Annual Maximum Rainfall Trends in Canada - Environment and Climate Change Canada's Updated V3.20 Datasets Show Few Significant Trends

Previous posts have presented overall trends in annual maximum observed rainfall amounts at Canadian climate stations (link: https://www.cityfloodmap.com/2020/12/design-rainfall-trends-in-canada.html).

Environment and Climate Change Canada (ECCC) periodically updates trend analysis on the annual maximum series (AMS) for each station, and these series are used to derive design intensities, i.e., Intensity-Duration-Frequency curves. Data area available here (link):  https://climate.weather.gc.ca/prods_servs/engineering_e.html

In May 2021 additional analysis has been included in these Engineering Climate Datasets. The "Whats_New_EC_IDF_v3-20.pdf" file includes a summary table that provides counts of stations and additional station years added with recent update:

The v-3.20 update adds 490 station-years of data. There are currently 17,133 station-years of data. The average length of record is 25.3 years. This represents a slight decrease in record length compared to 25.5 years in v-3.10, as new short record stations are factored in.

Trends in annual maximum rainfall have not change all relative to the 2020 v-3.10 data. The majority of station data show no statistically significant trend. The table below compares trends in earlier datasets, averaged across all durations. 

Trend in Maximum Rain    v3.20       v3.10       v3.00         v2.30

Significant Increase              4.16%     4.28%       4.18%        4.09%

Significant Decrease             2.25%     2.24%       2.33%        2.30%

No Significant Trend          85.73%    85.80%     85.55%      86.37%

No Calculation                      7.86%      7.68%       7.94%        7.24%

 

The v-3.20 datasets  have the following trends within various durations:

Excluding 'No Calculation" data, annual maximum observations with 'No Significant Trend' represents 92-94% of series, with an average of 93.0%. Excluding the 'No Calc' data results in an average of 4.5% significant increases and 2.5% significant decreases.

The following chart shows that majority of station data show no statistically significant trend.

There has not been any appreciable change in the annual maximum rainfall trends considering earlier datasets (see v-2.30 to v-3.00 update https://www.cityfloodmap.com/2020/02/annual-maximum-rainfall-trends-in.html)

In Canada, the number of stations with annual series used to derive extreme rainfall statistics continues to increase. A previous post explored how manually-operated climate stations have been declining while this increase occurs (link: https://www.cityfloodmap.com/2020/06/do-we-have-enough-climate-stations-in.html). In 1990 there were only 11,268 station-years of data. Today with 17,133 station-years of data we have 52% more information to guide assessments of extreme rainfall.  

This table summarizes the rise in climate station count and rise in station-years as new data is added.

Some have confused the decline in manual stations with a decline in overall data (see the above post). This chart shows the rise in climate stations with IDF data used for engineering design (orange lines), along with the decline in manual station data (blue bars).

Are More 100 Year Storm Happening? Yes and No. A Proliferation of Rain Gauges Can Now Record More 100 Year Storms, But Fixed Locations Show No Increase

There are many sensational media stories about ghost storms and ninja storms hitting urban areas, and a steady claim that we are experiencing more extreme rainfall, that is, higher intensities for a given probability (called return period), or greater frequency of given design intensities. Often it is stated that we are experiencing more 100 year storms today and that is a "new normal" brought on by a changing climate.

How does the number of climate stations, or rain gauges, that are in operation affect the number of observed extreme events. Well, let's look at Toronto for example.  Several past extreme events were reported in the Staff Report on Impact of July 8, 2013 storm on the City's Sewer and Stormwater Systems dated September 6, 2016: (https://www.toronto.ca/legdocs/mmis/2013/pw/bgrd/backgroundfile-61363.pdf)

During the May 12, 2000 extreme rainfall event, Toronto operated 16 rain gauges as shown on the staff report map below.

Fifteen years later, during the August 19, 2005 storm, the City operated 31 rain gauges as shown below, so almost double the number of rain gauges.  Look at the higher density of gauges in north Toronto where many higher August 19, 2005 rainfall depths were observed.

Then 8 years later, during the July 8, 2013 storm the city operated even more rain gauges, i.e., 35 in total.

And then a few years later, on August 7, 2018, the city operated 43 rain gauges - even more than 2013. I don't have a map but here is a super-cool graph summarizing Toronto Open Data rainfall totals at those gauges over a period of 5 minutes to 24 hours.

And now today as of July 17, 2019, Toronto has 45 active rain gauges as shown in the following map presented to the Ministry of Environment Conservation and Parks' stormwater stakeholder group participating in development of minimum standards for ECA pre-approval.

So let us summarize the trend in the number of rain gauges in the chart below.

Astute blog readers will notice that the number of rain gauges has increased almost 300% since the year 2000. Yes, almost three times the number of rain gauges now. Obviously, more extreme events can be observed and recorded when the number of rain gauges increases dramatically.

The following table shows that in the year 2000, there was a rain gauge every 39.4 square kilometres (16 gauges per 630.2 square kilometres). By 2019, there is a gauge ever 14 square kilometres.

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So what is happening at fixed locations where rain intensities are measured? In Toronto and Mississauga, many trends are downward according to the Engineering Climate Datasets:

 

As a result, design intensities for short durations have been decreasing since 1990:

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To recap, many more rain gauges today mean we 'see' more storms - these are typically needed to support basement flooding Municipal Class EA studies (rainfall needed to calibrate hydrologic and hydraulic simulation models), to guide operational activities too.  Many municipalities have installed rain gauges to support inflow and infiltration management programs.

We have a "finer mesh net" to catch these events and add them to our records - we have almost 3 times more rain gauges in Toronto since 2000.

But no. Storm are not becoming more intense. If we see more of them, it is because we are looking harder for them with more extensive monitoring efforts. Given this expanding intensive network of rain gauges today, it is not uncommon, statistically speaking, to observe many 100-year storms over a short time period.  This earlier post explores those statistics in the GTA - https://www.cityfloodmap.com/2019/03/are-six-100-year-storms-across-gta-rare.html.