contains the following files:
 Bureau of Meteorology (2006) Guidebook to the Estimation of Probable Maximum Precipitation: Generalised Southeast Australia Method.
 GSAM datasets needed to produce estimates of PMP
 GSDM manual and worksheet
 Temporal patterns
GSAM procedure

Choose the correct zone
The first step is to confirm the appropriate zone/s based on the catchment location and area. 
Obtain raw depths
Select raw PMP depth values for the standard durations using the appropriate table or envelope curves. 
Calculate catchment adjustment factors
 The Moisture Adjustment Factor (MAF) is the ratio of the extreme precipitable water (EPW_{catchment}) to the standard precipitable water (EPW_{standard}), where EPW_{catchment} is obtained by calculating the catchment average based on the national grid.
 The Topographic Adjustment Factor (TAF) is determined by the catchment average based on the national grid.

Calculate preliminary GSAM PMP depths
Multiply the raw depth values for each standard duration by the catchment adjustment factors. Include short duration depths from GSDM if appropriate.Preliminary PMP depth = Raw PMP depth × MAF × TAF

Calculation of final GSAM PMP estimates
Combine GSAM and GSDM depths and smooth inconsistencies using an enveloping curve. Read off final PMP values for all standard durations.
There is also information including the spatial and temporal distribution of the GSAM PMP for design purposes, as well as some guidance on seasonality.
 West Coast of Tasmania Method of Storm Transposition and Maximisation

The West Coast Tasmania method (WCTas) was developed for estimating PMP for the West Coast of Tasmania, to meet requirements of Hydro Tasmania. Analysis of large and extreme rainfall events for this region found that these were caused by different rainfall mechanisms from the GSAM region so a different method was required. This method selected was a Storm Transposition and Maximisation method rather than a generalised method like the GSAM, GTSMR and GSDM. Storm transposition implies a displacement of the characteristics of a storm from the location where it occurred to a target location if the storm could just as easily have occurred there. Maximisation is the process of adjusting the rainfall upward assuming maximum moisture inflow into the storm. The geographical boundaries between the Tasmanian GSAM Coastal Zone and the West Coast of Tasmania are given in the above diagram.
Development of the West Coast of Tasmania Method of Storm Transposition and Maximisation took place between 1996 and 2000 and is documented in the Hydrology Report Series, Report No. 7.
WCTas PMP estimates
WCTas PMP estimates may now be obtained by following the procedures outlined in the current publication available for download:
Xuereb, K.C., Moore, G.J. and Taylor, B.F. (2001) Development of the Method of Storm Transposition and Maximisation for the West Coast of Tasmania, HRS Report No. 7, Hydrology Report Series, Bureau of Meteorology, Melbourne, Australia, January 2001
WCTas procedure

Selection and transposition of storm/s
The first step is to select the appropriate storm/s for each duration and transporting the isohyets to the target location to determine the maximum rainfall over the catchment. 
Maximisation
Maximise catchment rainfall using the appropriate maximisation factor for dewpoint temperatures, considering barrier effects. 
Apply rainfall intensity field
Apply the selected rainfall intensity field of 48 hour 50 year ARI (2% AEP) Rc = Rs * IFDc/IFDs Where: Rc = denotes the rainfall grid over the target catchment
 Rs = denotes the rainfall grid over the storm location
 IFDc = is the rainfall frequency intensity grid over the target catchment
 IFDs = is the rainfall frequency intensity grid over the storm location.

Restricted to unrestricted adjustment
Multiply restricted observed rainfall depths (9am – 9am) by the restricted/unrestricted conversion factor. 
Calculation of final WCTas PMP estimates
Combine WCTas and GSDM depths and smooth inconsistencies using an enveloping curve. Read off final PMP values for all standard durations.
There is also information including the temporal distribution of the WCTas PMP for design purposes, as well as some guidance on seasonality.

Selection and transposition of storm/s

Frequently Asked Questions
2016 Design Rainfalls
Expand all / Collapse all
Key Questions
 How do I find an FAQ?

