Design Rainfall Trends in Canada - Extremes not Trending with Means - Time to Check Theories on Shifting Rainfall IDF Statistics

Previous posts have reviewed changes in design rainfall intensities, that is, the statistics used to define IDF (intensity-duration-frequency) curves (  These rainfall characteristics are the most commonly applied data in urban storm drainage system design, and are often directly related to the peak runoff rates accommodated in flow conveyance infrastructure systems.  These data are also used to derive hyetographs, temporal rainfall "storm" patterns or event time series, used in hydrologic analysis of urban collection systems (i.e., storm, partially-separated sanitary or combined), as well as urban and rural catchments. 

The most recent update of Canadian climate station data includes "IDF Files" in the Engineering Climate Datasets.  Probability distributions are fit to the observed annual maximum intensities to estimate design intensities at various 'return periods', i.e., event probabilities.  Low return periods characterize frequent events, such as 2-year storms that have a 1/2 or 50% annual exceedance probability.  Meanwhile high return period characterize rare events, such as 100-year storms that only have a 1/100 or 1% probability.

The most recent update to Environment and Climate Change Canada's IDF Files adds 8.7 years, on average, to climate station records that exist in both the earlier V2.00 and the newest V3.10 sets - here is a link to those IDF Files:  As the V2.00 data had a 29.1 year average record length, the longer V3.10 records with an average of 37.8 years of data are 23% longer.  This improves the reliability of the derived statistics, allowing higher return period event intensities to be estimated.  A rule of thumb is that return periods that are double the record length can be reliably estimated - so 38 years of data can be used to estimate 76 year return period intensities with good confidence.

The following chart illustrates the change in median design rainfall intensity from the earlier V2.00 to the most recently updated V3.10 datasets. 

Climate Change Canada Extreme Rainfall Trends
Design Rainfall IDF Trends Canada - Environment and Climate Change Canada IDF Files / Engineering Climate Datasets

Small frequent rain intensities, the 2 year rates observed in an average year, and represented by the green markets, have increased by almost up to 1% as a result of the longer records being added.  The less frequent 5 year and 10 year intensities, represented by purple and blue markers respectively, are generally unchanged overall.   The rare 50 and 100 year intensities, represented by the orange and red markers, have decreased the most, although the absolute change is limited.

The following table shows the % change in intensities from V2.00 to V3.10. 

Extreme rainfall intensity shifts in Canada
Rainfall intensity changes in Canada at long-term stations (median IDF curve intensity trends)

The percentage changes are small when new data is added. When one compares the newest data added to the earlier V2.00 data a larger percentage change is apparent.  The following graphic illustrates this where the initial series, similar to the V2.00 data, is represented by the blue markers and the whole current series is represented by the orange markers.  While the average of the whole data series, represented by the orange dashed line, is only slightly above the initial series average (blue dashed line), the new data average is significantly higher (green dashed line). 
Rain intensity shifts in Canada with new data
Shifts in rainfall statistics with new longer periods of record (observations)

When the percentage changes in the above table are factored to account for the the changes in the new data relative to the initial series, the percentages increase by 4.3 times more, as shown in the table below. 

Rainfall intensity shifts in Canada - recent observations and effects on design intensities for frequent and rare events
Rainfall intensity shifts in Canada - new observation comparison with earlier Environment and Climate Change Canada Engineering Climate Datasets

While individual stations will vary, the above table shows that on average the rare 50 to 100 year rainfall intensities in the almost 9 years of new data, added between the V2.00 and V3.10 datasets, are 2.4% to 2.2% lower than the earlier V2.00 values.  These changes are over 4 times greater than the shift in V2.00 to V3.10 values. In contrast the 2 year intensities are 2.6% higher in the new data relative to the V2.00 data.   Again this is over 4 times the V2.00 to V3.10 shift in the previous table.

As the confidence interval for rainfall statistics is wide relative to these changes, they may not represent statistically significant changes in rainfall intensity.  Some change may be significant however.  For example, the Toronto City confidence limits are shown below.

