Greater Toronto Area (GTA) maximum yearly rainfall trends are shown in the following charts for Toronto City (downtown 'Bloor Street' gauge), Pearson Airport (Mississauga) and Buttonville Airport (Markham):
A review of Pearson Airport climate station extreme rainfall trends considering raw data was provided in a recent National Post Op Ed - see link. Other raw data for the GTA was analyzed in a previous post.
The long term series for Toronto and Mississauga show trends that are flat (no change) or decreasing - for Toronto, the 12 hour rainfall amounts are decreasing significantly. The Buttonville Airport data has not been extended by Environment Canada in the version 3.0 datasets however the City of Markham has done so with raw data and identified decreases in short duration intensities (see IDF discussion at end of this post).
In other Southern Ontario regions trends are generally not statistically significant and can be up or down (the only possibilities really). Here are trends for Ottawa, Kingston, Hamilton and London:
Kingston has a long term record of almost 100 years. While records are not extended since the version 2.3 dataset, the trends are basically flat over the period of record as shown below:
In Edmonton, the longest record goes back a century at the Blatchford climate station. The annual maximum rainfall observed there is decreasing or flat for duration of up to 12 hours, with no significant trends.
1) It is best to look at the trends over the long term and rely on the longest periods of record for estimating extreme value statistics in engineering design. An earlier review of these statistics for long term stations in Southern Ontario show small, high frequency rain intensities decreasing slightly on average and large, low frequency intensities mixed (i.e., only small increases and decreases that are insignificant in hydrologic analysis).
2) Most urban storm drainage systems are small and 'flashy' responding to short duration rainfall intensities that correspond to the 'time of concentration' of the catchment of up to a couple hours (but typically less). Trends in rainfall maximum amounts over those duration can contribute to changes in flood flows and flood losses (damages) - overall, hydrologic changes (more urbanization over decades, more intensification within earlier development) greatly overshadow any meteorologic changes. My paper in the Journal of Water Management Modeling "Thinking Fast and Slow on Floods and Flow" explores some of this as do earlier posts.
3) Small local wastewater systems may be sensitive to short duration high intensity rainfall trends as well, especially where rooftop drainage improperly or illicitly contributes inflows to those collection systems. Flow monitoring data can show a 'flashy' response in extraneous flow rates that stress system capacity and contribute to basement flooding / sewer back-up risks.
4) Large surface water drainage collection systems (channel systems and local creek tributaries), as well as wastewater collection systems may be most sensitive to longer duration rainfall intensities (cumulative volumes). For this reason, some municipalities (Ottawa, York Region) have adopted long duration design rainfall hyetographs to assess system capacity.
5) Despite the lack of overall extreme rainfall trends in the regions screened above, some other regions in Canada may have other trends. An earlier review of the version 2.3 datasets across the country showed some regions with more increasing than decreasing trends - see post here with regional summaries of trends direction and significance. See post here with a review of long term station trends (shows more increases than decreases in Maritimes and Newfoundland).
Stay tuned for a review of IDF updates with the version 2.3 datasets. Previous work in Southern Ontario municipalities using earlier data and some updated data (City of Markham's Toronto, Mississauga and Markham gauge review, for example) has not shown appreciable changes in IDF values - see previous post.
Below is an initial review of 5-minutes design rainfall intensities for return periods of 2-year to 100-years considering extended datasets:
The Markham design intensities (at Buttonville Airport) are decreasing since the 2003 values for all return periods considering extreme rainfall observations data up to 2016 (raw data from Environment Canada and analysis by City of Markham for 2016 values).