There is lots of useful information in the answers on this page. If you can't find the answer to your question straight away, try expanding all text using the button near the top of this page and then searching the expanded text using the built in search function (Ctrl + f). Pressing both of these at the same time will display a text box in the top corner of your browser. Type in the key word that you are looking for to locate all of the places that this occurs in the text on this page.
If you are still not able to find an answer to your question, please send an email to ifdrevision@bom.gov.au.
 How were the 2016 design rainfalls estimated?

The 2016 design rainfalls 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 Lmoments for the rainfall frequency analysis; Bayesian Generalised Least Squares Regression for deriving subdaily rainfall statistics from daily rainfall values; GISbased methods for gridding data; and an 'index rainfall procedure' for regionalisation of point data.
 How do the 2016 design rainfalls compare to the ARR87 IFDs?

The 2016 design rainfalls 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 2016 design rainfalls and the ARR87 IFDs are summarised in the table below:
Method New IFDs ARR87 IFDs Number of rainfall stations Daily read  8074
Continuous  2280Daily read  7500
Continuous  600Period 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 yearsDaily read >= 30 years
Continuous > 6 yearsSource 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 Lmoments LogPearson Type III (LPIII) distribution fitted using method of moments Extension of subdaily rainfall statistics to daily read stations Bayesian Generalised Least Squares Regression (BGLSR) Principal Component Analysis Gridding Regionalised atsite distribution parameters gridded using ANUSPLIN Maps handdrawn to atsite 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 probabilityAs is to be expected, the differences between the data and methods adopted have resulted in differences between the 2016 design rainfalls and the ARR87 IFDs. These differences vary not only across Australia but across durations and probabilities. It is emphasised that the 2016 design rainfalls are only one input to design flood estimation. The full impact of the 2016 design rainfalls can only be assessed by considered all inputs into the design process (including design temporal patterns, areal reduction factors and losses).
Information sheets summarising the nature and extent of the differences for each capital city are provided below.
Comparison of 2016 design rainfalls with ARR87 IFDs
 How do I incorporate climate change into the 2016 design rainfalls?

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 2016 design rainfalls is provided in Book 1; Chapter 6 Climate Change Consideration of ARR2016 http://arr.ga.gov.au/arrguideline.
 Where can I find out more about the 2016 design rainfalls?

An overview of the methods used to derive the new design rainfall 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 There is also a list of publications on the design rainfalls available here which contain details of the derivation for each probability range, as well as some of the comparisons that were undertaken.
 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/arrguideline.
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 available with the 2016 design rainfalls?

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 design rainfalls, therefore equivalent values are not available for the 2016 design rainfalls.
 How can I get the grids of the 2016 design rainfalls?

At present, the Bureau is not making the grids that underpin the 2016 design rainfalls available for the whole of Australia. If required for a project, design rainfall grids for standard durations and probabilities can be downloaded in ascii format for rectangular extents up to 10000 grid cells. This functionality is available through the Design Rainfall System page, using the Extent search function.
 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 design rainfall values used for?

Design rainfall 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 2016 design rainfalls

 How do I access the 2016 design rainfalls?

Follow the steps as outlined on the new design rainfall website to obtain a basic design rainfall estimate for a specific point:
 Select the location required for analysis (subcatchment centroid, site coordinates, location coordinates) in decimal degrees, degreesminutesseconds, or Eastings and Northings. These coordinates can 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.
 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 nonstandard durations to suit your analysis requirements.
 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.
The website also has functionality to extract multiple points, grid extents for standard probabilities and durations and coefficient tables. A map selection tool is also available to select point/s and extents for analysis.
 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 do I use the coefficients extracted from the design rainfall system to generated design rainfalls?

