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Intensity Duration Frequency (IDF) curves relate short-duration rainfall intensity with its frequency of occurrence and are often used for flood forecasting and urban drainage design.

As most people have experienced, a very intense storm can bring lots of rain over a very short period of time, overwhelming storm drains, flooding basements, and washing out bridges and roads. Therefore, when engineers and hydrologists are designing safe and reliable infrastructure, they need to know how often these damaging rainstorms occur.

One way to determine this is to use Historical Rainfall Intensity-Duration-Frequency (IDF) Curves. Historical IDF Curves show the frequency (probability of occurrence) of extreme rainfall rates persisting for short periods of time (5, 10, 15, 30, 60 min; 2, 6, 12, and 24 h) based on historical rainfall data recorded at a specific weather station. They do not reflect rainfall amounts or rates over wider areas such as entire cities or river basins. The curves are produced to support the development of infrastructure capable of handling extreme rainfall events thereby reducing the risk of flooding and the associated damage to property, and risk to people. IDF Curves are most often used by professional groups such as: engineers, water resource managers, urban and regional planners.

Use with caution: Historical IDF Curves alone cannot be used to assess future extreme rainfall. Since IDF Curves are based on the analysis of historical rate-of-rainfall data, they do not explicitly incorporate any projected future trends due to a changing climate. 

Due to the sparsity of sub-daily hourly or minute-by-minute rainfall observations and the high levels of uncertainty associated with future projections of extreme rainfall at a specific location and over short time periods high spatial and temporal resolution, Environment and Climate Change Canada does not produce future IDF curves.

For further technical information on how IDF Curves are produced, please refer to Environment and Climate Change Canada’s Engineering Climate Datasets page or contact the Engineering Climate Services Unit at [email protected].

Climate Normals 1981-2010 are used to summarize or describe the average climatic conditions of a particular location. At the completion of each decade, Environment and Climate Change Canada updates its climate normals for as many locations and as many climatic characteristics as possible. The climate normals offered here are based on Canadian climate stations with at least 15 years of data between 1981 to 2010.