Precipitation Intensity duration Frequency Curves

What is an IDF curve?

An IDF curve is used to illustrate the relationship between intensity, duration, and frequency of rainfall events.  They are used for designing stormwater systems, predict flooding events, and predicting the ways in which rainfall patterns could change with climate change.  

How to interpret an IDF curve:

To read an IDF curve it is important to know what each of the axis and lines indicates.  The vertical or Y axes shows the intensity of a rainfall event.  In other words, how much rain falls within a given time frame usually one hour during a single rainfall event.  The horizontal or X axis shows the duration of the rainfall event, or how long it lasts.  Lastly, each line indicates a different return period or frequency.  This tells us how often a rainfall event of a given intensity and duration is expected to occur.   If a rainfall event has a return period of 5 years, that means that there is a 1/5 or 20% chance that a storm of that magnitude will happen in any given year.  

Climate Projections:

IDF curves are designed using historical rainfall patterns in a given region, and thus are unique to each area.  However, climate change is expected to cause major shifts in the precipitation patterns we observe.  Climate projections can be made and plotted as IDF curves to indicate what the potential impacts of changing rainfall patterns may be.  

Unfortunately due to limited weather stations there are no IDF projections for Salt Spring Island specifically.  Below are IDF curves for North Cowichan and Saturna weather stations based on projected climate data under two climate scenarios.  SSP 126 indicates the best case future conditions, and SSP 585 indicates the worst case scenario.  

As we can see, rainfall events are expected to increase in intensity with a changing climate.  This can lead to increased risk of flooding, damage to crops, vegetation, and infrastructure, as well as increased runoff and soil erosion.  It is important for communities to properly prepare for shifts in weather.  This can be done by ensuring effective stormwater drainage systems are in place, and mitigations against flooding are implemented.