Canada's renewable fresh water supply is shrinking, according to a new report which says the southern part of the country lost enough water to fill 1.4 million Olympic-sized swimming pools every year over the past three decades.

A Statistics Canada study of southern Canada's water yield – the amount of water that falls as rain, melts from snow and ice packs and flows through rivers and streams – found that it has declined 8.5 per cent since 1971.

That amounts to 3.5 cubic kilometres a year on average, the federal statistical agency said.

The Statistics Canada study looked at water yields between 1971 and 2004 across southern Canada, where 98 per cent of Canadians live.

Heather Dewar, an analyst for the agency, said water yield does not include large existing bodies of water like the Great Lakes.

"It's all the water that flows into the system," she said in an interview with

Gary Sprules, a biology professor at the University of Toronto who specializes in the ecology of lakes, said that decline represents a serious shortfall in our water supply.

"I think that's a significant loss," he said. "That's close to 10 per cent in a generation and a half and that's troublesome."

He said much of the loss is likely due to the global warming trend, which has changed spring runoff patterns across the continent. "Global climate change has led to changes in rainfall patterns in our part of the world and the amount and time of ice cover on our lakes and rivers."

Sprules said more research needs to be done to find out where the water is going and why it's disappearing from the system

Dewar said Statistics Canada did not track the reasons for the decline and was careful to note that the study was not a prediction of future water supplies.

"The water yield figures are quite variable and this is not necessarily a projection of what's going to happen in the future," she said.

But she said one thing that surprised researchers in the study was the wide variation between Canada's different regions.

The Pacific Coastal drainage region has the highest water yield in the country, followed by the Newfoundland and Labrador drainage region. Drainage regions in the Prairies and north of the Prairies produce the least water.

"We think of Canada as a water-rich nation," Dewar said. "But when you look at a region like the Prairies, it's almost as dry there as in Australia … and in the Pacific it's quite wet."

The Statistics Canada study did not offer any reasons for the drop. "There are a lot of factors that play into that decline," Dewar said.

But the agency did tally who has been using our water.

In 2005, Canadians withdrew about 42 cubic kilometres of water from the environment, more than 90 per cent of it to support economic activity. Only about 9 per cent was used directly by the residential sector.

The most water overall was used by power generation plants, mostly for cooling turbines that produce electricity. In 2005, the power generation sector used almost 28 cubic kilometres of water, most of which was returned to the system within a short period of time.

The manufacturing sector, used almost six cubic kilometres of water, or about 14 per cent of all water use and agriculture was responsible for nearly 5 per cent of total water withdrawals in 2005, mostly to irrigate crops or water livestock.

Home water taps accounted for 9 per cent of total water use, or about 3.8 cubic kilometres, in 2005.

Drinking water plants provided water to about 28 million Canadians in 2005.

Canada's total annual renewable freshwater supply is about 3,470 cubic kilometres, roughly equivalent to the volume of Lake Huron.