Southern Ontario Extreme Rainfall Intensity Trends - Update From Environment Canada Engineering Climate Datasets

Environment and Climate Change Canada has updated and extended the Engineering Climate Datasets as noted in the last post. This post shows the updated trends in extreme rainfall intensities across long-term southern Ontario climate stations - the good news is that intensities have not increased. This means that infrastructure built in the last few decades is not undersized considering current rainfall design intensities.

Previously trends in some southern Ontario intensity-duration-frequency (IDF) values and annual maximum series were evaluated in my paper "Evidence Based Policy Gaps in Water Resources: Thinking Fast and Slow on Floods and Flow" in the Journal of Water Management Modeling: https://www.chijournal.org/C449

Further analysis of trends in long-term stations has been presented in this blog and in the National Research Council of Canada's "National Guidelines on Undertaking a Comprehensive Analysis of Benefits, Costs and Uncertainties of Storm Drainage and Flood Control Infrastructure in a Changing Climate" - that guideline included trends in southern Ontario up to Version 3.10, as presented in the earlier post: https://www.cityfloodmap.com/2022/02/nrc-national-guidelines-on-flood.html

The southern Ontario IDF trend data has now been updated based on the Version 3.3 dataset released in May 2023 and includes some station updates to 2021- 9 of the 21 stations were updated. The following table and charts show the trends in 2-year to 100-year design rainfall intensities.

The table below shows changes in average intensity - decreases since 1990 are shaded in green and increases are in red. Note the the trends are weighted by record length. Across all durations and return periods the average decrease is - 0.33 %.  That is a slight decrease from the Version 3.20 datasets, meaning less intense rainfall when more recent data has been included. On average 30 statistics decreased while 20 statistics increased.

It is noteworthy that none of the 2-year intensities increased and the largest increase was 0.8% for 100-year intensities at one station for durations of 30 minutes and 1 hour. Overall for 21 stations 100-year intensities were virtually unchanged with the average intensities decreasing 0.1% after 30+ years, and the median increasing 0.2%. Skewed data statistics should increase over time with longer records - check out this post for more on that: https://www.cityfloodmap.com/2016/02/ontario-climate-change-trends-going.html

The following chart shows the range of changes for each return period as well as the average change. The decreases are greater than any increases for the 5 to 100-year events.

(note: above chart average intensity change dashed line corrected Nov. 10, 2024)

The following chart provides more of a breakdown by duration. One can see the red 5 minute intensities decreased on average for all return periods. The 2 hour to 24 hour intensities decreased for most return periods and where there were increase they were minor compared to other decreases. For the 5-year to 100-year return periods the 15 minutes to 1 hour intensities increased, but by no greater than 0.8%. These increases and decreases are basically insignificant in terms of impacts on infrastructure design.

This last table is annotated to show how various statistics are used in design. Infrastructure that has been designed considering short duration intensities like local sewer systems are now subject to virtually the same 2 to 10-year design intensities that existed over 30 years ago. Ponds designed for long duration higher return periods (e.g., 100-year events) are now subject to virtually the same intensities, or design event volumes, they were subject to decades ago as well. 

Environment Canada IDF Curve Update - Version 3.30 Release Adds Stations and Extends Data Records

Environment and Climate Change Canada has released the Version 3.30 of the Engineering Climate Datasets: https://collaboration.cmc.ec.gc.ca/cmc/climate/Engineer_Climate/IDF/

The number of stations and the length of average station record has increased with 276 updated stations, 38 new stations and 9 joined stations. This brings the total number of stations to 714, a significant increase relative to the time I started working.

The following table and charts illustrate how the number of stations and station-years of record have increased over time.

The number of "station-years" of data in the current data set (some data up to 2021) is 66% greater than the approximate number of station years in earlier data up to 1990. That is good to see.

Average record length is now up to 26.2 years, and the average 'last year' of data is just past 2007 (there are 3 stations that stopped in the 1940's, which brings the average last year down).

 

This chart shows how the number of stations and station-years of data have increased over the past several decades. Note that the years on the x-axis for 3.1, 3.2, and 3.3 are the respective release years (2019, 2021 and 2023), and not the newest data year included.

