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) .. note '20 Year' should be '25 Year'

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

Town of Oakville Class Action Lawsuit Over Wider Floodplains and Flood Damages - Is Urbanization or Climate Change the Cause?

The CBC reported on a $1B class-action claim that alleges Oakville property owners are at flood risk due to 'over-development'.  The article appeared last week:

A resident interviewed for the story said that floodplain development restrictions have grown over time, restricting development activities on private property.

The mayor of Oakville explained the change in floodplains in the story: "He said that flood plains are continuously adjusted according to developing science and that the mapping in a century-old neighborhood like South Oakville would naturally require some changes over the years."

It is true that changes in analysis methods can affect floodplain extents.  Most likely the first high-level hydraulic models, using the USACE's HEC-2 program, were coded on punch cards in a consultant's office, and models were compiled and simulated on mainframe computers off-site (I know, I saw the old punch cards in our office storage in the early 1990's).  Personal computers came into offices in the 1980's to run the same simulations.

So floodplains have been estimated for many decades but not when centuries-old neighbourhoods in South Oakville were developed. 

Documentation from the US Army Corps of Engineers speaks to the computer requirements identified in the 1982 HEC-2 manual (image at right lists mainframe computers used on the top and emerging microcomputer PC's at the bottom).  The image below it represents bridge hydraulic model parameters in the USACE's Hydrologic Engineering Centre's HEC-2 hydraulic model - that input would be used to prepare punch cards in the early 1980's.  So forty years ago modelling was pretty basic right? And there was no such modelling 100 years ago.  

Hydrology models that determine flow rates in rivers have undergone similar upgrades over the decades just like HEC-2 hydraulic models.

So again, floodplains were not mapped 100-years ago in the 1920's in South Oakville.  Floodplain limits have not been changing on their own since then, unless the upstream land uses changed resulting in more flow or unless storms are bigger now.  According to Wikipedia, Conservation Halton, who has the role of mapping floodplains and regulating hazards (i.e., under O. Reg. 162/06: HALTON REGION CONSERVATION AUTHORITY: REGULATION OF DEVELOPMENT, INTERFERENCE WITH WETLANDS AND ALTERATIONS TO SHORELINES AND WATERCOURSES under Conservation Authorities Act, R.S.O. 1990, c. C.27), has been around (in one form or another) only since the 1950's according to their web site:

"Conservation Halton was formed in 1956 as the Sixteen Mile Conservation Authority followed by the formation of the Twelve Mile Conservation Authority in 1957. In 1963 these conservation authorities amalgamated to form the Halton Region Conservation Authority which later became known as Conservation Halton."

So floodplain mapping in South Oakville has likely not been in place for more than 40 to 50 years.  The 2014 report National Floodplain Mapping Assessment - Final Report prepared for Public Safety Canada charts the ago of floodplain mapping in Canada showing mapping started in the mid 1970's - see excerpt below:

The CBC article discusses the causes of increased floodplain extents.  The key factor noted in the class action lawsuit is urbanization that can increase runoff volumes and runoff rates, thus increasing river flow rates and river flood levels.  High flood levels result in wider, more extensive floodplains.

Two reports by the Intact Centre on Climate Adaptation (TOO SMALL TO FAIL: Protecting Canadian Communities from Floods (2018), and Preventing Disaster Before It Strikes: Developing a Canadian Standard for New Flood-Resilient Residential Communities (2017)) lists other stormwater management and flood-related lawsuits in Canada.  So lawsuits related to flooding are not new.

So has there been development in Oakville and upstream of Oakville that could have increased flood risks?  First there has been development as shown in the following images.  The 1960 development limit is based on Statistics Canada dwelling age of construction in census dissemination areas (very approximate), the 1971, 1991, 2001, and 2011 development limits are from Statistics Canada as well.  The 2015 limits are according to Version 3 SOLRIS land use mapping from the Province of Ontario.

