Extreme Rainfall and Climate Change in Canada and Ontario - Compiled Engineering Reports, Research Papers and Presentations by Esteemed Hydrologists Indicate No Change

We have sent a request to the Globe and Mail regarding a recent article on climate change and extreme rainfall and flooding. Specifically we have requested that a correction be made to this statement:

"Moreover, extreme weather related to climate change appears to be increasing the frequency and severity of flooding events."

The article was entitled "How we can better mitigate flood risk in Canada", January 15, 2018, by Glen Hodgson, a senior fellow at the Conference Board of Canada. This is a link to the article .. or should we say the "Opinion Piece":

Certainly everyone is entitled to their own 'opinion', but not their own 'data'. With that in mind we have asked the Globe for a correction based on the following data and analysis (we have added to the original list of reports since making the request):


The following engineering reports, thesis documents, peer-reviewed papers, presentations by Canada's most esteemed hydrologists and comments by Environment and Climate Change Canada staff clearly refute the Globe article statement that the frequency and severity of extreme weather is increasing and that "extreme weather related to climate change appears to be increasing":

i. City of Guelph – Ward 1, Frequency Analysis of Maximum Rainfall and IDF Design Curve Update, Earthtech, 2007. Concludes that city would “prefer to retain the existing curves and higher values” as updated rainfall intensities were lower.

ii. Updates of Intensity-Duration-Frequency (IDF) Curves for the City of Waterloo and the City of Kitchener, University of Waterloo, 2012: indicates “new IDF curves tend to be lower .. for rainfall durations up to one to two hours”.

iii. Rainfall Intensity-Duration-Frequency (IDF) Curves RFP11-080 Engineering Design Criteria and Standards Update (DRAFT), fabian papa & partners inc., 2012. Found that “City’s existing criteria ... are more conservative than recently compiled statistics” and “up-to-date Environment Canada IDF Curves will result in lower storm intensities”.

iv. Trends in Canadian Short‐Duration Extreme Rainfall: Including an Intensity–Duration–Frequency Perspective, Environment Canada, Atmosphere Ocean 2014: indicates “lack of a detectable trend signal” across Canada an no regional increases in Ontario.

v. Precipitation Intensities for Design of Buried Municipal Infrastructure, Yi Wang, University of Guelph, Ph.D Thesis 2014: identifies 24 significant increases and 41 significant decreases.

vi. Waterloo Sanitary Master Plan, Volume 1, Appendix A Climate Change, Stantec, 2015. The study did not identify any rainfall intensity changes but adopted a different rainfall pattern to be more conservative in design.

vii. Changes in Rainfall Extremes in Ontario, International Journal of Environmental Research, University of Guelph, 2015. The paper that identified “results of this study indicate that the +ve and -ve changes in annual rainfall extremes are similar in the order of magnitude.”

viii. Short Duration Frequent Rainfall Show No Change in Southern Ontario IDF Design Intensities - No Change in Averages Suggests No Change in Extremes, Muir, 2018. This analysis of Environment and Climate Change Canada’s IDF statistics from 1990 to the current Version 2.3 Engineering Climate Datasets shows no change in 2-year to 10 year 5-minute to 2-hour rainfall.


ix Floods in Southern Ontario Have Changed, University of Guelph, MNRF Floodplain Technical Workshop, Vaughan, March 7, 2018. Professor Emeritus Dr Trevor Dickinson presented  last week that:

"In fact:
- the number of rainfall events has not increased,
- the total amount of rainfall occurring over the growing season has not increased, &
- to date, there is no evidence that rain storms are more severe."

x Environment and Climate Change Canada (ECCC) has commented to the CBC/Ombudsman to dispute insurance industry statements that we have more storms (see letter to me at this link):

That was in response to this story that had no fact-checking on extreme weather frequency:

And which had this correction made based on ECCC and real data: "However, Environment Canada says it has recently looked at the trends in heavy rainfall events and there were "no significant changes" in the Windsor region between 1953 and 2012." 

xi Canadian Underwriter editors dispute insurance industry statement on more frequent / severe storms after fact-checking with ECCC: 

"Associate Editor’s Note: In the 2012 report Telling the Weather Story, commissioned to the Institute for Catastrophic Loss Reduction by the Insurance Bureau of Canada, Professor Gordon McBean writes: “Weather events that used to happen once every 40 years are now happening once every six years in some regions in the country.” A footnote cites “Environment Canada: Intensity-Duration-Frequency Tables and Graphs.” However, a spokesperson for Environment and Climate Change Canada told Canadian Underwriter that ECCC’s studies “have not shown evidence to support” this statement."

Please issue a correction to the article. Thank you.

Robert J. Muir, M.A.Sc., P.Eng. 

BONUS - the City of Ottawa has also reviewed IDF curves in October 2015 and found the following:


"The IDF curves used for sizing of storm sewers and stormwater management designs that are currently in use at the City and provided in the Ottawa Sewer Guidelines (OSG) is based on rainfall data collected at the Ottawa McDonald Airport from 1967 to 1997 by Environment Canada (further referred to OSG IDF Curves).  Environment Canada updated the IDF curves using data to 2007 (further referred to as 2007 IDF Curves).  Over the recent past years, the update was brought to ISD’s attention for adopting into the OSG.