There have been suggestions that the number of stations has declined, adversely affecting the ability to assess changes in extreme rainfall intensities - see discussion in an earlier post https://www.cityfloodmap.com/2020/06/do-we-have-enough-climate-stations-in.html

Mekis et. al noted that the number "Manual" stations has declined as shown in the following chart - that can reflect the change in technology to automated stations that replaced manual stations (remember those analog strip charts?). The number of stations with IDF data has increased though, from only 532 stations in the 1990 data set to 714 now - a 34% increase. Those increases in station numbers are shown against the decrease in manual stations below. 

The average record length has been increasing overall as well (see table above), resulting in more reliable trend data today. Note that the addition of many new stations tends to pull the average record length down as shorter record length stations are factored in. Overall, more data is better and the new station records can extend over time.

Annual Maximum Rainfall Trends in Canada - Environment and Climate Change Canada's Updated V3.20 Datasets Show Few Significant Trends

Previous posts have presented overall trends in annual maximum observed rainfall amounts at Canadian climate stations (link: https://www.cityfloodmap.com/2020/12/design-rainfall-trends-in-canada.html).

Environment and Climate Change Canada (ECCC) periodically updates trend analysis on the annual maximum series (AMS) for each station, and these series are used to derive design intensities, i.e., Intensity-Duration-Frequency curves. Data area available here (link):  https://climate.weather.gc.ca/prods_servs/engineering_e.html

In May 2021 additional analysis has been included in these Engineering Climate Datasets. The "Whats_New_EC_IDF_v3-20.pdf" file includes a summary table that provides counts of stations and additional station years added with recent update:

The v-3.20 update adds 490 station-years of data. There are currently 17,133 station-years of data. The average length of record is 25.3 years. This represents a slight decrease in record length compared to 25.5 years in v-3.10, as new short record stations are factored in.

Trends in annual maximum rainfall have not change all relative to the 2020 v-3.10 data. The majority of station data show no statistically significant trend. The table below compares trends in earlier datasets, averaged across all durations. 

Trend in Maximum Rain    v3.20       v3.10       v3.00         v2.30

Significant Increase              4.16%     4.28%       4.18%        4.09%

Significant Decrease             2.25%     2.24%       2.33%        2.30%

No Significant Trend          85.73%    85.80%     85.55%      86.37%

No Calculation                      7.86%      7.68%       7.94%        7.24%

 

The v-3.20 datasets  have the following trends within various durations:

Excluding 'No Calculation" data, annual maximum observations with 'No Significant Trend' represents 92-94% of series, with an average of 93.0%. Excluding the 'No Calc' data results in an average of 4.5% significant increases and 2.5% significant decreases.

The following chart shows that majority of station data show no statistically significant trend.

There has not been any appreciable change in the annual maximum rainfall trends considering earlier datasets (see v-2.30 to v-3.00 update https://www.cityfloodmap.com/2020/02/annual-maximum-rainfall-trends-in.html)

In Canada, the number of stations with annual series used to derive extreme rainfall statistics continues to increase. A previous post explored how manually-operated climate stations have been declining while this increase occurs (link: https://www.cityfloodmap.com/2020/06/do-we-have-enough-climate-stations-in.html). In 1990 there were only 11,268 station-years of data. Today with 17,133 station-years of data we have 52% more information to guide assessments of extreme rainfall.  

This table summarizes the rise in climate station count and rise in station-years as new data is added.

Some have confused the decline in manual stations with a decline in overall data (see the above post). This chart shows the rise in climate stations with IDF data used for engineering design (orange lines), along with the decline in manual station data (blue bars).

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 (https://www.cityfloodmap.com/2020/07/how-have-rainfall-intensities-changed.html).  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: https://climate.weather.gc.ca/prods_servs/engineering_e.html.  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. 

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. 

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). 

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 - 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: https://www.cityfloodmap.com/2020/05/annual-maximum-rainfall-trends-in.html

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: https://www.cityfloodmap.com/2020/06/yes-were-getting-more-extreme-rainfall.html

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.

Why?

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 https://www.cityfloodmap.com/2020/10/town-of-oakville-class-action-lawsuit.html or this https://www.cityfloodmap.com/2016/08/land-use-change-drives-urban-flood-risk.html.