Its pretty clear that there has been development.  The urban area in Oakville in 1971 was about 3500 hectares.  In 2001 it was 8800 hectares.  In 2011 it was 9200 hectares. So that is a significant increase.

Secondly, has the development caused floodplain impacts?  Conservation Halton describes several flood mitigation measures that have been put in place decades ago to mitigate some earlier, long-standing flood risks.  These measures include (according to their web site):


"Conservation Halton’s dams, along with many of the major dams within other conservation authorities across the GTA were built in direct response to the devastation associated with Hurricane Hazel (October 1954). Most of these facilities were constructed in the 1960’s and 1970’s, however none have been built since then as a more passive approach to hazard management, including land acquisition and regulation, were adopted instead of costly engineered structures."

  • Scotch Block Reservoir
  • Hilton Falls
  • Kelso
  • Mountsberg
Flood Control Channels

"Conservation Halton built three flood channels between the late 1960’s and 1970’s to safely move water through our communities and into Lake Ontario as quickly as possible. The three channels are Hager-Rambo in Burlington, Milton and Morrison-Wedgewood in Oakville. The channels are designed to move large flood flows which may result from rapid rainfall or a longer rain event away from historically developed flood sensitive / prone areas."

So works are in place to address earlier-noted flood risks, say up to the 1960's and 1970's.  More recent development has been supported by robust planning and risk mitigation measures, including effective stormwater management.  There is a risk that development that has occurred between the 1970's and the early 2000's could have increased flood risks - after that time more robust mitigation are generally in place to account for cumulative watershed effects, e.g., due to higher runoff volume.  Intensification within existing development areas can also increase runoff and contribute to higher flood risks.

The CBC story discusses the role of different factors saying "At its core, the claim blames increased flood risk in South Oakville on urban development. But there are other factors that can affect an area's risk for flooding, and the most important of those may be climate change."

Is climate change the most important factor? Have observed rainfall volumes increased during storms or have design intensities for rare, extreme rainfall events increased?

To answer those questions one can review the published Engineering Climate Datasets from Environment Canada to evaluate how annual maximum rainfall amounts and design intensities have changed over the years.  The data on observed maximum annual rainfall, measured over various durations of 5 minutes to 24 hours, show no increase at long-term climate stations surrounding Oakville.  The Pearson Airport climate station to the east of Oakville shows no increases in observed annual maxima going back to the 1950's (see Environment Canada chart below).

When observed rainfall extremes decrease as noted above, so do the derived design rainfall intensities.  The next table shows how design rainfall intensities over a 5-minutes duration have decreased since 1990.

There are decreases for 2-year intensities, for which there are a lot of observations, and decreases for rare 100-year intensities too (note: the intensities inched up temporarily after the July 8, 2013 storm but have trended back down now).

The Town of Oakville actually uses the downtown Toronto rainfall gauge for their design guidelines.  A recent study for the Town confirmed that the Toronto gauge data can be used to design in the future as well.  Town consultant Wood assessed future rainfall and Town’s existing design intensities (Review of Future Rainfall Scenarios, December 2018), and asked and answered this question:

"1. Should the Town of Oakville maintain its rainfall standard based on the Toronto City Environment
and Climate Change Canada station or move to a database within the boundaries of the Town?

Recommendation: Maintain the Toronto City ECCC station as the basis for the Town’s design IDF

The IDF relationship is the Intensity-Duration-Frequency characteristics used to design drainage systems).  The Town's consultant recommended using the Environment Canada data that is showing decreasing annual maximum rainfall. 

Specifically what is happening at the Toronto station used for Oakville drainage design? Annual maximum measured rainfall is generally declining for all durations - the 12-hour duration rainfall even has a statistically significant decrease (bottom middle chart below).

These observed decreases result in engineering design intensities that decrease as well. Over a 5 minute duration, these design intensities have been decreasing since the 1990 IDF updates for the Toronto rainfall gauge.  The rare 50 and 100 year rainfall intensities are decreasing the most a shown in the table below.