The 95% confidence interval for the 5 minute 100 year intensity of 42.9 mm/hr is plus or minus 16% of the expected value of 261 mm/hr.  A decrease of 6.8% in that statistic (the decrease in 5 minute 100 year intensities in new vs. V2.00 data) could be statistically significant.

To evaluate significance of trends, the Engineering Climate Datasets have included trends on annual maximum series observations since the V2.30 datasets.  These are explored for the entire V2.30, V3.00 and V3.10 datasets in this post:

Above and below are summaries of these trends.

Trend in Maximum Rain     v3.10        v3.00         v2.30
Significant Increase                 4.28%       4.18%        4.09%
Significant Decrease                2.24%      2.33%        2.30%
No Significant Trend             85.80%     85.55%      86.37%
No Calculation                         7.68%      7.94%        7.24%

Trends at long-term stations in Canada are shown in this post:

An example of these trends is shown below for Alberta.

These annual maximum series (called AMS) trends are taken from summaries in Environment and Climate Change Canada's IDF Files, 'tucked away' text files called "idf_v-3.10_2020_03_27_trends.txt", and that look like this:

The AMS trends described in the text file above are also shown graphically on charts for each station and can be used to check if AMS trends may be significant.  We can look at an example to see if a large change in IDF values has an underlying significant change in AMS used to derive IDF values.

A summary of changes over time at the Toronto City climate station for 5 minute durations is shown below. The Toronto 5 minute 100 year design intensities have decreased from 268.5 mm/hr in the V2.00 period to 2007 to 261.0 mm/hr in the V3.10 (and V3.00) period up 2017.   That is a decrease of 2.8% since 2007.  

This appears to be a large change. But is the underlying AMS observed data change significant?  The AMS trend chart for Toronto City is shown below and shows that the 5 minute trend in the top right chart is decreasing but the trend is not significant.  Significant trends are illustrated with a blue trend line.

The 12 hour trends in the middle chart on the bottom is a significant trend in the AMS of observed rainfall though.  How have the IDF values over a 12 hour duration changed at the Toronto City Station? Does this significant trend in AMS result in a large change in derived IDF values for that duration?  The following table shows trends in IDF values for 12 hour durations from recent Engineering Climate Datasets.

Surprise !

Despite the 2 year frequent storm intensity decreasing for 1990 to 2017, the IDF value from 2007 to 2017 does not change - it does not decrease in step with the AMS trend that is significantly decreasing.


This has to do with the effect of the extreme 2013 event on the Gumbel probability distribution used to derive IDF intensities from the AMS series.  The standard deviation of the 12 hour AMS was only 13.8 mm up to 2007 and this increased to 14.8 mm up to 2017.  This extends the tails of the distribution, resulting in the increased 100 year 12 hour design intensity from 7.2 to 7.5 mm/hr.  

What about other durations and the decreasing 5 minute intensities in the earlier table? What is the effect of 2013 on other durations?

The extreme 2013 event increases the 100 year 24 hour design intensity, but the frequent 2 year 24 hour intensity has decreased since 1990, and has stayed steady since 2003, as shown below.

As for the short-duration design intensities that govern urban drainage / conveyance system design and performance, those are decreasing overall since 1990, despite some increases from 2007 to 2017 in the 1 hour data. 

Most infrastructure in the Toronto area was built by 1990, considering that almost 90% of buildings were already in place by 1991.  Therefore most infrastructure was designed considering rainfall data that was available before 1990.  The performance of that infrastructure during short duration rainfall events would be affected by the design assumptions and the likely limited rainfall data available at the time.  It would also be affected by land use changes that have been significant over pervious decades as explored in previous posts like this or this

In examining the Toronto IDF trends it is worth thinking about a common assumption that changes in mean values are reflected in changes in extremes.  The 12 hour and 24 hour data show that decreases in 2 year values can be accompanied by increases in 100 year values - a change in the opposite direction.  The data across Canada in the very first table above show a reversed trend - while 2 year intensities have increased overall and for all durations, the 20, 50 and 100 year intensities have all decreased overall and for all durations with one single exception (15 minute 100 year intensities increased).  A presentation on the Institute for Catastrophic Loss Reduction's report Telling the Weather Story for the Insurance Bureau of Canada (IBC) is on the IBC YouTube channel (, and makes the assumption that changes to averages also affect the extremes.  This graphic is used to explain how a shift in probability distribution (1 standard deviation in this illustration) is supposed to effect the extreme probabilities (see 13:10 into the presentation):