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 (https://www.youtube.com/watch?v=aRppaOquP5E), 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: https://www.cityfloodmap.com/2020/07/can-we-use-daily-rainfall-models-to.html).

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:

https://cbc.radio-canada.ca/fr/ombudsman/revisions/2020-11-19

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

https://site-cbc.radio-canada.ca/media/5702/review-robert-muir-november-19-2020-sw.pdf

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, english@rcinet.ca Posted: Wednesday, June 3, 2020 13:27, Last Updated: Thursday, November 19, 2020 10:04

link: https://www.rcinet.ca/en/2020/06/03/climate-change-environment-canada-confirms-rain-and-weather-extremes/

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: 

https://www.cityfloodmap.com/2020/07/how-have-rainfall-intensities-changed.html

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:

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:

https://www.cityfloodmap.com/2019/06/cbc-correcting-claims-on-extreme.html

***

IMPORTANT FOLLOW-UP

Radio-Canada management just deleted the problematic Marc Montgomery article: https://ici.radio-canada.ca/nouvelle/1752927/climate-change-rci-ombudsman-revision

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

<photo>

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

By

Marc Montgomery

english@rcinet.ca

Posted:

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’

<photo>

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.

<photo>

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

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.   

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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.

How Have Rainfall Intensities Changed in Canada Over the Past 10 Year? Not Much. Extreme 100-Year Rainfall and Short Duration Intensities Causing Flooding Are Lower

Environment and Climate Change Canada's Engineering Climate Datasets including rainfall intensity duration frequency (IDF) statistics are regularly updated as observation records become longer, and more and more stations have sufficient data to analyze.

What do the recent updates show? There is no new normal in design rainfall intensities.  Over the past 10 years, the severity of extreme rainfall has decreased on average.

Short duration sudden rainfall rates responsible for flooding in urban areas have also decreased overall - only the frequent, low intensities show an overall increase, which can be expected given additional precipitation in Canada. Of course some regions may have different trends (a previous post has shown that the southern Ontario frequent intensities (i.e., 2-year return period) have decreased).

Where do design intensities, the statistics in IDF curves and tables, come from?

Annual maximum series (AMS) of recorded rain intensity are collected for duration intervals of 5 minutes to 24 hours.  These series are used to derive probability density functions to describe the frequency distribution of rainfall, and that can be used to determine specific 'return period' design intensities.  The return period is the inverse of the probability of a rainfall intensity (or volume) over a certain duration occurring during a given year.  So a 100-year intensity has a 1/100 or 1% chance of being exceeded each year, while a 2-year intensity has a 1/2 = 50% chance per year. Storm sewers are designed to convey 2, 5 to 10-year return period rain intensities - 5-year is most common.  Flooding, especially extreme flooding, occurs at higher return periods becoming more severe above the 25-year return period and increasing for 50 and 100-year intensities.

The recent version 3.10 update to IDF statistics analyzes rainfall data up to 2017.  These intensities can be compared to the version 2.00 datasets that included data up to 2007.  A total of 226 stations were analyzed to check for changes in intensity - this total includes about 72 stations that have been relocated, but by not more than 5 km from their previous location.  The same trends are apparent for all the exact match stations (92 stations) and stations with new IDs but unchanged coordinates (154 stations).

The following chart shows the ratio of new intensities to old intensities for these 226 stations, so 1.0 means no change in design intensities.

Extreme Rainfall Trends in Canada - Design Intensities by Duration and Return Period

What are the take-aways?

1) rainfall design intensities are generally unchanged over the past 10 years, considering 3313 station-years of additional data,

2) extreme rainfall intensities, the 100-year rates (red markers in the chart), have decreased - the shortest duration intensity governing urban flood risk has dropped the most,

3) short duration intensities that govern sewver design, 5-year return period intensities (purple markers) over 5-minute to 2 hour durations are unchanged on average,

4) 2-year intensities (green markers), the low intensity rainfall that is exceeded in 50% of years, has increased slightly - these intensities do not govern infrastructure design and are unrelated to urban flash flooding or flood damages.

Popular media has focused on theoretical changes in rainfall intensity, sometimes confusing those projections with actual changes in rainfall intensity that have been measured or observed.  See this review of recent CBC coverage in the Financial Post.  Increasing damage amounts are erroneously linked to changes in rainfall due to a changing climate.

