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 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) |
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) |
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