Understanding rainfall statistics

This article presents a step-wise approach to give design practitioners an idea of where figures like the 90th percentile event and concepts like 5 mm retention = 50% annual water balance target = 0.5 runoff coefficient come from.

The Ontario Ministry of the Environment Conservation and Parks, and other regulatory agencies perform much more rigorous statistical analyses than the example presented, and their resultant stormwater management targets vary accordingly.

Example: City of Barrie, using years 2015 to 2017

Daily rainfall records from BARRIE LANDFILL weather station were obtained for the years 2015 - 2017, from the Environment Canada historical data record.

The example will only consider rainstorm events, although the records also contain data for 'precipitation', which could be used instead if snowfall were also of interest to the analyst. (For example, if the analyst wished to calculate annual retention or runoff from a facility.)

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

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.

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.

Any form of rain or snow.