From the Summary:
"An evaluation by the Aquatic Studies Branch of the site of a newly constructed stormwater collection system located on Highway 97 near Kelowna and discharging into Okanagan Lake was requested by the Waste Management Branch, Okanagan Region. The evaluation indicated that average monthly precipitation for the Kelowna area is about 28.5 cm. Over the 6.6 hectare catchment area, a flow of about 1900 cubic metres per month could be generated from this precipitation."

See the Evaluation [PDF].