New design rainfalls for Australia

The Bureau of Meteorology has completed its re-work of design rainfall intensities for Australia.  These things are used all over the place in engineering design, from sizing that new drain the council just put in your street, to sizing the humongous pumps some coal miner needs to keep their pit dry; from inputs to the flood study keeping development off a hazardous floodpain, to choosing a new dish for your telco’s satellite comms.  The previous design rainfalls were produced by the Bureau an age ago, way back in 1987, and published in a familiar great big book of hand-drawn (gasp) contour maps of the whole continent.  There’d be a worn-out copy in just about every design office in the country.

The early goss on this multi-year project was to expect substantial design rainfall increases, aligned with the meme which claims climate change must be responsible for all that flooding we’ve been having recently.  Before you sign up there, best recall that only a few years back climate change just had to be responsible for the extreme droughts wracking the continent. Then the Pacific Ocean quietly flipped, as it has a thousand times before.

Climate change is a slow-motion train wreck. There’s plenty of real and obvious effects to contemplate right now, like warmer nights (less frost), more frequent heat waves, a rapidly drying south-west and ongoing reductions in snowpack.  And there’ll be real enough nasties to come, but our need to generate immediate, urgent “crises” is just a simple human one. It has naught to do with atmospheric physics.

So what do the new design rainfalls look like? First, some things to get straight:

  1. This was not a climate change attribution study.  The purpose wasn’t to compare recent extreme rainfalls with those in the past, although naturally enough the BOM team checked on that:

    “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.”

    So they didn’t allow for trends and didn’t censor the data to only include more recent rainfall events.  The full historical datasets available at the time were used for the old and for the new studies.

  2. What it did do was add another 30 years to the database of heavy rainfall events used to derive the design figures.  So the new results are based on (typically) over a hundred years of daily rainfall data compared with say 70 years for the earlier set.  And there’s now vastly more short duration rainfall data available (rain over minutes to hours) due to the proliferation of digital automatic weather stations (like the one at Perisher).
  3. The methods used in the two studies were different (LPIII fits to partial series in the old; GEV fits to annual series in the new).  Not too much emphasis should be placed on that though, because both are good methods, and they really ought to give pretty similar results — at least for rarer, more extreme rainfalls.

BOM has published comparison maps for capital cities, but out of interest I’ve of course done my own little comparison.  I chose to look at east coast locales, focusing on places where there should have been good local data (so minimising any interpolation differences between the two studies):


As you can see, there really isn’t much difference, at least along the east coast at the 100 year average recurrence interval level.  (Whoops, we’re now supposed to say “1% annual exceedence probability” … that’s much clearer, isn’t it.)

1 comment to New design rainfalls for Australia

  • Warrie

    Thanks Gerg for this update. When I can find a spare few hours I’ll wade into the BoM report. As for “hand drawn – gasp” LOL. Of course “1% annual exceedence probability” isn’t clearer at all LOL as well. Oh and thanks for putting up something intelligent rather than the dominating blather on some forums. Cheers… W