Short Duration Frequent Rainfall Show No Change in Southern Ontario IDF Design Intensities - No Change in Averages Suggests No Change in Extremes

It is often stated that changes in average conditions are an indicator of changes in extreme conditions. This makes sense for rainfall statistics as a change in typical conditions, such as an increase in rainfall intensities, can be accompanied by higher extreme values as well (i.e., the whole distribution shifts). Since extreme values are somewhat elusive to those recording rainfall intensities at Canadian climate stations - that is, they are rare and may not be readily observed in short records or sparsely-spaced climate stations - we can look at the trends in the more abundant and frequent short duration rainfall statistics as an indicator of where the extreme values are heading.

The following table summarizes trends in short duration rainfall intensities for long term Southern Ontario climate stations (below latitude of 44 degrees). Stations have at least 30 years of record. The change in 2-year 5 minute rainfall intensity and 5-year 10 minute rainfall intensity have been calculated using a starting point of then Environment Canada's 1990 IDF tables (obtained from Environment and Climate Change Canada in 2017), and an ending point of the Version 2.3 Engineering Climate Datasets.

Climate change rainfall
Change in average and frequent rainfall intensities in southern Ontario.
The review indicates that there has been no increase in frequent short duration rainfall intensities. In fact the most frequent 2-year (i.e., average), 5-minute duration rainfall intensities have decreased somewhat. This is welcome news considering the potential for frequent storms to cause erosion in southern Ontario streams. This also suggests that extreme rainfall intensities have not changed as a result of the average rainfall intensities changing. That is, there is no consistent shift in the average rainfall intensities at long term climate stations.

The Insurance Bureau of Canada and the Institute for Catastrophic Loss Reduction have reported that average rainfall intensities have shifted by an entire standard deviation (thus making extreme 40 year storms become more frequent 6 year storms) - this has been refuted by Environment and Climate Change Canada (see Canadian Underwriter editor's note). The data in the above table indicate no such shift.

It is a commonly held belief that rainfall intensities have increased dramatically as a result of climate changes effects. Recently the Globe and Mail reported "It is hard to ignore the growing relationship between climate change and the resulting impact of severe flooding events." .. actually its hard to explain the role of changing climate given rainfall intensity data in some regions. It may be best to ignore rainfall and focus on other flood risk drivers like urbanization and intensification.

Datasets from Environment and Climate Change Canada refute the belief that rainfall is becoming more extreme.


The following tables show the 2 to 5 year IDF trends for 5 to 10 minutes (first table), and 5 to 10 year trends for 1 hour and 2 hours (second table)

1990 (pre-version 1) IDF Dataset Worksheets have been prepared for Ontario stations:
Ontario Disk 1 Volume Tables :
Ontario Disk 2 Volume Tables :
Ontario Disk 3 Volume Tables :
Ontario Disk 4 Volume Tables :

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