The polynomial coefficients underlying the design rainfalls can be displayed for all point locations.
The coefficients can be applied to estimate the design rainfall depth for a full range of durations from 1 minute to 7 days.
To calculate the design rainfall depths from the coefficients, use the following equation:
Depth_{P} = e^(C_{0} + C_{1} ln(T) + C_{2} ln(T)^{2} + C_{3} ln(T)^{3} + C_{4} ln(T)^{4} + C_{5} ln(T)^{5} + C_{6} ln(T)^{6})
 Depth = Design Rainfall for the specific probability and duration (in mm)
 P = Probability
 C_{x} is the polynomial coefficient (0 − 6) extracted from the coefficient grids
 T = duration (in minutes)
While this equation appears to be in the same format as the one provided previously, there are three main differences to be aware of when applying the equation. Some spreadsheets and software that use the coefficients may need to be updated to reflect these changes.
 Duration is in minutes rather than hours
 Output is now in Depth (mm) instead of intensity (mm/hr)
 While the daily rare design rainfalls only have four coefficients, the same equation can be used by including the zeros in the tables for the last three coefficients (C_{4} − C_{6})
Please note that there are two sets of coefficients for the Rare design rainfalls (1 in 200 − 1 in 2000 AEP). As different methods were used to derive the two sets of rare design rainfalls, there are two different polynomials for each standard probability which will give incorrect results if extrapolated outside the appropriate range of durations. The daily coefficients should be used to estimate rare design rainfalls from 1 day to 7 days, while the subdaily coefficients should be used to estimate the rare design rainfalls from 1 minute up to 1 day. There is only one set of coefficients for each of the standard probabilities in the Very Frequent and IFD probability ranges (12 EY − 1% AEP) which applies to all durations from 1 minute to 7 days.
It is recommended that only three significant figures are used when undertaking calculations using design rainfalls generated in this way.
 How accurately do I have to specify the coordinates when I estimate a design rainfall?

The 2016 design rainfalls are gridded at a resolution of 0.025 degrees of latitude and longitude, which is approximately 2.8 km^{2} at the equator.
The 2016 design rainfall webpage provides a design rainfall for the nearest grid cell to the search location. If the input coordinates are not specified accurately enough, the design rainfall estimate could be provided for a grid cell that is not the closest to the desired location. In some cases there may not be much difference in design rainfall 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 design rainfalls or use the same one for both projects. You will need to consider what the design rainfalls 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 design rainfall grid point.
 What if I have set up a local database of design rainfall values for specific locations that I work with regularly?

Using design rainfalls from the website to set up a local database is not recommended as any locally stored values will not necessarily remain current. The design rainfalls on the Bureau's website may be periodically updated as new information becomes available.
However, for major projects, it may be necessary to store design rainfalls values as part of the documentation to support decisions. If you need to do this, you should clearly label the design rainfalls with the date they were extracted from the design rainfall website.
 Will the old (ARR87) IFD values still be available?

The ARR87 IFDs have been superseded by the release of both the 2016 edition of Australian Rainfall and Runoff and the 2016 design rainfalls. It is now recommended that the 2016 design rainfalls are used for design purposes in conjunction with the design methodologies in ARR2016. Access to the ARR 87 IFDs will be provided until June 2020.
 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 design rainfall curves 'backwards'?

The 2016 design rainfalls 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 righthand side of the table or chart.
 New IFD website features

 What can I do with the new design rainfall website that I couldn't before?

 A table containing design rainfalls for the full range of probabilities can now be downloaded.
 Design rainfall grids for standard durations and probabilities can be downloaded in ascii format for rectangular extents up to 10000 grid cells.
 Point/s and extents can be selected from an interactive map.
 An envelope of observed storm rainfall can be added to the design rainfall table and chart.
 The standard durations reflect the temporal patterns available in ARR2016.
 Subdaily rare design rainfalls are now available.
 Design rainfall estimates can be extracted for nonstandard durations and all probabilities.
 Polynomial coefficients are available as part of the location search.
 Multiple points (up to 50) can be entered and downloaded at the same time for larger projects.
 Seasonality guidance is provided in the form of Winter factors for the parts of Australia that are dominated by winter rainfall.
 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 (EY) to 1 in 2000 annual exceedance probability (AEP).
 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 2016 design rainfalls 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 km^{2} 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 2016 design rainfalls is 'variable'. In some regions the 2016 design rainfalls 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 2016 design rainfalls, how can I be sure that the new ones are right?

