Climate Trends in Ontario

Climate-related impacts

Since 1995, Ontario has had a weather-related state of emergency almost every single year . The City of Windsor saw extreme events that caused severe flooding in 2007, 2010, 2016 and 2017 . The Ottawa region experienced one extreme event every year for five years, and in the Greater Toronto Area (GTA), there have been four extreme rainfall events in the past ten years . Such high intensity events produce heavy rainfall in relatively short periods of time. While it is reasonable to expect runoff to be produced under such conditions – particularly when rain falls which exceeds a soil’s hydraulic conductivity - the production of stormwater is exacerbated in urban areas where the overwhelming majority of surfaces are impervious. The problems associated with managing stormwater volumes are exacerbated when dense storm sewer networks efficiently convey stormwater runoff volumes from a large contributing upland area to a single outlet location, such as a storm-sewer outfall in a river or stream.

In July 2013, the GTA experienced its most severe storm event in 60 years. Nearly five inches (126 mm) of rain fell in a two-hour period. In comparison, during Hurricane Hazel (a devastating event in 1954 where 81 lives were lost), the two-hour maximum precipitation was 91 mm and the total amount of rainfall was 285 mm over nearly two days . Conventional municipal drainage systems could not carry stormwater away fast enough. Roads and highways were overcome with floodwater closing major transportation corridors including Highway 427. GO Train passengers were stranded, and power outages and basement flooding were widespread with property damage of more than $1 billion.

It is nearly impossible to ascribe the cause of a single event to the broader issue of climate change, the trend is clear: an increasing number of high-intensity, short-duration (HISD) events are impacting our urban areas, exacerbating the stresses on overtaxed stormwater infrastructure. The figure highlights a series of seven recent extreme rainfall events which have struck the Greater Toronto and Hamilton Area (GTHA). On August 10, 2012, a large storm tracked across Lake Ontario parallel to the Canadian shoreline. Situated only 15 km southeast of Mississauga, this event lasted 6.5 hours and had estimated sustained intensities of 150 - 200 mm/hr. While the impacts of extreme rainfall events on urban areas cannot be ignored, the increasingly prolonged, dry inter-event periods necessitate that stormwater infiltration and percolation be maximized in order sustain base flows in support of aquatic ecosystems.

Urban flooding and extreme rainfall garner the most attention in discussions pertaining to stormwater management, it is crucial that consideration also be given to the management of our water cycle during dry periods as well. Collectively, we need to be able to manage extreme rainfall events such as the July 8, 2013 storm, combined rain and snow events such as that which caused the Bow River flood in Calgary in 2013, and extended periods of drought as occurred in southern Ontario in 2007. Drought preparedness is required if we are to sustain riverine baseflows, ensure the security of drinking water resources and optimize both water and waste water infrastructure.

As municipalities grapple with these new climate realities and their associated costs, they are rethinking how to manage stormwater using a variety of innovative solutions. The figure illustrates what has already happened in Ontario under conditions of prolonged drought. The Island Lake Reservoir, located near the Town of Orangeville, saw significant drawdown during the summer of 2007 after a period of prolonged drought. Reliant on groundwater for its municipal supply, continued pumping by the Town led to a significant drawdown within the reservoir. This was problematic not just for the ecosystem of the Lake, but for the downstream wastewater treatment plan as well, which relies on discharges from the reservoir in order to ensure that treated effluent can safely be assimilated by the receiving watercourse.

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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.

A parameter that describes the capability of a medium to transmit water.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The point, location, or structure where wastewater or drainage discharges from a sewer pipe, ditch or other conveyance to a receiving body of water.

Any form of rain or snow.

The continuous movement of water from the oceans to the atmosphere (by evaporation), from the atmosphere to the land by condensation and precipitation, and from the land back to the sea (via groundwater and stream flow); also referred to as hydrologic cycle.

A biological community, including humans and their natural environment.

(a) A natural well-defined channel produced wholly or in part by a definite flow of water and through which water flows continuously or intermittently. Also, a ditch, canal, aqueduct, or other artificial channel for the conveyance of water to or away from a given place, as for the draining of a swamp.

(b) A stream or current of water. Legally, a natural stream arising in a given drainage basin but not wholly dependent for its flow on surface drainage in its immediate area, flowing in a channel with a well-defined bed between visible banks or through a definite depression (as a ravine or swamp) in the surrounding land, having a definite and permanent periodic supply of water (the stream may be intermittent), and usually, but not necessarily having a perceptible current in a particular direction and discharging at affixed point into another body of water.

(c) A legal right permitting the use of a flow of a stream (especially of one flowing through one’s land) or the receipt of water discharged upon land belonging to another.

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.