The speaker indicates "if we shift the means, we necessarily shift the frequency of occurrence of those extreme events".  That theory, or "necessity", is not showing up in the IDF shifts in Canadian data however.  In fact the shift in means, represented by the 2 year rainfall intensities, are going up overall, but this is accompanied by a decrease in the extremes (20, 50 and 100 year rainfall intensities) overall.  The shifts in means and extreme are not in the same direction.  And we see examples of the opposite occurring too - Toronto 2 year intensities decreasing with 100 year intensities increasing for some durations.

Its always good to check theories with data.

Warming in Canada is assumed to bring more intense rainfall in theory, but data has not shown an increase as illustrated above with the Engineering Climate Datasets.  Shifts in means or averages "necessarily" shift extremes too, again in theory, but not in Canadian rainfall observations, again as shown above.  Shifts in daily rainfall, often simulated in climate models, are assumed to predict changes in short duration extremes affecting urban flooding as well - but Canadian data shows that in regions across the country the shifts in daily rainfall can be opposite to the shifts in short duration rainfall (see this post that drills down into IDF shifts in several regions of the country and there 24 hour intensities and short duration ones are going in opposite directions:

Theories need to be tested with experiments, i.e., real observational data, of course.  Richard Feynman perhaps said it best: "It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."

Radio-Canada Ombudsman Finds Standards Violations in Inaccurate Reporting on Extreme Rainfall Trends in Canada

Radio-Canada found one of their journalists has again violated the Journalistic Standards and Practices when reporting on historical extreme rainfall trends linked to climate change.  This is Ombudsman Guy Gendron's review in french:

And here is the link to the english translation on that page:

The violation relates to Radio-Canada's standards for reporting accuracy and for handling corrections related to this story:

Climate change: Environment Canada confirms rain becomes more extreme, By Marc Montgomery, Posted: Wednesday, June 3, 2020 13:27, Last Updated: Thursday, November 19, 2020 10:04


The review is quite harsh.  The Ombudsman found errors in the original article and in the corrections to the article based on our complaint, and summed it up this way (my bold):

"When it comes to accuracy, the JSP expects us to “seek out the truth” and “invest our time and our skills to learn, understand and clearly explain the facts.” The article failed at all these levels. The article’s errors, omissions, imprecisions and inaccuracies are so numerous that I believe it is an aggravated case of failure to comply with the JSP. Since this seems to have been a repeat offence on the same subject by journalist Marc Montgomery, I cannot hold him solely responsible. This is also an issue of inadequate editorial oversight.

Given the scope of the observed failures, I doubt it is possible to correct an article that is so flawed through and through. If Radio Canada International decides to amend it, I strongly recommend that the task be assigned to another journalist, preferably someone who has demonstrated their ability to cover environmental topics.

Moreover, I think the problems encountered cannot be attributed to the complainant’s overly high expectations in terms of technical considerations that would be incomprehensible for average readers. News stories must simplify concepts that are sometimes complex, but without distorting the meaning."

In the thorough review, the Ombudsman checked the references that the journalist cited and found that the article distorted the meaning, misleading readers.  For example he wrote:

"All in all, I believe the article misleads readers with respect to ECCC’s position on the frequency and severity of rainfall events by implying that the study “confirms what many have been saying,” namely that rain “becomes more extreme,” at least as far as Canada is concerned."

He noted that the article had material that was "incongruous" and "awkward", and that corrections had lacked "transparency" and "humility" and one instance was deemed "false".  When describing the amount of errors associated with the documentation and attribution of corrections he wrote:

"It would be difficult to qualify all these errors, omissions, imprecisions and inaccuracies as having occurred merely by chance."

The Ombudsman seems to suggest that there are other factors, or perhaps motivations, that result in such extensive problems with the reporting.

Here is how the review concludes listing the shortcomings in meeting Radio-Canada journalistic standards:

"Here is a summary of the article’s main shortcomings:

The title is still problematic.