To the west of Oakville, in Hamilton, the annual maximum rainfall observations at the Royal Botanical Gardens show decreases or no change in rainfall since the 1960's:

The Hamilton Airport observed trends are also lower for short durations (see chart below). Trends for long durations are flat since the early 1970's.

Looking wider beyond those four stations above, a review of Southern Ontario trends shows in a previous post shows the trends at 21 long-term climate stations: This is a summary figure and table that show decreases in frequent storm intensities and virtually no change in extreme infrequent storm intensities:

Southern Ontario IDF Rainfall Intensity Trend Chart by Duration - Environment and Climate Change Canada's Engineering Climate Datasets, Pre-Version 1.00 (up to 1990) to Version 3.10 (up to  2017)


Development has increased significantly since the 1960's, and has doubled since mitigation works were constructed in the early 1970's to 2001 after which stormwater management measures have become more robust.  So development seems to be an important factor.

Rainfall extremes have not changed since the 1950's and 1960's at surrounding climate stations, or in southern Ontario in general. So rain does not appear to be a factor resulting in higher and wider floodplains - while Milli Vanilli can Blame it on the Rain (see below), CBC could do some fundamental fact checking on the topics in the story.

The CBC story suggests "it's difficult in general to "decouple" the effects that climate change and urbanization have on flood risk" and "determining that one played more of a role than the other is challenging" - perhaps in general it is difficult, and perhaps it is challenging.  But the difficult work has been done in this case already.  Statistics Canada has mapped urbanization growth in Oakville, and Environment and Climate Change Canada has charted and analyzed extreme rainfall trends in the region as well.   

Given the specific data here, CBC does not appear to offer any support for this statement "At its core, the claim blames increased flood risk in South Oakville on urban development. But there are other factors that can affect an area's risk for flooding, and the most important of those may be climate change."


Here is a higher resolution video showing the land use progression in Oakville (you can enlarge it once it starts to play):

Can We Use Daily Rainfall Models To Predict Short Duration Trends? Not Always - Observed Daily and Short Duration Trends Can Diverge

One can assess trends in rainfall intensities over various durations and return periods using Environment Canada's Engineering Climate Datasets.  National trends based on updating 226 station IDF curves were shown in an earlier post.

What are the trends in regions of Canada that have experienced significant flooding in the past?  And do the trends projected by models for long durations (1 day precipitation) match observed data trends?  No - some 24-hour trends are decreasing despite models estimating they will go up (or have gone up because of increasing temperatures).

Also, what is happening with observed short duration intensities, the ones responsible for flooding in urban areas, compared to the observed 1-day trends?

The data show short duration and long duration trends diverge. Therefore relying on models of 1 day precipitation to estimate what is happening with short duration, sudden, extreme rainfall should be done with caution.

A couple charts help illustrate these observed data trends and show what is wrong with relying directly on models to project local extreme rainfall.

This is the trend in observed rainfall for southern Ontario climate stations, using median changes in IDF statistics:

Southern Ontario Extreme Rainfall Trends

Long duration intensities are decreasing and short duration intensities are decreasing even more.  The extreme intensities (red dots = 100 year, orange dots = 50 year) decrease more than the small frequent storm intensities (green dots = 2 year).  Observed data diverges from Environment Canada models that suggest intensities are going up due to a warmer climate (see recent CBC article).

These are the trends for Alberta observed rainfall when new data are added and are reflected in the most current v3.10 datasets:

Alberta Extreme Rainfall Trends

In Alberta, long duration intensities decrease significantly (100 year is down by 4% on average).  Meanwhile the short duration intensities increase.  The long duration decrease is contrary to Environment Canada's simulation models that estimate 1 day rainfall at a sub-continental scale.

In northern Ontario, trends are different than in southern Ontario as shown below:

Northern Ontario Extreme Rainfall Trends

In northern Ontario the long duration intensities have increased but short duration intensities have decreased on average.  So we see short and long duration rainfall trends are diverging when we consider new data.

