More wildfires and changes in run-off a challenge for irrigation district

A lack of tree cover can boost run-off volumes but they come sooner and with much more sediment

Chris Williams (in blue cap) and Erin Cherlet are part of a team of researchers who have been measuring the impact of wildfires on run-off.
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As wildfire experts peer into the future, they see more — and more severe — forest fires because of a changing climate.

But some are also looking to the past and measuring the impact that forest fires have on the quantity and quality of run-off — two factors that could have major implications for Alberta’s irrigation districts.

“If you’ve got climate change broadening out the fire season and maybe melting your snowpacks a little earlier to begin with, and on top of it you’ve got a fire-affected landscape where it melts even sooner, you could have effects that are just exacerbated by one another,” said Chris Williams, a research hydrologist and PhD student with the Southern Rockies Watershed Project with the University of Alberta.

“It’s easy to see that the effects could make it as far as the irrigation providers, but the scales are large, so there’s a lot of variability there.”

Williams has been involved with a long-term study of the effects of the Lost Creek Fire in the Crowsnest Pass area in 2003.

“Every year since 2005, around the time of peak snow water equivalent, we’ve gone out and we’ve measured snow courses,” said Williams. “We set up five snow courses in a reference stand — a subalpine forest that was still healthy and then we set up five in a burned area. The big roll-up number from those 10 years of work is 78 per cent more snow water equivalent in the burn stand than in the reference stand.”

More water may sound like a good thing, but when that water runs off — and in what volume — are key factors, and the research is not reassuring on that score.

Snow water equivalent is the amount of water you’d be left with if you melted all the snow. The 78 per cent increase equates to 152 millimetres of more snow water equivalent in the burn compared to the healthy forest. (Each snow course is a 100-metre-long transect in the forest where depth measurements and snow core samples are taken.)

The main reason for the difference, is that a subalpine canopy (mostly fir and spruce with some lodgepole pine) is dense and a lot of snow lands on branches. Much of the intercepted snow is sublimated (that is, goes from snow to water vapour without melting) and so never makes it to the ground.

On the other hand, once fire destroys that subalpine canopy, a lot more snow comes to rest on the forest floor.

Then comes spring

Not surprisingly, snow melts faster when there aren’t any trees to shade it. As well, dark ash and char mixed in with the snow accelerates the melt even more, said Williams. In the April to October ice-free months from 2005 to 2014, researchers measured the average run-offs of three burned and two unburned watersheds in the Crowsnest Pass. The preliminary results show a clear difference.

“Fifty per cent of the water yield in the burned streams was arriving at our gauging stations by about June 10,” said Williams. “In the reference watersheds only about 20 per cent of the run-off was arriving by that date.

“So in the reference watersheds, it was taking an extra nine to 10 days to basically have an equivalent amount of water yield pass our gauging stations.”

Researchers have also taken thousands of water samples since 2004. A study encompassing 2004 to 2007 showed significant increases in sediment in burned catchments than in reference catchments.

“Total suspended sediment concentration and yields were eight times greater in the burn catchments than in the reference catchments,” said Williams.

These levels change throughout the year, with less sediment in the winter when flows are low, and more in spring or during storm floods. During a storm, sediment loads from the burned areas were up to 100 times larger.

This effect persists for a long time after a forest fire.

“We never really saw a lot of recovery,” said Williams. “We were still getting quite high sediment loads even after a decade of research there in the burned streams.”

What lies ahead

“Most of the literature is showing fairly steady increases in fires since the 1980s in terms of area burned and also the severity — the intensity of the wildfires,” said Williams.

In 2017, British Columbia set a new record for the total area burned by fire — breaking one that dated back to the 1950s. That record lasted just a year as a new one was set in 2018.

“If you put the two years together, more area was burned in B.C. in 2017 and 2018 than there was over the last 30 years combined,” said Williams. “It seems to be a problem that’s certainly not getting better.”

In 2017, the Kenow Wildfire burned more than 38,000 hec­tares, half of them in Waterton Lakes National Park. In that case, lakes and reservoirs buffered the St. Mary River Irrigation District from the effects of fire, said manager Terrence Lazarus. But it was a reminder that more wildfires are one of the challenges resulting from climate change, he added.

“The bigger picture for us is how to manage these changes because coupled with that is changes in the climate and changes in the seeding patterns and changes in when the crop grows in the ground, that sort of thing,” said Lazarus. “I think it just talks to adaptability. We’ve always had to be adaptable.”

The difference will be the scale and speed of change, he added.

“The fire’s one symptom of the disease,” said Lazarus. “Once you add it up, the adaptability piece is going to be huge as the basin characteristics change. I believe it’s all part of the same story about our changing climate. We are seeing the symptoms of that particular disease.

“Alberta’s always been unpredictable, but those unpredictable parameters are definitely widening.”

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