Environment and Climate Change Canada Updates Annual Maximum Rainfall Trends and IDF Statistics in Engineering Climate Datasets - Decreasing Extreme Rainfall Trends Continue in GTA

Environment and Climate Change Canada has posted updates to Annual Maximum Rainfall Trends / Series and derived Intensity Duration Frequency curve data on their website - see link. This is version 3.0 of the Engineering Climate Datasets and indicate trends in extreme rain intensities over various durations that are considered in engineering design such as municipal infrastructure design or building design. Many climate stations have had their records extended to as recently as 2017 however some maintain only the previous records in version 2.3. The following is a summary of some the updates looking at trends in storm severity based on recorded data.

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:

The Ottawa trend are downward for short duration affecting flash flooding in urban areas. For long durations the trends are mixed - the Ottawa CDA RCS gauge has a high recent value for the remnants of hurricane Francis in 2004 and many low values at the turn of the century that drive the 24 hour rainfall trend up at that gauge - nonetheless, trends for durations of 2 hours and less that reflect convective thunderstorms are decreasing at that gauge. At the Ottawa Airport, decreasing trends are
strong and are statistically significant for durations of 10 minutes, 15 minutes and 1 hour.

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:

Hamilton Airport has a moderate length record and show decreasing annual maximum rainfall amounts wince the early 1970's (see below). The longer record Botanical Garden gauge show no overall trends going back to the 1960's.

The London Ontario period of record goes back to the 1940's. Trends in annual maximum rainfall are up, flat, or down depending on the duration as shown below in the extended series: 

 In Windsor, the record has not been extended to cover recent extreme events in 2016 and 2017 (version 3.0 is the same as version 2.3). The available series show decreasing annual maximum rainfall going back to the 1940's and statistically significant decreasing trends for many durations including 10 minutes, 2 hours, 6 hours and 12 hours. It is likely those trends will 'level out' when the 2015 and 2016 storm are included.

The University of Windsor series in shown below to illustrate that short record are unreliable to make observations on overall trends - increasing trends through the 1970's in some of the charts below do not reflect overall decreasing trends over many decades as shown in the charts above.
Other records from across Canada has been undated in the version 3.0 dataset. A few are shown below. The Calgary Airport trends since the 1940's are decreasing, flat or increasing slightly - there are no significant trends in any direction for any duration.

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.
 If we look at a shorter duration gonig back to the 1960's at the Edmonton Airport, trends are up:
 And if we look at shorter records since the 1980's like at Stoney Plain CS, the trends are down:

Take aways:

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 downtown Toronto design rainfall intensities are decreasing since the 1990 values for all return periods considering extreme rainfall observations data up to 2017.

The Mississauga design rainfall intensities (at Pearson Airport) are decreasing since the 1990 values for all return periods considering extreme rainfall observations data up to 2017.

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).


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