 A comparison of the OSG and the 2007 IDF curves revealed that for short durations the intensities from the 2007 IDF Curves are less than the OSG IDF Curves on average by 5% and 10% for the 5 year and 100 year curves respectively.  However, for the longer durations the intensities for the 2007 IDF Curves are actually greater than the OSG IDF Curves on average by 11% and 17% for the 5 year and 100 year curves respectively.  The intensities at the shorter durations would influence storm sewer sizing while the longer durations will influence stormwater pond sizing.  The lower intensities at the short durations will tend to result in smaller storm sewer sizes while the larger intensities at higher durations will tend to increase stormwater management pond requirements.  


Given that the percentage differences in intensities between the IDF curves is within the margin of error associated with data collection and hydrologic assessments, it was ISD’s opinion not to update the OSD IDF curves.  As part of the Stormwater Levels of Service review, the need to revisit the IDF curve selection was identified."

Because stormwater ponds are not designed with IDF curve statistics alone, i.e,. they are designed by simulating temporal patters of storms in hydrologic/hydraulic models, it is questionable if an IDF shift in long durations alone would alter the design of a stormwater management pond. That is the design storm (or hyetograph) pattern would influence the pond performance more than the input IDF used to adjust storm volume. Also, some municipalities assess pond performance assuming the outlet is completely blocked, as an operational worst case scenario, such that all collected runoff is routed through the emergency spillway. In this extreme operating condition, a small change in rainfall volume may not affect overall performance (i.e., spillways operate efficiently as weirs, able to accommodate additional flow release with limited increase in operating level (pond water surface elevation).

ANOTHER BONUS - the City of Hamilton has also reviewed IDF curves in 2015 and found no change that warranted updates in their current standards (R.Muir personal communication with Hamilton engineering staff May, 2018).

YET ANOTHER BONUS - the City of Markham has also reviewed IDF curves in 2018 based on extended rain data analysis considering local data (Buttonville Airport) and data for the long-term Toronto Bloor Street climate station (upon which Markham's standards were based on). This review is summarized in a recent Municipal Class Environmental Assessment Study for Don Mills Channel flood remediation (see report here: https://www.markham.ca/wps/portal/home/about/city-hall/city-projects-initiatives/current/stormwater-management/don-mills-flood-reduction/don-mills-channel-flood-reduction).

The report notes: 

The City of Markham has recently completed a review of past and current climate data and a number of other climate change resources in order to assess the resiliency of the City’s wastewater collection systems. To assess IDF impacts the City of Markham first reviewed national and regional rainfall trends in Environment and Climate Change Canada’s (ECCC’s) Engineering Climate Datasets (version 2.3) and local research and determined that ‘no historical changes in rainfall intensity are expected based on the analysis of national and regional (southern Ontario) datasets’ (Xu and Muir, 2018). This is consistent with extreme rainfall trends analysis by ECCC that indicate ‘a general lack of a detectable trend signal’ nationally (Shephard et.al, 2014).
As part of the assessment, the City of Markham updated local IDF curves for the long-term Toronto City climate station that its design standards are based on, as well as the Toronto International Airport (Pearson) and Markham Buttonville Airport stations IDF curves. The findings related to the wastewater system resiliency assessment, which are also relevant to storm drainage infrastructure, were as follows:
■ “The Pearson station 100-year data showed no change since the ECCC 2013 dataset, and a decrease since the 1990 dataset (average decrease of 3.2%). The Buttonville station 100-year data showed an average increase of only 1.1%. Therefore 100-year short-duration intensities are considered to be stationary for the purpose of the existing system capacity assessment under today’s climate - past rainfall intensities (IDF data) maybe used to assess current wastewater system wet weather performance.”

YET ANOTHER BONUS - the City of Welland has also reviewed its design IDF curves in 2012 in this study. It's consultant found that:

"The following general conclusions stem from the analysis:
• the 1963 IDF curves is conservative relative to the estimates made in the 2000 IDF curves.
Thus, adoption of the 2000 curves would effect a relaxation of planning standards for many
types of infrastructure.
• the 1963 curves were conservative relative to the current (2000) estimates and even relative
to the projected (2020 and 2050) values for many duration/return interval combinations. In
those instances, it is reasonable to retain the 1963 intensities."

So even looking into the future, the city's design rainfall intensities from the 1960's are more conservative. So of course if there has been flooding in Welland it's due to other design considerations (like return period level of service for sewer design, etc.) and not because rain is now, or will be, higher than design intensities.