The differences between the old and 2016 design rainfalls 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 2016 design rainfalls.
Both the old and the 2016 design rainfalls are estimates, but the 2016 design rainfalls 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 2016 design rainfalls are more accurate than the old ones?

The 2016 design rainfalls 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 design rainfalls provide a better overall statistical fit to the current rainfall database than the ARR87 design rainfalls.
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 design rainfalls 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 2016 design rainfalls 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 design rainfall values have decreased – how can this be?

The 2016 design rainfalls 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 catchmentwide rainfall, rather than local flooding from local rainfall.
The differences between the two sets of design rainfalls are the result of estimation differences. They do not imply trends over time. It is more correct to consider the 2016 design rainfall estimates as being greater or lesser than the previous ARR87 design rainfall estimates rather than rainfall events increasing or decreasing since the ARR87 design rainfall were estimated.
 Comparing the 2016 design rainfalls to atsite frequency analyses

 Why don't the values from my observed rainfall event plot along one of the lines from the 2016 design rainfalls?

The 2016 design rainfalls 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 design rainfalls.
The lines on the design rainfall chart 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 atsite frequency analysis that I did for a specific rain gauge match up with the design rainfalls extracted for that location?

Although atsite frequency analysis of the Annual Maximum Series (AMS) of observed rainfall was an integral part of the method adopted for the 2016 design rainfalls, it was only one of many steps used to produce the new gridded, regional design rainfall estimates.
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 design rainfall values. Although each independent event in the AMS is a record of the actual rainfall recorded by a rain gauge, 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 atsite comparisons. Regionalisation was applied to the measured rainfall data to effectively smooth out the effects of sampling uncertainty.
More information can be found in the this paper GreenJolly2018.pdf
 Integrating the 2016 design rainfalls with hydrologic and hydraulic design methods

 Can I keep using my hydrologic and hydraulic design spreadsheet for projects in the future?

The 2016 design rainfalls 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, depths and coefficients. It is recommended that you check regularly for updates to the design rainfalls; 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 design rainfall values an methods in current design software be updated?

Several software companies have been notified of the Design Rainfall Revision project. For further information on using the new design rainfall values in your software package, contact your supplier directly.
 Can I use the Probabilistic Rational Method with the 2016 design rainfalls to estimate peak flow rates?

No, the Probabilistic Rational Method was calibrated using the ARR87 IFDs not the 2016 design rainfalls. 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 design rainfall website also has functionality to change design rainfall outputs between depth and intensity. Consider updating the hydraulic calculation sheet anyway as the standard durations and coefficients have changed.
 Estimating the probability / severity of an observed rainfall event

 How do I estimate the probability of an observed rainfall event using the 2016 design rainfalls?

There is a tool includes as part of the design rainfall system that can be used to estimate the probability of an observed or forecast rainfall event, based on the 2016 design rainfalls. The approach is outlined below:
 Obtain rainfall data detailing the maximum depth of rainfall for the duration/s of interest 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.
 Select a Single Point search through the design rainfall system and enter the coordinates of the rain gauge.
 Select Observed Rainfall comparison from the Analysis tab.
 Enter the maximum rainfall depths for the required standard durations and select Update.
 View the observed rainfall storm envelope on the chart or table for the IFD probability range. If the event extend above or below this probability range, change to the Very Frequent or Rare design rainfalls.
Note that restricted 24 hour totals (9am to 9am) may underestimate the 24 hour rainfall total for an event, which will result in an underestimate the probability of the event.
More information on the new probability terminology (EY and AEP) can be found on the Frequently Asked Questions page.
 Which design rainfalls should I use to estimate the probability of an observed rainfall event – the ARR87 design rainfalls or the 2016 design rainfalls?

The 2016 design rainfalls 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 design rainfalls may differ from the AEP assigned to the same event using the ARR87 design rainfalls, you should specify which design rainfall version you are using.
 Rainfall data used for the 2016 design rainfalls

 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 design rainfalls, 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 were set for the rainfall sites to be included. 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 design rainfalls 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 database for the 2016 design rainfalls?

