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| Toronto: temperature up - rainfall down |
Downtown Toronto shows the most consistent decrease in observed rainfall intensity and corresponding decrease in design rainfall intensity. These design intensities are based on extreme value statistics as derived by Environment Canada using the raw weather observations at the climate station.The tables at right show intensities for "Toronto City" Climate Station ID 6158355 (aka "Toronto" 6158350) for three recent Environment Canada IDF curve updates:
- Up to 1990 (47 years of record)
- Up to 2003 (57 years of record)
- Up to 2007 (61 years of record)
Design rainfall intensities are shown for short durations of 5 minutes, 15 minutes and 1 hour in the three tables.
Changes in rainfall intensity are also shown up to 2003 and up to 2007 in the right two columns. All design intensities have decreased for all durations and for all return periods from 2 to 100 years (i.e., common storms to rare storms). Is the Ontario government muzzling scientist and engineers from sharing this, to justify cap and trade and Bill 172 to fight flooding through climate change mitigation? That would seem misguided, not evidence-based as storm intensity is not increasing.
The most extreme (rare) "100 year" rainfall intensities that have a 1% chance every year have decreased between 3.7% and 5.5% for 1 hour to 5 minute durations, respectively. The more common "2 year" intensities with a 50% chance every year have decreased as well, dropping 2.5% to 4.1%.
A similar climate change in severe weather is observed at the Pearson International Airport climate station in Mississauga, Ontario. The Environment Canada label for the this station ID 6158733 is "TORONTO INTL A", or "TORONTO LESTER B. PEARSO ONT", which inaccurately identifies Toronto as the municipality where the gauge is located.
- Up to 1990 (38 years of record)
- Up to 2003 (51 years of record)
- Up to 2007 (54 years of record)
- Up to 2013 (60 years of record)
Changes in rainfall intensity are also shown from 1990 up to 2003, 2007 and 2013 in the right three columns. Design intensities have decreased for shorter durations (5 and 15 minutes) for all return periods from 2 to 100 years (i.e., common storms to rare storms).
For longer duration intensities, the trend is mixed with "2 year" common intensities decreasing 4.1% and rare "100 year" intensities increasing 10.9%. The increase reflects the fact that a record rainfall was recorded in the last year of the record (July 8, 2013). Subsequent years have shown below average extremes - in 2014 and 2015 the maximum daily rainfall totals were only 27.6 mm and 37.4 mm, respectively, both less than the average "2 year" value of 47.6 mm. These recent trends would result in a return to lower design intensity values. The chart below shows the decreasing trend in daily rainfall totals up to 2013 and up to 2015. The downward trend in 24 hour maximum rainfall is stronger when 2014 and 2015 are included (dashed blue line). Yet, even if the recent lower daily rainfall totals are excluded, the trend is downward (dashed red line).
Looking at longer durations, 12 and 24 hours, the overall Mississauga Pearson trends are flat or lower as well since 1990:
What do the Toronto and Mississauga "Toronto Pearson" IDF trends mean for drainage design? Hopefully nothing. Although downtown Toronto trends have been steadily decreasing, the most recent 2013 storm is not included and could moderate the downward trend in severe rainfall statistics. Although Mississauga Pearson has some increasing intensities, this is skewed by the timing of the most recent record and absence of even more recent below average data points. And although some Mississauga intensities increase (above 5 year, 1 hour data), short duration values remain below downtown Toronto values in an absolute sense, meaning the Mississauga values are just 'catching up' to the historically higher Toronto ones. To illustrate this, up to 1990 the 5 minute Toronto 100 year design intensity was 21% above the corresponding Pearson value. In 2007, that Toronto statistic was still 20% higher.
Given decreasing or as yet inconclusive or mixed IDF trends, municipalities relying on the long term Toronto and Mississauga Pearson rainfall statistics should focus on other elements of drainage design besides IDF curve updates to reduce urban flood risk. These elements include review of runoff coefficients that increase with intensification and infill development or denser urban design, and return period factors to increase conservatism when IDF values are used in rational method design. Design hyetographs based on IDF values should also be reviewed for conservatism, especially how mass curves are used to distribute IDF rainfall totals over design storm periods - preservation of peak rainfall intensities within the hyetograph pattern would have a significant influence on peak runoff rates for flashy urban catchments.
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“It's tough to make predictions, especially about the future.” ― Yogi Berra
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Update to 2016 Pearson Airport Daily Maximum Rainfall Totals: earlier decreasing trend continues in 2016:
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A comparison of IDF values in southern Ontario between the pre-version 1 1990 datasets and the current version 2.3 datasets shows an overall decrease in frequent rainfall and essentially no change in infrequent rainfall - here are a few results from that post:
IDF values should increase over time as sample bias is reduced with longer data sets that better characterize extreme events in the population. Samples (aka observations) from skewed populations like rainfall require long records with many observations for these extremes to be reflected in the IDF statistics - a previous post explores this in context of the variability we see in rainfall observations. The IDF review above considers climate station records with 30 years of observations or more in which sample bias is expected to be low (see Toronto example in previous post).
A comparison of IDF values in southern Ontario between the pre-version 1 1990 datasets and the current version 2.3 datasets shows an overall decrease in frequent rainfall and essentially no change in infrequent rainfall - here are a few results from that post:
IDF values should increase over time as sample bias is reduced with longer data sets that better characterize extreme events in the population. Samples (aka observations) from skewed populations like rainfall require long records with many observations for these extremes to be reflected in the IDF statistics - a previous post explores this in context of the variability we see in rainfall observations. The IDF review above considers climate station records with 30 years of observations or more in which sample bias is expected to be low (see Toronto example in previous post).
