Intensity-Duration-Frequency Curves

Intensity-Duration-Frequency Curves

Reading IDF Curves: A precipitation event with a duration of 12 minutes, and intensity of 5 inches/hour is predicted to occur once every 5 years (Bentley, N.D)

IDF curves may shift under future climate scenarios. In this example, the future 50-year storm is predicted to have a higher rate of rainfall than historical storms of the same return period. LID practices should be designed to accommodate this shift. Image credit: ECCC (2025)

Intensity–Duration–Frequency (IDF) curves are a standard tool in hydrology that describe the statistical relationship between rainfall intensity, storm duration, and frequency of occurrence (return period) to determine the likelihood of extreme rainfall events (Martel et al., 2021).

• Intensity: the rate of rainfall (mm/hr)
• Duration: the length of the rainfall event (minutes to days)
• Frequency: how often a given storm is expected to occur (e.g., a 10-year storm has a 10% chance of occurring in any given year)

IDF curves are developed from long-term rainfall records and models and are widely used to estimate design storms for stormwater management. IDF curves are crucial for sizing LID BMPs because they:

1. Define design storms: LIDs must be able to manage runoff from specific return-period events (e.g., 2-year or 10-year storms). IDF curves provide the rainfall intensity and depth to use for those design events.
2. Determine runoff volumes: By applying rainfall depth (from IDF data) to a given catchment, engineers estimate the stormwater volume that LIDs need to capture, infiltrate, or detain (USDA, 2021).
3. Assess peak flows: LID facilities are often designed to reduce peak discharge. IDF curves help simulate storm hydrographs and assess how much peak flow reduction is required (City of Toronto, 2006) .
4. Account for climate change: Historical data is no longer an accurate predictor of future extreme rainfall (CSA, 2025). Since IDF curves are based on historical data, climate change-scaled IDFs can be used to design LIDs that are resilient under future rainfall conditions (ECCC, 2025).

Any form of rain or snow.

The average time between years when an precipitation event of specified magnitude is met or exceeded.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

The rate of rainfall in millimeters per hour.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

The greatest volume of stream flow occurring during a storm event.

the maximum flow rate occurring during a flood event measured at a given point in a river, street, or pipe system.

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The greatest volume of stream flow occurring during a storm event.