If popular media were to focus on observed data, and actual trends in extreme rainfall statistics, like the trends reviewed above, it would have to temper claims of a new normal in extreme weather.  Data do not show increases the critical rainfall intensities - in fact, on average, extreme intensities have decreased.

Changes in v2.00 to v3.10 dataset intensities are shown in the tables below.

Extreme Rainfall Trends in Canada - Engineering Climate Datasets - IDF Curves

The analysis above is based on assessing the effect of adding additional data to the v2.00 IDF data intensities.  It is also possible to assess the effects of new data by splitting the series into old and new halves to compare IDF intensities and look for trends.  The following charts show the change in two long-period climate stations in the Toronto area.   Rainfall volumes are shown for a 24 hour period - intensities would be simply the volumes divided by 24 hours.

Toronto Pearson International Airport Climate Station - Changes in 24 Hour Rainfall Frequencies

For the Toronto Pearson International Airport climate station, the return periods of the old period volumes (blue line) have shifted right in the new data set, meaning longer return periods for a given volume, i.e., lower frequency.

The chart also compares how a climate model has predicted return periods have changed from 1961 to 2010, covering approximately a similar period.  Those model frequency shifts were reported by the CBC (link: https://www.cbc.ca/news/technology/extreme-rainfall-climate-change-1.5595396) and considered a 1 degree warming scenario. The climate model predicts lower return periods for a given volume, meaning that volume occurs more frequently - that is not consistent with observed local data at this station that has shown significantly longer return periods in the new period.

Toronto City Climate Station - Changes in 24 Hour Rainfall Frequencies

For the Toronto City (downtown) climate station, the return periods of the old period volumes (blue line) have shifted slightly right in the new data set, meaning slightly longer return periods for a given volume, i.e., slightly lower frequency.

The chart again compares climate model return periods for 1961 to 2010.  Again, the model, which represent a large area, and not necessarily the specifics of the Toronto area predicts lower return periods for a given volume, meaning that volume occurs more frequently - that is not consistent with observed local data at this station that has shown no significant change.

It is possible to look at the change in intensity as opposed to the change in frequency.  The following chart for Toronto Pearson International Airport climate station presents the same data but expresses the changes in terms of intensity, as opposed to frequency.

Toronto Pearson International Airport Climate Station - Changes in 24 Hour Rainfall Volumes

Often you can read in media reports that both the frequency and intensity increased over time - this is a peculiar way to express changes as that data can be used to show a change in one or the other but realistically not both at the same time.  To show the change in frequency and the change in intensity would mean allocating the change in some proportion to the two.

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Do we have enough weather stations to analyze trends in observations - yes! - we are getting more and more stations and data over time - see previous post regarding additional Environment Canada stations since 1990.

In addition, municipalities are adding 100's of stations to support local studies as described in another post. More rain intensity data than ever before.

Although the data shows less extreme rainfall in Canada, some confuse models that predict future conditions and measured data.  The CBC misinterpreted a model predicting that 50 year storms would happen every 35 years in a time period out to 2015, and reported that this projections has already happened - read more about that here.

Yes, we're getting more extreme rainfall, and it's due to climate change, study confirms .. well not so fast

CBC News has a new report "Yes, we're getting more extreme rainfall, and it's due to climate change, study confirms" https://www.cbc.ca/news/technology/extreme-rainfall-climate-change-1.5595396

The byline is "Federal scientists predict more frequent and severe rainfall in future", referring to this research paper Human influence has intensified extreme precipitation in North America by Megan C. Kirchmeier-Young and Xuebin Zhang
https://www.pnas.org/content/early/2020/05/26/1921628117

The research paper refers to "heavy rainfall", i.e., Kirchmeier-Young the lead author and research scientist at Environment and Climate Change Canada stated "We're finding that in North America, we have seen an increase in the frequency and severity of heavy rainfall events."

Kirchmeier-Young also refers to "extreme rainfall" and makes a connection to urban flooding in the CBC article:

"And as we continue to see warming, we will continue to see increases in the frequency and severity of extreme rainfall," Kirchmeier-Young said. "And heavy rainfall is one of the major factors in flash flooding, particularly in urban areas."

The CBC relates extreme weather to rising insured flood damage trends in Canada since the early 1980's.