The Bureau has made every effort to include data recorded for all large rainfall events in order to ensure the 2016 design rainfalls are based on as complete a database 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 design rainfall estimates. Therefore, records from private rain gauges were not included in the 2016 design rainfalls.
 Seasonality guidance

 How do I use the winter factor?

Winter factors have been provided for practitioners who use seasonal depths for design purposes. This is only applicable in areas where rainfall predominantly occurs in the winter months (May to October). The factors are provided for guidance purposes only and should be used with care. As such, they are provided to the nearest 0.05.
Due to the way the winter factors have been derived, the value varies across AEP and, to a lesser degree, duration. Therefore factors have been provided for the following:
 50%, 20%, 10%, 5%, 2%, 1%, 1 in 200, 1 in 500, 1 in 1000, 1 in 2000 AEP
 1 day, 7 day durations
To calculate winter depths:
Winter depth_{[aep,duration]} = Design rainfall depth_{[aep,duration]} × winter factor_{[aep,duration]}
 For AEPs more frequent than 50%, use the 50% AEP winter factor.
 For durations less than 1 day, use the 1 day winter factor.
 For durations between 1 day and 7 days, linearly interpolate the winter factor:
Winter factor_{[aep,xday]} = winter factor_{[aep,1day]} + (xday1) × ((winter factor_{[aep,7day]} − winter factor_{[aep,1day]})/6)
If using this method, ensure that the calculated values are consistent (i.e. depths increase with duration and increase with lower AEPs) – where necessary, adjust depths by a small amount.
Note, winter factors have been derived using a simplified method and should only be used for general guidance. In particular, values have a high degree of uncertainty for AEPs rarer than 2%.
 How were the winter factors derived?

Seasonal design rainfalls have not previously been provided. Similarly, seasonal depths are not typically derived on a large scale internationally due to the extensive data requirements. However, a Partial Duration Series was created during the design rainfall revision project and this has provided a data source for estimating an approximate winter depth using a simplified version of the design rainfall method.
For the purposes of producing seasonal depths, winter is defined as May to October. The following steps describe the adopted approach:
 A 'seasonal maxima series' was created from the Partial Duration Series at each rainfall station where available for sites with greater than 30 years of daily rainfall data
 Seasonal GEV distribution parameters derived and atsite depths calculated
 The ratio of the atsite seasonal depth to the IFD depth (winter factor) was determined
 The winter factors were gridded, smoothed and rounded to the nearest 0.05
 Values are provided for the area of Australia where winter rainfall is dominant
Winter factors have been provided for the following:
 50%, 20%, 10%, 5%, 2%, 1%, 1 in 200, 1 in 500, 1 in 1000, 1 in 2000 AEP
 1 day, 7 day durations
The winter factors vary across AEP and, to a lesser extent, duration. While the shorter duration, higher AEP winter factors have a relatively consistent spatial pattern, factors for AEPs lower than 2% exhibit a high degree uncertainty. Care should be taken when applying winter factors to ensure that practitioners do not have inappropriate confidence in the depths.
 Probable Maximum Precipitation (PMP) guidance

 What is PMP?

Probable Maximum Precipitation (PMP) is defined by the Manual for Estimation of Probable Maximum Precipitation (WMO, 2009) as:
"...the theoretical maximum precipitation for a given duration under modern meteorological conditions."
Hydrologists use a PMP magnitude, plus its spatial and temporal distributions, to calculate the Probable Maximum Flood (PMF) for the catchment of a dam. The PMF is used to design the dam.
The World Meteorological Organization has published a manual on estimation of PMP:
World Meteorological Organization (2009) Manual for Estimation of Probable Maximum Precipitation, 3rd edition, WMO  No. 1045, Geneva, ISBN 9789263110459
This publication is available through the WMO library website
 What are Generalised Methods?