Climate modellers may suggest that simulated 1 day precipitation can guide what happens during short durations too.  Observed data suggest otherwise.  Trends actually diverge.

In brief, for this sample of regions shown above, we see these trends:

Location                 Short Duration Trend         Long Duration Trend

Southern Ontario      Larger Decrease                        Decrease
Northern Ontario            Decrease                              Increase
Alberta                            Increase                               Decrease

Remember "All models are wrong, some are useful".  Climate models do not accurately project changes in extreme rainfall in Canada based on observed data.  Furthermore, simulated 1 day precipitation trends from models cannot be used to assume short duration trends related to flooding in urban areas - short and long duration rainfall trends are observed to change in opposite directions in sample regions across Canada.

When using 1 day rainfall trends to estimate short duration trends, given the actual observed data trends above, it may be appropriate to conduct sensitivity analysis on potential shorter duration trends, especially if those shorter durations influence system behaviour (e.g., 'flashy' urban drainage systems).  Those short duration trends trends may be in an opposite direction or magnitude than the 1 day trends. For example, in Northern Ontario the 1 day 100-year intensities have increased 2% as a result of the most recent IDF data updates, however the intensities for durations of 2 hours or less have mostly decreased.

The following chart compares the 30 minute, 1 hour and 2 hour 100-year intensity trends with the 24 hour 100-year trends at 226 climate stations across Canada.

The correlation of short duration trends with 24 hour trends is weak with R-squared value of 0.12 for 2 hour trends, 0.06 for 1 hour trends and 0.006 for 30 minute trends.  This suggests that short duration trends are not correlated with 24 hour trends.   


Given recent flooding in British Columbia, it is worthwhile looking at trends in design rainfall intensities in BC. This chart shows that extreme rainfall intensities (red dots with 100-year return period, orange dots with 50-year return period) have not increased for most durations - the 12 hour duration intensities are up slightly on average (less than 0.5% increase), while other duration intensities have decreased by more than 2 % on average (5-minute 100-year intensities).

For the longest duration of 24 hours, intensities have decreased on average - typical 2-year intensities are unchanged on average while the moderate and extreme intensities (5-years, 10-year, 25-year, 50-year and 100-year) have decreased on average.

The following tables shows trends in observed annual maximum rainfall over various durations at BC climate stations with long-term records. These are called the Annual Maximum Series (AMS). Trends in derived design rainfall intensities above (e.g., 2-year to 100-year rainfall rates) follow these trends in AMS.

Super Models vs Dowdy Data - How Climate Models Diverge From Observations On Extreme Weather

A recent special article in the Financial Post noted the difference between models and observations on extreme rainfall: link

Recent reporting by CBC and Radio Canada International (RCI) have reported shifts in extreme rainfall frequency, stating that there is confirmation that a warmer climate is now making extreme rainfall more frequent and intense.  The confirmation, however, was from models analyzed by Environment Canada, and not actual measured rainfall.

As pointed out in the Financial Post article, both CBC and RCI confused models with actual observed data in stating broad confirmations.  They overlooked limitations in the models to represent local events and extreme events, omitted data that showed all the models were wrong in some regions (projected increasing rainfall when data showed decreasing rainfall), and failed to mention that other climate effects like less snow in a warmer climate can decrease flood risk, mitigating precipitation increases.

Fundamentally, observed rainfall frequencies and model frequencies are not consistent, despite RCI and CBC reporting.  The following tables show the clear difference between what models project could happen and what actual data show has happened.

This first table relates to the recent CBC and RCI reporting on a North American climate model.  The model predicts that 100 year storms become 20 year storms (i.e., for a given intensity), meaning more frequent.  Alternatively, the model says that intensities of a given frequency are higher.  In contrast, the observed data for Canada show a slight decrease in 100 year intensities at 226 climate stations, meaning storms of a given intensity are are not more frequent, but rather slightly less frequent when recent data are factored in.