Let's review:

1) What 'heavy rainfall' events were reviewed in Kirchmeier-Young's research paper?

2) Is 'heavy rainfall' for a climate researcher the same as 'extreme rainfall' for an engineer?

3) Do 'heavy rainfall' and precipitation trends follow 'extreme rainfall' trends used in engineering design?

4) Do 'heavy rainfall' events studied in the research paper cause damaging flood events, and flash flooding 'particularly in urban areas?

5) What are the trends in 'extreme rainfall' in Environment and Climate Change Canada's Engineering Climate Datasets, the data used by engineers to analyze and design infrastructure to manage flash flooding risks in urban areas?

6) What does Kirchmeier-Young's research paper reveal about previous extreme rainfall and flooding events in Canada - has climate change increased runoff that could aggravate flood damages?

1) What 'heavy rainfall' events were reviewed in Kirchmeier-Young's research paper?

The research paper abstract indicates "Here, we address the question of whether observed changes in annual maximum 1- and 5-d precipitation can be attributed to human influence on the climate."

What does "1- and 5-d precipitation" mean?  This is the amount of rainfall over one to five days, so 24 to 120 hours. While precipitation can include snowfall too, the focus is on rain.

Note, the research paper actually refers to 'heavy precipitation' and not 'heavy rainfall'.

The authors have confirmed that short-duration rainfall was not reviewed, only annual maximum daily rain.

2) Is 'heavy rainfall' for a climate researcher the same as 'extreme rainfall' for an engineer?

No.

The research paper states:

"We focus on the annual maxima of 1-d (Rx1day) and 5-d (Rx5day) rainfall. Rx1day is important for flash floods as well as infrastructure design. Rx5day is relevant to large-scale river flooding."

A training session on the use of rainfall intensity design curves from a climate scientist (link: http://projects.upei.ca/climate/files/2012/07/IDFtraining-Auld-final.pdf) indicates that shorter times influence flooding (underline and all-caps emphasis are in the original material, not added here):

 "An urban centre could experience flooding from heavy rains falling over a SHORT period of time, such as A 5 TO 30 MINUTE PERIOD."

"• A rural highway with deep ditches on its shoulders would not likely be impacted by an intense rainfall lasting only 5 to 15 minutes, although the paved road itself would see ponding of water.
• A heavy rainfall event lasting 1 to 6 hours might be more significant for filling the ditches and overflowing the roadway."

So short durations are important for flooding.

From the insurance industry perspective, an Institute for Catastrophic Loss Reduction paper in Journal of Flood Risk Management notes the importance of short-duration rainfall (link: https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/jfr3.12168) states

"Subdivisions built before the 1970s are less likely to be serviced by major systems (Watt et al., 2003), and are thus more vulnerable to overland flooding from extreme short-duration rainfall events."

A civil engineer will tell you that rarely that the a 1-day rainfall is not 'important for flash floods'.  Why? Because urban flooding is caused by short-duration rainfall.  Designers of storage facilities such as stormwater manage ponds may consider design rainfall events up to 24 hours.

In the Canadian Water Resources Journal, authors of Flood processes in Canada: Regional and special aspects (link: https://www.usask.ca/hydrology/papers/Buttle_et_al_2016.pdf) representing six universities across Canada, INRS-ete, and Environment Canada review "key processes that generate floods in Canada":

"Similarly, floods can be generated across most of the country by rainstorms with large depths and/
or intensities (Figure 1). Thus, convective and frontal systems can generate large short-duration rainfall intensities (Alila 2000) which can occur in all regions (Table 1). Nevertheless, the significance of such storms to flood generation varies across the country, with the greatest
depths and intensities for short-duration events in southern parts of Canada and the smallest in the Arctic. These short-duration events are often responsible for flood generation in relatively small drainage basins, given the greater chance of high-intensity rainfall occurring over
the entire basin (Watt et al. 1989)."

"Short-duration events are often responsible for flood generation".

"Small drainage basins" is equivalent to urban drainage systems. In the municipality where I was Manager, Stormwater our storm sewer drainage systems averaged just over 50 hectares in size.  Urban drainage systems that are 'flashy', responding quickly to rainfall running off hard surfaces, are characterized in engineering design by a 'time of concentration' that is the response time of the drainage area, and which is used to determine the extreme rainfall durations relevant to infrastructure design.  It is never 24 hours or one day.  Typical times of concentration are measured in minutes and up to hours.