The choice of photos and the selected excerpts from the ECCC study imply that the title confirms an already observed increase in extreme weather events in Canada.

The juxtaposition of the citation from Blair Feltmate of the Intact Centre on Climate Adaptation – dating from 18 months earlier – enhances that false perception.

The wording for the link to the ECCC report incorrectly credits Natural Resources Canada.

The June 9 note explaining the first changes to the article falsely and unnecessarily identifies Xuebin Zhang for the “suggestion” concerning changes to the title.

That note lacks transparency about the changes made and it lacks clarity in its formulation.

In addition, that note states that the title was “very slightly modified,” which is superfluous in any explanation for a correction, especially a title.

The September 21 note explaining that a sentence regarding damage claims from extreme weather had been removed also fails to be transparent and frank by attributing it “to another study” rather than dismissing it as did the ECCC report, which is actually cited as a reference at the bottom of the article.

Lastly, various details illustrate the lack of rigour applied to this article even though it was reviewed and corrected multiple times: one sentence ends with a semi-colon and another with a comma, Ontario is misspelled as “Ontarion,” and the legend under the third photo is poorly written (containing an extra and).

Guy Gendron French Services Ombudsman, CBC/Radio-Canada November 19, 2020"

The reference to the title being problematic is fundamental since it says "Climate change: Environment Canada confirms rain becomes more extreme" but the cited material does not support that when it comes to extreme rainfall that drives flood damages.  The Ombudsman rightly acknowledges that the Environment Canada research paper cited confirms that 1 to 5 day "heavy rainfall" has increased (let's call that more "soggy days" that will keep our lawns green, and grow mushrooms), but the study did not confirm that the extreme short duration rainfall that leads to flooding has increases - increases in those rare, severe events have only been projected in models and not confirmed with observations.  The Ombudsman cites the Canada's Changing Climate Report on this fact, noting the cited paper does not confirm extreme events has increased:

"It therefore does not confirm that those extreme events have increased in Canada, as the RCI article would lead us to believe. In fact, according to Canada’s Changing Climate Report, which journalist Marc Montgomery should have read because he references it in his article, current data does not show any increase in those extreme events. Here is what the report specifies on page 155:

For Canada as a whole, observational evidence of changes in extreme precipitation amounts, accumulated over periods of a day or less, is lacking.

Later, a chapter is devoted to “extreme precipitation” and the first section covers “observed changes.” 

The opening sentence of that section on page 168 is:

There do not appear to be detectable trends in short-duration extreme precipitation in Canada for the country as a whole based on available station data."


CityFloodMap.Com blog readers will know that we have reported on these extreme rainfall trends across Canada and confirm that rare, extreme rainfall intensities (say with return periods of 20, 50 or 100 years), have decreased at 226 Environment Canada climate stations, according recent Engineering Climate Datasets updates.  This is a blog post with further details on actual overall changes in extreme design rainfall intensities:

Here is a summary graph:

The orange to red dots are 20 to 100 year intensity changes from the Version 2.00 rain intensity datasets to the Version 3.10 datasets.  They are on average less than 1.0, meaning decreasing intensities overall in the new data: 5-minute duration 100-year return period rainfall rates are 1.6% lower on average.  Short duration trends are on the left and long durations on the right.  The green dots are small frequent storm trends - those are up a bit for short durations, but not so much for long durations.  The biggest decreases are for short duration extreme rainfall (the orange and red dots on the very left).

And here are the data behind the chart above:

Extreme Rainfall Trends Climate Change Canada

As we can see the 20 to 100 year rainfall intensities have decreased by 0.4% to 0.6% across all durations overall.  Shorter duration intensities for durations of 5 minutes have decreased the most for these rare events: the 20-year to 100-year intensities have decreased 1.2% to 1.6% respectively on average.

As the intensity of small storms has increased (0.6% for the 2 year return period) one can see that the Environment Canada study cited by Radio-Canada can be right about 1-5 day heavy rainfall ("soggy days" from small storms) increasing without the rare, flood-causing 20 to 100-year extreme rainfall increasing.


Some history.