YET ANOTHER BONUS - the City of Niagara Falls has also reviewed its design IDF curves and found lower rainfall intensities. The following is an excerpt from the MOECC (now MECP) LID Guidance Manual (second draft, page 129):

"Many Ontario municipalities have conducted climate change and/or IDF analysis studies to provide direction for municipal infrastructure planners in light of climate change risks. Of note is the City of Niagara Falls which conducted an IDF curve update and climate change analysis as part of their 2015 Master Drainage Plan Update Study. Updated IDFs for four of the five climate stations within the City were found to generate rainfall volumes and intensities that were slightly lower than those generated by the previous IDF curves (Hatch Mott MacDonald, 2015). Additional analysis conducted for Niagara Falls found that the “average annual rainfall volumes for the past 15 years (2000 to 2014) were actually 5.5% lower than the long term average, and significantly lower (by 12.6%) than the average annual rainfalls in the 1970’s, 80’s and 90’s; and the frequency of the larger rainfall events (> 25 mm) that cause most of the stormwater management and combined sewer overflows problems were all significantly lower than the long term average (by 15-25%)” (Hatch Mott MacDonald, 2015)."

YET ANOTHER BONUS - The Windsor/Essex Region Stormwater Management Standards Manual reviewed City of Windsor Airport IDF trends - see December 2018 report:  https://essexregionconservation.ca/wp-content/uploads/2018/12/WE-Region-SWM-Standards-Manual.pdf

"Table A-3.9.1b showing Windsor Airport extreme rainfall trends from 1995 to 2015 continues to illustrate a decreasing trend for short-duration events from 5min to 30min duration for nearly all return periods. The trends illustrate an increasing trend in 1 hour, 2 hour, 12 hour and to a lesser extent the 24 hour durations."

The review includes data up to 2015 - Table A-3.9.1b is shown below:

YET ANOTHER BONUS - The Ontario Ministry of Transportation (MTO) completed a comprehensive study entitled “The Resilience of Ontario Highway Drainage Infrastructure to Climate Change”  in 2015 that indicates consistently decreasing intensities predicted for short durations affecting urban flooding:

“The IDF predictions in the 2014 UR study (2) also give rainfall predictions with significant variability with location, storm duration and return period (frequency) which can be compared to the 2007 MTO IDF curves. Predicted storms with durations less than 6 hours are less intense than those observed in 2007, for all return periods. Longer duration storms do not always hold to this pattern, with the 6 and 24 hour storms often predicted to become more intense, particularly in Northwestern Ontario.” 

The study noted ranges in predicted increases and decreases for bias-corrected climate models:

“In some areas rainfall intensity increased from 0% to just above 30% where in other areas there were rainfall intensity reductions in the from 2 to 10%.”

The Ontario Ministry of Transportation noted in the report that modern drainage infrastructure including sewers, culverts and bridges are resilient to increases in design flows that may occur due to climate change: 

“An overwhelming percentage of the storms sewer networks tested appeared to have sufficient excess capacity to hand the increases in design flow rates up to 30%. Similarly, the sample of highway culverts analysed showed adequate capacities, for a large percentage of the culvert, to handle the rage of low rate increases investigated without the need to be replaced. The bridges tested also appeared to suffer no risk to structures as a result of the flow increases.”

In response to this low risk, the Ministry’s Highway Standards Branch has developed a policy for assessing risks based on future climate that incorporates flexibility in design. The 2016 memorandum entitled Implementation of the Ministry’s Climate Change Consideration in the Design of Highway Drainage Infrastructure states: 

“Designers are to exercise engineering judgement to determine whether the infrastructure will meet current and future design criteria through appropriate sizing of the infrastructure or through providing allowances for future adaptation measures.”

The Ministry’s approach of allowing for future adaptation as opposed to increasing infrastructure capacity as a result of future criteria (i.e., higher rainfall intensities) is similar to the ASCE approach that advocates the Observational Method that promotes “Design for low regret, adaptability, and robustness, and revisit designs when new information is available.”



A separate post explores IDF trends in the Version 3.0 Engineering Climate Datasets for long term southern Ontario climate stations - previous post link. Here are the results showing an overall decrease of 0.4% in all design intensities, and decreases for all return periods (bottom row) - smaller storms with more observations decreasing the most - and decreasing for most durations (right column) - especially the shorter durations of less than 1 hour:


Update: The Globe and Mail responded by 'working their Google' and citing examples of flood damages increasing, diverting from the question on extreme weather frequency. We have explained that flood damages can increase for many reasons and that they have fallen prey to 'attribute substitution'. Check out this recently published paper on heuristic biases and challenges in framing and solving problems related to extreme weather and flooding:

The CBC Ombudsman has recently reviewed data provided on this topic, cited Environment and Climate Change Canada statements and agreed as shown in this post https://www.cityfloodmap.com/2019/01/cbc-ombudsman-decision-finds-lack-of.html

The Minister of Environment and Climate Change Catherine McKenna has also weighed in on this topic indicating that there is no evidence of changes in extreme precipitation (i.e., short duration rainfall affecting flooding).

The letter (at right) was to clarify comments made by Prime Minister Trudeau following Gatineau 2017 flooding that "The frequency of extreme weather events is increasing, and that's related to climate change".

The Minister's letter, drawing from the recent Canada's Changing Climate Report, does not support the Prime Minister's statement on extreme weather frequency.


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