The Ontario Ministry of Transportation describes the design rain storms that may be used to analyze rural and urban areas, including the duration of the storm (link:   http://www.mto.gov.on.ca/english/publications/drainage/stormwater/section10.shtml):

Storms of duration up to 24 hours are applicable to rural land uses.  Storms of up to 4 hours (including flashy Chicago hyetograph temporal distributions) are applicable to urban areas.  The SWMM Knowledge Base, a discussion forum for the standard U.S.EPA Stormwater Management Model and other modelling platforms, provides insight into what storm durations practicing civil engineering / urban system modelling professionals use.  In the discussion thread "Design storm duration" (link: https://www.openswmm.org/Topic/3967/design-storm-duration) a duration of 24 hours is deemed by one practitioner to be 'ultra conservative' ("ultra conservative choice of a 24-hour storm but it hardly can be justified when no detention storage is involved"), another states that in small urban systems the 5-minute rainfall governs peak flows ("a small (25 acre) urban, very impervious, drainage area was that the peaks were almost the same no matter the duration, and that they were driven by the peak 5-minute rainfall"), and Ben Urbonas, Ben Urbonas,
President of Urban Watersheds Research Institute and Owner, Urban Watersheds, LLC (LinkedIn: https://www.linkedin.com/in/ben-urbonas-2a319338/)  notes the use of durations of 2-6 hours ("All of our design storms are front loaded intensity types and range from 2-hour to 6-hour durations depending on watershed area.").

Marsalek and Watt's paper Design storms for urban drainage design in the Canadian Journal of Civil Engineering shows design storm durations of often 1 hour duration, sometimes up to 6 hours (US Soil Conservation Service (SCS) for rural areas, as highlighted in their Figure 1.

Marsalek and Watt tablulate design storms with duration and categorize the use of the storms for different hydrological studies, including urban/sewer design and other applications, such as the study of large rural basins. Table 2 from their urban drainage review shows durations of up to 1- 12 hours for Canada's Atmospheric Environment Service's (AES) storm, and 1, 3 and 4 hour storms for sewer sizing in other jurisdictions (see highlights below).

Practitioners in Ontario, Canada will know that longer duration storms are considered for large regional wastewater systems that have a slow response to long-high volume storm events.  These govern large trunk sewer system performance, but not local sewer system performance that is dominated by short duration rainfall.  Even small wastewater system trunks may be governed by short duration rainfall intensities where there are direct inflows, which is common for many flood prone systems.  Analysis of trunk system response in the Kitchener-Waterloo Region showed wastewater trunk peaks flows for most-highly correlated to the 5-minutes rainfall intensities in on Master Plan study (i.e., more than longer durations).

Rivard's paper in the Journal of Water Management Modelling entitled Design Storm Events for Urban Drainage Based on Historical Rainfall Data: a Conceptual Framework for a Logical Approach (link: https://www.chijournal.org/Journals/PDF/R191-12) summarize early work on characterizing storms in Canada and in the highlighted excerpt notes that 1- 12 hour durations represented convective (thunderstorm) and synoptic scale events. See the highlight to the right.

Rivard also summarized what storm durations are of interest for urban design graphically as follows:

So in small to medium basin, up to a 3 hour duration is critical, and for a very large urban basin, up to 6 hours.  Twenty four hour durations and longer are critical to large rural basins.

The statement "Rx1day is important for flash floods as well as infrastructure design." is therefore inconsistent with professional engineering practice in Canada.

Environment and Climate Change Canada publishes Engineering Climate Datasets including Intensity-Duration-Frequency statistics describing rainfall, both common, moderate and extreme, used from infrastructure design.  The durations analyzed are from 5-minutes to 24-hours.

So again, no, 'heavy rainfall' in a climate research paper is not the same as 'extreme rainfall' an engineer uses for infrastructure analysis and design. Rainfall over 1-5 days periods is not the same as extreme rainfall over minutes to hours used to design conveyance systems in urban areas - those 'flashy' systems with short 'time of concentration' characteristics.

