Southern Ontario Extreme Rainfall Trends - Environment Canada Engineering Climate Datasets IDF Tables

Environment Canada's version 3.10 update to Canada's rainfall IDF tables and curves shows more increases than decreases and slightly more significant increases than decreases - overall trends were shown in a recent post: https://www.cityfloodmap.com/2020/05/annual-maximum-rainfall-trends-in.html.

Regional trends may be up or down and warrant further review.  In Southern Ontario the IDF intensities for long term stations have been reviewed and compared with pre-version 1.00 statistics up to 1990 (similar to version 3.00 and version 2.30 comparisons shared in earlier posts).  The following table shows average changes since 1990, and the surrounding arrows suggest how these changes may influence infrastructure design, if at all.

Southern Ontario IDF Rainfall Intensity Trend Table - Environment and Climate Change Canada's Engineering Climate Datasets, Pre-Version 1.00 (up to 1990) to Version 3.10 (up to  2017)
The 21 stations assessed include:  Sarnia Airport, Chatham WPCP, Delhi CS, Port Colborne, Ridgetown RCS, St Catharines Airport, St Thomas WPCP, Windsor Airport, Brantford MOE, Fergus Shand Dam, Guelph Turfgras CS, London CS, Mount Forest (Aut), Stratford WWTP, Waterloo Wellington Airport, Bowmanville Mostert, Hamilton Airport, Hamilton RBG CS, Oshawa WPCP, Toronto City, Toronto International Airport (Pearson).

The above changes in design rainfall intensities since 1990 do not suggest any overall shift that would affect how municipal drainage infrastructure would be designed considering current weather conditions.  That is, overall intensities have decreased by 0.2% which in negligible.  The 5-minute, 2-hour and 6-hour intensities decreased consistently across all return periods.  The change however is also negligible.  On average frequent intensities, e.g., 2-year intensities expected every couple of years, and 5-year intensities, used for storm sewer design showed overall decreases.  Again the changes are negligible.  The 100-year intensities increased by 0.1% overall which is also negligible, especially considering the confidence limits with such statistics and the uncertainty in curve fitting (Gumbel distributions are used, other distributions would provide shifts in results).  One would expect rare intensities to increase over time for skewed distributions given sampling bias with short records (i.e., limited observations of extreme events are expected to lead to underestimates of 100-year statistics).

Of course considerations must be made to account for future changes and uncertainties.  Some cities and regions have incorporated allowance for climate change effects.  In Quebec a 18% allowance is standardized.  Some cities (e.g., Ottawa) include a 20% stress test to evaluate any unacceptable conditions that warrant design changes to address future potential risks.  Others incorporate stress test hyetographs in the design process - the Windsor/Essex Ontario standards include a stress test event that has 39% greater volume than the standard 100-year design storm (NB - the daily volume is increased to account for that additional volume distributed uniformly across 24 hours, while peak hyetograph intensities are only nominally affected). 

The chart below shows the IDF trends at these long-term record Southern Ontario stations.

Southern Ontario IDF Rainfall Intensity Trend Chart by Duration - Environment and Climate Change Canada's Engineering Climate Datasets, Pre-Version 1.00 (up to 1990) to Version 3.10 (up to  2017)
It is clear that that the short duration intensities (red and orange bars representing 5 and 10 minute durations) have decreased the most as shown in the table above. The chart and table below shows a more simplified version of the above chart indicating the range of changes observed.

Southern Ontario IDF Rainfall Intensity Trend Chart by Duration - Environment and Climate Change Canada's Engineering Climate Datasets, Pre-Version 1.00 (up to 1990) to Version 3.10 (up to  2017)
The above chart shows that 2-year and 5-year IDF rainfall intensities have decreased most consistently among all stations.  Those intensity estimates benefit from many observations each year to determine the statistics.  Rare event intensities from 25-year to 100-year return periods have more equal increases and decreases yet the decreases are greater.  The magnitude of the changes, both increases and decreases are on average negligible.  Even the greatest increase of 1.2% from 1990 to 2017 (27 years) is negligible for the purpose of hydrologic analysis and drainage infrastructure design.



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