Prior to submitting the above complaint that Mr. Gendron reviewed, colleagues and I had provided both Radio-Canada and CBC Ombudsmen with a detailed review of their June 2019 coverage of the cited Environment Canada research.  Our review contained several recommendations to help improve the accuracy of their reporting on the topic of extreme rainfall and key causes of flood damages.  That review was supported by input from Environment Canada as well, specifically related to the June 2019 reporting.  Thank you to those in the academic engineering community and professional engineering practice for contributing to that review.  It appears the Radio-Canada and their readers can benefit from that shared knowledge and insight into a complex topic that has been consistently mis-reported in the past.  See a previous review of Radio-Canada and CBC reporting errors on this topic in this post:



Radio-Canada management just deleted the problematic Marc Montgomery article:

For those interested in seeing the original article that was removed by Radio-Canada management, here is the text:


A sudden intense downpour in Montreal in 2011 overwhelmed the sewers causing some to turn into geysers powerful enough to lift a car (via CBC-YouTube)

Climate change: Environment Canada confirms rain becomes more extreme


Marc Montgomery


Wednesday, June 3, 2020 13:27

Last Updated:

Thursday, November 19, 2020 10:04

A new study by Environment and Climate Change Canada (ECCC) confirms what many have been saying, that climate change has made rainfall events more frequent and more severe and the changes are dominated by human activity. This includes burning of fossil fuels, but also development onto natural green spaces for such things as agriculture and expansion of cities.

The report in the scientific journal Proceedings of the National Academy of Sciences (USA) is entitled, ‘Human influence has intensified extreme precipitation in North America’


Residents near the Ottawa River in Cumberland, a community in Ottawa, shore up their properties against a flood in 2019. Communities all along the Ottawa River were threatened by record water levels. (Judy Trinh/CBC)

The study indicates that, “Recent years have seen numerous flooding and rainfall-related extreme events in North America, totaling billions of dollars in damages”;

The report begins with a statement that humans have strongly contributed to the changes noting that while past studies have, “identified an anthropogenic influence on extreme precipitation at hemispheric scales, this study finds robust results for a continental scale. We establish that anthropogenic climate change has contributed to the intensification of continental and regional extreme precipitation”.

The report shows that the so-called ‘one-in-20, 50 or 100-year’ events can be expected to occur with far greater frequency with just a 1-degreeCelsius temperature increase over pre-industrial averages, an increase that has already occurred. It notes with that increase a so-called ‘100 year’ event might occur every 20 years. With a 3-C increase, such extreme events would occur with even far greater frequency,

In January last year, Blair Feltmate, head of the Intact Centre for Climate Adaptation at the University of Waterloo in Ontario indicated that Insurance Bureau of Canada payouts for extreme weather claims have doubled every five years since the 1980’s.

Contributing factors to higher claims include higher property values, building onto known flood plains and building over green spaces and wetlands that help absorb rain mitigate flooding. Still experts say the extreme weather and flooding is the main factor says Natalia Moudrak, director of climate resilience at the Intact Centre.


A sudden intense storm with a possible tornado swept through southwestern Ontario in September 2019, with record breaking rainfall and leaving broken trees and damage and many without power. (submitted by Steve Biro-via CBC)

Moudrak says the study further underlines the need for changes to city zoning laws, redrawing of flood plain maps, change in stormwater management and designs, and for building codes to change to reflect new extreme weather realities, a process she says that has already begun in many cases but needs to continue.

Several experts have said preservation and restoration of wetlands and green spaces should also be taken into future plans for flood controls.

Additional information – sources

CBC: Chung/Hopton/Reid: Jun 3/20: Yes we’re getting more extreme rainfall and it’s due to climate change. study confirms

Natural Resources Canada: Canada’s Changing Climate Report 2019

NOTE: Jun 9: the title has been very slightly modified to specify ‘rain’ at the suggestion of ECCC scientist Xuebin Zhang who also suggested noting human activity as fossil fuel burning and development onto green spaces.

Sep 21: a sentence regarding damage claims from extreme weather mistakenly referred to another study than the one cited here and as such has been removed.

Categories: Environment & Animal Life

Tags: climate change, environment, extreme weather, study