The statement in the research paper "Rx1day is important for flash floods as well as infrastructure design." is questionable.  One-day rainfall is way at the fringe of influence on flash flooding.

3) Do 'heavy rainfall' and precipitation trends follow 'extreme rainfall' trends used in engineering design?

Kirchmeier-Young's research found that 1-day duration simulated precipitation from various models has increased over past decades, and this trend follows observations from HadEX2 (a global gridded dataset).

We can compare the HadEX2 trends across North America, and subregions shown in the research paper, with extreme rainfall trends based on Canadian climate station observations.  Let's start with the 1-day, 24-hour annual maximum rainfall trends across Canada.

The chart below shows how annual maximum rainfall has changed according to Environment and Climate Change Canada's version 3.10 Engineering Climate Datasets for all storm durations from 5-minutes to 24-hours.

For 24-hour durations, 4.9% of all stations have a significant increase, 91.2% have no significant change, 2.3% have significant decreases and 1.5% of stations had no data.

Comparing to earlier datasets:

                                                     Version 2.30           Version 3.00            Version 3.10

No significant 24-hour trend            91.5%                    91.1%                         91.2%

Significant 24-hour increase              5.3%                      5.4%                           4.9%           

So the percentage of data that has no significant trend is relatively steady, and represents over 90% of the data. The percentage of data that has a significant increase in 24-hour rainfall is decreasing relative to the earlier datasets.

Canadian Engineering Climate Dataset trend data does not show increases consistent with the research paper.

4) Do 'heavy rainfall' events studied in the research paper cause damaging flood events, and flash flooding 'particularly in urban areas?

No.  Flash flooding is due to short duration, high-intensity rainfall.

The severe thunderstorms that are responsible for urban flooding and that occur over minutes to hours are different than the storms that occur over hours to days as indicated in the RSI IDF training presentation noted above:

For this reason, those interested in turban flooding drivers should look at short duration rainfall extremes - see below.

5) What are the trends in 'extreme rainfall' in Environment and Climate Change Canada's Engineering Climate Datasets, the data used by engineers to analyze and design infrastructure to manage flash flooding risks in urban areas?

Short duration rainfall is responsible for urban flash flooding.  Environment and Climate Change Canada's Engineering Climate Datasets indicate the following on annual maximum rainfall trends across Canada:

The short durations from minutes to a couple hours have low percentages of significant increase, just like the 24-hour data noted above.  The amount of significant increases expected due to chance is 2.5% increasing and 2.5% decreasing.

In a review of an earlier dataset by Environment Canada's Shephard et. al in 2014 (link: https://www.tandfonline.com/doi/pdf/10.1080/07055900.2014.969677) these amounts of changes were deemed not significant:

"Based on this IDF single station analysis, and the more general single station climate results from the 1965–2005 period presented in Section 4a, we conclude that the annual maximum short duration rainfall values across Canada typically do not show a significant trend."

And more recently in Canada’s Changing Climate Report, such changes in short duration extreme precipitation were explained by chance (link: https://changingclimate.ca/CCCR2019/):

"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. More stations have experienced an increase than a decrease in the highest amount of one-day rainfall each year, but the direction of trends is rather random over space. Some stations show significant trends, but the number of sites that had significant trends is not more than what one would expect from chance (Shephard et al., 2014; Mekis et al., 2015; Vincent et al., 2018)."

The short duration intensities used for infrastructure design, derived based on annual maximum series, have not increased in many regions based on compiled studies (see previous post: https://www.cityfloodmap.com/2018/03/extreme-rainfall-and-climate-change-in.html).  A review of design intensities in southern Ontario shows overall increases in short duration values (see previous post: https://www.cityfloodmap.com/2020/05/southern-ontario-extreme-rainfall.html).

So no change in how infrastructure is designed based on short-duration design intensities (that is, not including checks or 'stress tests' for future changes).

6) What does Kirchmeier-Young's research paper reveal about previous extreme rainfall and flooding events in Canada - has climate change increased runoff that could aggravate flood damages?

Nothing.  The storms that lead to widespread urban flooding are not addressed in the research paper.  The processes driving 1-5 day rainfall are different than those driving short-duration rainfall.  There are no significant increases in the short-duration rainfall that causes flooding based on Engineering Climate Datasets as shown above.

