Extreme Rainfall Trends in Canada - Engineering Climate Datasets for Long Term Climate Stations Show Increases and Decreases

Environment and Climate Change Canada's Engineering Climate Datasets includes trends in observed annual maximum rainfall over durations of 5 minutes to 24 hours.  Version 3.00 of the data was released in early 2019.  The following table shows trend direction and significance for stations across Canada.  It represents 3993 station-years of data, with an average of 47 years of data at 85 stations.

Some observations:

- out of 85 stations with trends over 9 durations, 7.9% of trends are statistically significant increases

- 1.8% of trends are statistically significant decreases

- the total of significant increases and decreases (7.9+1.8=9.7%) is mostly explained by chance (5% could be explained by random chance, due to the natural variability of the data)

- there are more increases than decreases with the exception of Ontario where southern Ontario has more decreases than increases, while northern Ontario has more decreases

- southern Ontario has 50% more significant decreases than increases

- Alberta is almost even with increases and decreases, and has no statistically significant increases, and just one significant decrease

- statistically significant increases are more prevalent for long durations over 1 hour (10%), than for short durations of 1 hour or less (6.4%) .. so significant increases for short durations are slightly above the % explained by randomness in the natural variability, in contrast, long durations have more significant increases than would be expected by chance

- statistically significant decreases are more prevalent for short durations of 1 hour or less (2.1%), than for long durations of over 1 hour (1.5%)

A review of these trends based on earlier v2.30 datasets, specifically stations with 20 years of record between 1965 and 2005, was presented by Shephard et. al in Atmosphere-Ocean in 2014:

"Summary statistics in Table 6 show that for all durations fewer than 5.6% and 3.4% of the total number of stations have significant increasing and decreasing trends in the AMS amounts, respectively. The highest percentage of stations with significant trends from any duration is 7.8%
(5.6% + 2.2%) for the 24-hour duration, which is close to the nominal 5% significance level. Based on this IDF single station analysis, and the more general single station climate results from the 1965–2005 period presented in Section 4a, we conclude that the annual maximum short duration rainfall values across Canada typically do not show a significant trend. Thus, for most of the single station IDF stations across Canada there is no evidence indicating that the stationarity assumption used in the traditional national EC IDF calculations has been violated. These results are not unexpected given the typical high variability and relatively short time series of the extreme short-duration rainfall observations."

Therefore Environment and Climate Change Canada find 'no evidence' that data used in IDF calculations is changing (values are stationary), and significant trends are generally no more than the natural variability would suggest.