Previous posts have reviewed changes in design rainfall intensities, that is, the statistics used to define IDF (intensity-duration-frequency) curves (https://www.cityfloodmap.com/2020/07/how-have-rainfall-intensities-changed.html). These rainfall characteristics are the most commonly applied data in urban storm drainage system design, and are often directly related to the peak runoff rates accommodated in flow conveyance infrastructure systems. These data are also used to derive hyetographs, temporal rainfall "storm" patterns or event time series, used in hydrologic analysis of urban collection systems (i.e., storm, partially-separated sanitary or combined), as well as urban and rural catchments.
The most recent update of Canadian climate station data includes "IDF Files" in the Engineering Climate Datasets. Probability distributions are fit to the observed annual maximum intensities to estimate design intensities at various 'return periods', i.e., event probabilities. Low return periods characterize frequent events, such as 2-year storms that have a 1/2 or 50% annual exceedance probability. Meanwhile high return period characterize rare events, such as 100-year storms that only have a 1/100 or 1% probability.
The most recent update to Environment and Climate Change Canada's IDF Files adds 8.7 years, on average, to climate station records that exist in both the earlier V2.00 and the newest V3.10 sets - here is a link to those IDF Files: https://climate.weather.gc.ca/prods_servs/engineering_e.html. As the V2.00 data had a 29.1 year average record length, the longer V3.10 records with an average of 37.8 years of data are 23% longer. This improves the reliability of the derived statistics, allowing higher return period event intensities to be estimated. A rule of thumb is that return periods that are double the record length can be reliably estimated - so 38 years of data can be used to estimate 76 year return period intensities with good confidence.
The following chart illustrates the change in median design rainfall intensity from the earlier V2.00 to the most recently updated V3.10 datasets.
|Design Rainfall IDF Trends Canada - Environment and Climate Change Canada IDF Files / Engineering Climate Datasets|
Small frequent rain intensities, the 2 year rates observed in an average year, and represented by the green markets, have increased by almost up to 1% as a result of the longer records being added. The less frequent 5 year and 10 year intensities, represented by purple and blue markers respectively, are generally unchanged overall. The rare 50 and 100 year intensities, represented by the orange and red markers, have decreased the most, although the absolute change is limited.
The following table shows the % change in intensities from V2.00 to V3.10.
|Rainfall intensity changes in Canada at long-term stations (median IDF curve intensity trends) .. note '20 Year' should be '25 Year'|
|Shifts in rainfall statistics with new longer periods of record (observations)|
|Rainfall intensity shifts in Canada - new observation comparison with earlier Environment and Climate Change Canada Engineering Climate Datasets|
While individual stations will vary, the above table shows that on average the rare 50 to 100 year rainfall intensities in the almost 9 years of new data, added between the V2.00 and V3.10 datasets, are 2.4% to 2.2% lower than the earlier V2.00 values. These changes are over 4 times greater than the shift in V2.00 to V3.10 values. In contrast the 2 year intensities are 2.6% higher in the new data relative to the V2.00 data. Again this is over 4 times the V2.00 to V3.10 shift in the previous table.