|Toronto: temperature up - rainfall down|
The tables at right show intensities for "Toronto City" Climate Station ID 6158355 (aka "Toronto" 6158350) for three recent Environment Canada IDF curve updates:
- Up to 1990 (47 years of record)
- Up to 2003 (57 years of record)
- Up to 2007 (61 years of record)
Design rainfall intensities are shown for short durations of 5 minutes, 15 minutes and 1 hour in the three tables.
- Up to 1990 (38 years of record)
- Up to 2003 (51 years of record)
- Up to 2007 (54 years of record)
- Up to 2013 (60 years of record)
What do the Toronto and Mississauga "Toronto Pearson" IDF trends mean for drainage design? Hopefully nothing. Although downtown Toronto trends have been steadily decreasing, the most recent 2013 storm is not included and could moderate the downward trend in severe rainfall statistics. Although Mississauga Pearson has some increasing intensities, this is skewed by the timing of the most recent record and absence of even more recent below average data points. And although some Mississauga intensities increase (above 5 year, 1 hour data), short duration values remain below downtown Toronto values in an absolute sense, meaning the Mississauga values are just 'catching up' to the historically higher Toronto ones. To illustrate this, up to 1990 the 5 minute Toronto 100 year design intensity was 21% above the corresponding Pearson value. In 2007, that Toronto statistic was still 20% higher.
“It's tough to make predictions, especially about the future.” ― Yogi Berra
A comparison of IDF values in southern Ontario between the pre-version 1 1990 datasets and the current version 2.3 datasets shows an overall decrease in frequent rainfall and essentially no change in infrequent rainfall - here are a few results from that post:
IDF values should increase over time as sample bias is reduced with longer data sets that better characterize extreme events in the population. Samples (aka observations) from skewed populations like rainfall require long records with many observations for these extremes to be reflected in the IDF statistics - a previous post explores this in context of the variability we see in rainfall observations. The IDF review above considers climate station records with 30 years of observations or more in which sample bias is expected to be low (see Toronto example in previous post).