Why then have damages increased over decades? Possible reasons are:

a) growth in net written premiums: more insured properties = more losses

b) urbanization: more pavement means more runoff and impacts

The landmark case Scarborough Golf Country Club Ltd v City of Scarborough et al. (Ontario Court of Appeal, 1988, http://members.storm.ca/~river/letters/Scarboro%20Golf%20Club%20v%20City%20of%20Scarboro%20OCA%201988.pdf) decision indicates that Toronto-area urbanization markedly increased runoff stresses that caused runoff, erosion and flooding:

“Expert evidence confirmed the effect of the city&#39;s rapid urbanization and water control plans on the creek.”

“It is important to note that the case is not presented primarily as a complaint against flooding but
rather that the markedly increased flows and increased velocity of flow have caused and continue to
cause damage to the creek bed and the adjacent tableland.” and

“There can be no doubt that the storm sewer facilities and urbanization of the lands to the north of the Club are the cause of the effects just described and that the difference in flow and velocity of flow is very substantial.”

Cities are growing and there is more runoff as shown here in some regions:



c) inconsistent data: the data source for losses cited by CBC changed from 2008 onward

Changing data methods can lead to different results (see previous post on this: https://www.cityfloodmap.com/2018/06/catastrophic-losses-in-canada-have.html)



***

The research paper makes a reference to an attribution study for the 2013 Alberta flood.  It states:

"Additionally, event attribution studies have identified an increased probability of some individual extreme precipitation events in this region due to anthropogenic influence (4, 5)"

Reference 4 is:

B. Teufel et al., Investigation of the 2013 Alberta flood from weather and climate
perspectives. Clim. Dynam. 48, 2881–2899 (2017). (link: https://link.springer.com/article/10.1007/s00382-016-3239-8)

So we have one Canadian storm assessed. Findings are:

"Event attribution analysis suggests that greenhouse gas increases may have increased 1-day and 3-day return levels of May–June precipitation with respect to pre-industrial climate conditions. However, no anthropogenic influence can be detected for 1-day and 3-day surface runoff, as increases in extreme precipitation in the present-day climate are offset by decreased snow cover and lower frozen water content in soils during the May–June transition months, compared to pre-industrial climate."

So greenhouse gases may have increased precipitation, but that is offset by less snow, resulting in no change in runoff, compared to pre-industrial climate.

So with no change in runoff, can there be a change in flood damages attributed to the precipitation change?  The net effect is no increase in risk.

***

To wrap it up, CBC has relied on a research paper that looks at rainfall events (1-5 day precipitation) that are not related to urban flash flooding and are not related to the events that lead to significant damages (convective thunderstorms with peak intensities over minutes to hours).  The research does not review short-duration rainfall that is relevant to infrastructure design governed by short 'times of concentration' - i.e., they are 'flashy'.  The research does not appear to be consistent with trends in 24-hour annual maximum rainfall observed at Canadian climate stations and as published in Environment and Climate Change Canada's Engineering Climate Datasets - data show few statistically significant increases and the percentage of significant increases is decreasing slightly for 24-hour rainfall across Canada.  The short-duration rainfall intensities responsible for urban flooding show no consistent changes, and any significant changes are explained by chance, according to Environment Canada.

Many factors go into increasing flood damages.  Changes in rainfall does not appear to be one of those factors.  Media should take the time to dive deeper into the technical details they reference to improve the accuracy of reporting, so that the public is better informed about complex issues.
Urban flooding is a complex issue, and an important challenge to address that requires significant funding and attention.  A better understanding of the causes of flooding, and any changes in design rainfall, is required to mitigate flooding in the most objective, cost-effective manner.  CBC has relied more on model predictions than on actual data in the past, even confusing the two (see previous post: https://www.cityfloodmap.com/2020/05/what-covid-19-taught-us-about-observed.html).  In this recent report it has not met its JSP principle for accuracy by confusing longer-term precipitation and short-duration extreme rainfall.

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BONUS - Trends in short-duration rainfall, based on annual maximum observations, from Environment Canada's version 3.10 Engineering Climate Datasets are summarized below (link: https://climate.weather.gc.ca/prods_servs/engineering_e.html).  These tables consider stations with a long period of record and recently updated data for regions across Canada.