Extreme Rainfall in Canada - Trends in Modelled vs Observed Data for 100 Year Storm

The second table below is for the 50 year return period storm - it shows projected model return period shifts of 50 to 35 years from model.  The results are averaged across Canada.  In comparison, 226 climate stations across Canada have observed that results in a slight decrease in 50 year storm intensities.  Like the 100 year storm above, that means actual storm frequencies are lower now.  Old 50 year return periods are now longer than 50 years now.  

Extreme Rainfall in Canada - Trends in Modelled vs Observed Data for 50 Year Storm
The CBC reported the above 50 to 35 year model shift as actually having already occurred in its In Our Backyard interactive: (see flooding tab)
The CBC claimed that intensities in Toronto are greater today, resulting in more flooding.

While it is challenging to draw conclusions from trends at individual climate stations, shifts at a couple of  Toronto climate stations are shown in the 100 year and 50 year tables as well to check the CBC reporting.  The Toronto Pearson International Airport and Toronto City (aka Bloor Street) gauges have very long records to compare old and new intensities.

The old Pearson 100 year 24-hour storm intensity (top table) is now a  417 year storm, meaning it occurs much less frequently now.  Alternatively, the magnitude of the 100 year storm intensity has dropped from past to present, meaning such storms are less severe.  This decline occurred despite that climate station recording the large July 8, 2013 storm.  The 50 year storm is now a 108 year storm, again less frequent than before.

Clearly local data at Pearson Airport, just outside of Toronto is not changing the same way that the Canadian model projections are.  Observed frequencies are longer, while the model estimated them to be shorter.

The Toronto City climate station shows only small changes in 24-hour storm frequency.  The 100 year frequency is slightly shorter at 97 year. Meanwhile the 50 year frequency is slightly longer at 52 year.  These changes are nominal and represent no significant overall change.  They are consistent with the average changes at 226 stations across Canada that also showed no appreciable change when 10 additional years of data were analyzed.  Across Canada, 100 year and 50 year rainfall intensities decreased slightly overall - the 100 year intensities decreased 0.5% and the 50 year intensities decreased 0.6%.

Clearly local data at Toronto City, essentially downtown Toronto, shows no change in extreme storm frequency or intensity, contrary to the CBS's reported model estimates.

To not rely on just a couple Toronto stations, one can look at at changes in intensities at all long term southern Ontario climate stations that have recent data updates.  Comparing the Engineering Climate Datasets v2.00 with data up to 2007 and v3.10 with data up to 2017 one can see a slight decrease in 50 year and 100 year 24-hour intensities, on average.  The stations and their lengths of record are shown below:

Southern Ontario Long Term Climate Stations with Recent IDF Updates (v2.00 to v3.10) - Environment Canada Engineering Climate Datasets
Overall, there are 978 station-years of data to analyze trends.

In southern Ontario the 100 year 24-hour intensities decreased by 1.0% while the 50 year intensities decreased by 0.9%, when additional data was added.  This suggests that the regional trends in Toronto per the Toronto City climate station, showing no overall change, are consistent with other stations in the region.  The southern Ontario data does not support the North American or Canadian model estimates reported by CBC and RCI that expect shorter return periods and higher intensities.

So beware of media reports that mix up models with actual observed data.


The following image expand on the tables above, showing where CBC and RCI made reference to the climate model results, and the text used to describe 'confirmation' of changes in rainfall.  Links to comparison charts (some that were in earlier posts) and tables are also included, showing the actual observed data trends and indicating Environment Canada source material.

Click to enlarge:

Comparison of 100 Year Return Period Rainfall Trends in Canada - Climate Models vs Observed Data, CBC and RCI Reporting

Comparison of 50 Year Return Period Rainfall Trends in Canada - Climate Models vs Observed Data, CBC Reporting