Forest fires hurt prairie water flow and quality

In an average year, Alberta combats about 1,500 wildfires but most are small and cause few problems to the landscape.

However three-tenths of one percent of Alberta wildfires account for 98 percent of the total area burned and cause considerable and lasting damage to watersheds.

The nature of these mega fires seemed to change around 1999-2000, said researcher Uldis Silins of the University of Alberta, part of a pan-Canadian team investigating the after effects of fire on the environment for the last 15 years.

“We can’t take any single fire and relate it to climate change but we have seen changes since the late 1990s,” he said at the Alberta Soil Science Workshop held in Calgary Feb. 19-21.

Severe fires have been fought in western North America, Australia and Europe. American data showed since 2000, the cost to fight fires is exceeding government budgets.

In British Columbia, the fires of 2017 and 2018 broke long-standing records for the largest area burned.

“Those two years burned more forest in British Columbia than the past 30 years combined. That was four percent of the forested area of British Columbia,” he said.

Most of Alberta’s water supply is generated in the mountains so the effects on hydrology are significant, said Chris Williams, who is part of the Southern Rockies Watershed Project.

Researchers want to know how long the effects may last in terms of water flow, quality and general stream health.

They are finding different effects depending on local geology, said Silins.

They have found persistent effects on sediment production for more than a decade in their study areas in southwestern Alberta.

U.S. researchers have seen sediment clear faster, but the sedimentary geology and glacial history are different on the Canadian side of the border. Marine shale, mudstone and sandstone break down into finer particles compared to other bedrocks. B.C. has not seen fine sediment accumulations because there is more metamorphic rock.

Mountain snowfall is important for the Canadian prairie water supply.

Forests take up vast amounts of water. When a fire sweeps through a forested area, subsequent snowpacks are deeper. There has been an 80 percent increase in snow water equivalent over the last 10 years.

During summer, more rain water hits the ground because the tree canopy is gone. Those two events add up to a 50 percent increase in net precipitation in the last decade for these areas.

A change in hydrology was expected with higher peak flows due to that added accumulation of water.

“We have seen absolutely no discernible effect,” he said.

The only difference is the spring melt occurs sooner because the snow is exposed to more sunlight and starts to flow about 10 days earlier. The quantity is unchanged.

Fire destroys riparian areas but there is little change in water temperature because it was regulated by cold groundwater welling up into the streams.

They have also measured nitrogen and phosphorous levels after fires.

Stream nitrates were present but concentrations in spring started to drop during the lowest flows of mid-summer. Phosphorus was present and is easily transported with sediment. It has caused more eutrophication and more algae growth in water courses after a fire.

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