Generalised Methods of estimating Probable Maximum Precipitation use data from all available storms over a large region and include adjustments for moisture availability and differing topographic effects on rainfall depth. The adjusted storm data are enveloped by smoothing over a range of areas and durations. Generalised methods also provide design spatial and temporal patterns of PMP for the catchment.
The deterministic methods of estimating PMP have developed from ‘in situ maximisation’ through ‘storm transposition’ to the current‘ generalised’ methods.
The Generalised Methods available for Australia include:
 Generalised ShortDuration Method (GSDM)
 Revised Generalised Tropical Storm Method (GTSMR)
 Generalised Southeast Australia Method (GSAM) There is also a storm transposition and maximisation method available for the West Coast of Tasmania.
 How do I estimate a PMP?

Guidance on the PMP methods is available through the Bureau of Meteorology website: http://www.bom.gov.au/water/designRainfalls/index.shtml#pmp.
Here you can find the current documentation for each ofthe generalised method as well as the data required to apply these methods to produce PMP estimates.
 Climate change and the 2016 design rainfalls

 Do the 2016 design rainfalls incorporate/accommodate climate change models?

The 2016 design rainfalls are based on observed rainfall and therefore do not include the effects of future climate change. They should be seen as an assessment of rainfall probability at a given point in time. Advice on how to take climate change into consideration when using the 2016 design rainfalls is provided in Book 1; Chapter 6 Climate Change Consideration of ARR2016 http://arr.ga.gov.au/arrguideline.
 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 apriori, 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 (nonstationarity) to determine whether the full rainfall record should be used in deriving the 2016 design rainfalls.
Although the analyses found that the full record at some stations shows significant changes over time, there were no clear indications of trends or nonstationarity 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 straightforward 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 changerelated 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.

Rainfall Events
Details of world record rainfall events and notable point rainfall events for each Australian state.
An article which explains why 100 year rainfall events happen so often.

