Introduction to this Guideline

Canada has significant investments in stormwater, drainage, wastewater, and flood management systems. Every day, Canadians rely on this infrastructure to protect lives, property, and natural systems such as creeks, rivers, and lakes. In designing and managing these works, practicing professionals need to be concerned with the probability of occurrence of extreme values of rainfall amounts, often for specific storm durations. Rainfall IDF information commonly forms a critical input when applying the analytical techniques routinely used by practitioners.

IDF information is meant to describe the frequency (in terms of probability of occurrence) of extreme rainfall events of various rates and durations. The demand for rainfall IDF information has increased across Canada over recent years for a number of reasons. First, as the spatial heterogeneity of extreme rainfall patterns becomes better understood and documented, a stronger case is made for the value of "locally relevant" IDF information. Second, Canada continues to become increasingly urbanized. As urban areas expand, making watersheds generally less permeable to rainfall and run-off, many older water systems fall increasingly into deficit, failing to deliver the services for which they were designed. Understanding the full magnitude of this deficit requires information on the maximum inputs (extreme rainfall events) with which drainage works must contend. Finally, research now indicates that climate change will likely result in an increase in the intensity and frequency of extreme precipitation events in most regions in the future. As a result, IDF values will optimally need to be updated more frequently than in the past and climate change scenarios might eventually be drawn upon in order to inform IDF calculations.

Meanwhile, new approaches are emerging for enhancing knowledge of rainfall IDF characteristics. For example, radar imagery patterns are being investigated to retroactively "fill in the (data) gaps" for regions where conventional rainfall measurement instrumentation, and hence IDF data are missing. New approaches are also being considered for the derivation of regional, rather than single point, IDF curves.