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[[File:Lake_Ontario_2012.png|thumb|Six notable extreme rainfall events have occurred within the past thirteen years in the GTHA, resulting in damages due to flooding. This figure shows a notable extreme rainfall “near-miss” event, labelled “Lake Ontario 2012”.]]
[[File:Radar_tracking_August_2012.png|thumb|Radar tracking of the August 10, 2012 extreme rainfall event. The Lake Ontario nearshore experienced sustained intensities approaching 200 mm/hr, while the southern portion of Peel Region had no measurable precipitation. (Source: Risk Sciences International)]]
[[File:Lake_Ontario_Drought_2007.png|thumb|Drought conditions at Island Lake in the summer of 2007]]
Since 1995, Ontario has had a weather-related state of emergency almost every single year <ref>Swiss Re (in collaboration with Institute for Catastrophic Loss Reduction) (2010). Making Flood Insurable for Canadian Homeowners. Available at URL: http://www.iclr.org/images/Making_Flood_Insurable_for_Canada.pdf</ref>. The City of Windsor saw extreme events that caused severe flooding in 2007, 2010, 2016 and 2017 <ref>City of Windsor. 2012. Climate Change Adaptation Plan. Available at URL: http://www.citywindsor.ca/residents/environment/environmental-master-plan/documents/windsor%20climate%20change%20adaptation%20plan.pdf</ref>. 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 <ref>Environment Canada. 2014. Climate. Available at URL: http://climate.weather.gc.ca/</ref>. 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 stormsewer networks efficiently convey stormwater runoff volumes from a large contributing upland area to a single outlet location, such as a stormsewer outfall in a river or stream.