How were the 2016 design rainfalls estimated?
Summary of methods
A number of steps were used to derive the 2016 design rainfalls from observed rainfall data for Australia. The methods and rainfall data selected for each probability range varied as outlined in the following table.
Step Very Frequent design rainfalls IFDs Rare design rainfalls Number of rainfall stations Daily read: 15364
Continuous: 2722Daily read: 8074
Continuous: 2280Daily read: 3955 Period of record All available records up to 2012 All available records up to 2012 All available records up to 2012 Length of record Daily read: > 5 years
Continuous: > 5 yearsDaily read: >= 30 years
Continuous: > 8 yearsDaily read: >= 60 years Source of data Many organisations collecting rainfall data across Australia Many organisations collecting rainfall data across Australia Bureau of Meteorology Extreme value series Partial Duration Series (PDS) Annual Maximum Series (AMS) Annual Maximum Series (AMS) Frequency analysis Generalised Pareto (GPA) distribution, fitted using Lmoments Generalised Extreme Value (GEV) distribution, fitted using Lmoments Generalised Extreme Value (GEV) distribution, fitted using LH(2)moments Regionalisation Ratios based on 50% annual exceedance probability (AEP) Region of Influence (500 station years) Region of Influence (minimum 2000 station years) Gridding Regionalised atsite ratios, gridded using ANUSPLIN Regionalised atsite distribution parameters, gridded using ANUSPLIN Regionalised atsite distribution parameters, gridded using ANUSPLIN The approach adopted is outlined in Green et al (2015). These methods are summarised below, with links to key papers. Book 2, Chapter 3 of the 2016 edition of Australian Rainfall and Runoff ARR also contains more detailed information on the methods used to derive the 2016 design rainfalls, including the IFDs, Very Frequent and Rare design rainfalls.
A full list of publications on the design rainfall revision project and probable maximum precipitation methods is available here.
2016 IFDs
Collation of the rainfall database
A database containing data from all available rainfall stations was created. The number of continuously recording (subdaily) rainfall stations available was significantly greater than the number available for ARR87 due to the inclusion of data collected by other organisations, provided to the Bureau through the Water Regulations 2008 (Commonwealth).
Type Source Length of record ARR87 2016 IFDs Daily Bureau >= 30 years 7500 8074 Continuous Bureau > 8 years 600 754 Continuous Water Regulations data > 8 years n/a 1526 Rainfall records for each station were put through automatic and manual quality control procedures, described in Green et al (2011) . Some types of errors identified and corrected include accumulations, time shifts, missing data, and gross errors. The location information (latitude, longitude and elevation) was also checked.
Undertaking frequency analysis
The Annual Maximum Series (AMS) was extracted from the quality controlled database for:
 all dailyread stations with at least 30 years of record
 all continuouslyrecording stations with more than 8 years of record
An AMS is a list of the highest rainfall total recorded at a station for a specific standard duration each calendar year. Factors were applied to the dailyread rainfalls to change restricted 24 hour rainfall totals (9 am to 9 am) to unrestricted 24 hour rainfalls so these two data types could be combined across Australia.
A Generalised Extreme Value (GEV) frequency distribution was fitted to each AMS, as this distribution best represents Australian rainfall data. The three Lmoments (mean, coefficient of Lvariation (LCV) and Lskewness) were then used to summarise the statistical properties of each AMS (Hosking and Wallis, 1997; Green et al., 2012). To improve the spatial coverage of subdaily rainfall data, a Bayesian Generalised Least Squares Regression (BGLSR) was applied to infer subdaily Lmoments from those at the dailyread stations (Johnson et al., 2012a)
Regionalisation of rainfall data
Regionalisation was undertaken, to reduce the sampling uncertainty introduced by stations with shorter periods of record, by combining Lmoments from stations within a region of influence to give more weight to the stations with longer records. The regionalised LCV and LSkewness were combined with the atsite mean to estimate GEV distribution parameters (mean, shape and scale) and rainfall quantiles for all required exceedance probabilities at each station location. (Johnson et al., 2012b)
Preparation of final grids
To extend the regionalised GEV distribution parameters of mean, shape and scale to any point in Australia, the atsite values were translated to regular gridded rainfall estimates using thin plate smoothing splines using the ANUSPLIN algorithm. This enabled rainfall quantiles for the standard IFD annual exceedance probabilities to be estimated at any point in Australia. (The et al., 2014)
Very Frequent design rainfalls
Very Frequent design rainfalls are commonly used for stormwater quality purposes. The method adopted to estimate these was very similar to the IFDs outlined above; however the increased occurrence of these events meant that a different approach was required for the frequency analysis. A Partial Duration Series (PDS) was used to summarise the significant events for each rainfall station, after enforcing a minimum interevent time (MIT) to ensure independence. A minimum length of record of five years was used, with a threshold defined to select an average of one event per month of record (Xuereb and Green, 2012). A Generalised Pareto (GPA) distribution was then fitted to the PDS using Lmoments. Ratios of the atsite quantiles to the 50% AEP were extracted for each standard frequency and gridded using ANUSPLIN to form the Very Frequent design rainfall grids (The et al., 2015)
Rare design rainfalls
Estimating Rare design rainfalls required extrapolation outside of observed events, therefore, the minimum length of station record was set to 60 years. To place weight on the largest observed rainfall values in the AMS, a GEV distribution was fitted using LH moments. The GEV parameters were regionalised using a similar region of influence approach to the IFD method, but with an increased number of station years. To extend the regionalised GEV parameters to areas with no rain gauges, they were gridded using ANUSPLIN. To produce a continuous design rainfall frequency curve, the Rare design rainfalls were anchored to the more frequent design rainfalls at the 5% AEP (Green et al., 2016).
Subdaily rare design rainfalls
There are very few stations that have sufficiently long subdaily rainfall records to enable the derivation of subdaily rare design rainfalls using the methods adopted for the other design rainfalls. Therefore, a different approach was required that utilised the data and methods that are available. Gridded ratios were calculated across Australia by dividing the design rainfall depths for each standard rare probability by the corresponding 1% AEP depth. The ratios were derived using the 1day duration, as these daily rare design rainfall estimates were based on the most data  therefore giving the highest confidence. The 1% AEP grids for each standard subdaily duration were then multiplied by the gridded ratios to generate national grids of rainfall depths for each standard rare probability for each standard subdaily duration. Polynomials were fitted to each standard probability to smooth across durations, anchored to the 1 day rare design rainfall, to form the complete set of subdaily rare design rainfalls.
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