Risks may be high with Alberta’s snowpack


Prairie watersheds depend on mountain snow for their summer water supply, but rapid melting can be devastating

As scientists and provincial officials closely monitor the interaction between precipitation and snowmelt in Alberta’s Rocky Mountains in June, the shadow of the 2013 flooding looms over their calculations.

The frequency of floods like the one that hit Calgary and the region since June 20 eight years ago could be as low as every 60 or even 40 years, said John Pomeroy, professor and director of the Center for Hydrology of the ‘University of Saskatchewan. .

“The snowpack of the mountains is a bit like having extra reservoirs because it traps winter precipitation and releases it in May, June and even July, which is great for irrigation,” he said.

“But the risk is that as long as he stays up there, it’s still possible to have a big hot rain event and then (the combination) can cause flooding, so we have to live with the benefits and the dangers, I suppose. “

It’s impossible to predict more than seven to 10 days in advance if there will be an extreme rain event, “so really if it’s in the maps for us I think we just don’t know. until we get closer, ”said Clayton Weiss, Bow’s operations and infrastructure manager at Alberta Environment and Parks.

“We know that we will calculate the statistics and the probability that these things will happen again, but in any given year, it is not possible to predict these phenomena.”

However, for the reservoirs it manages, “the image of storage is very good throughout the Bow Basin …

The Alberta side of the Rockies, also known as the Eastern Slopes, contains the upper reaches of the southern and northern Saskatchewan river systems, which provide water to much of the Prairie provinces.

“In the Prairies, it looks like an extreme drought from Manitoba to Alberta,” Pomeroy said in an interview on May 28.

“It has eased a bit now because of the recent rains, but it’s not completely eased, and that’s why I’m saying the demand for irrigation water in southern Alberta is likely to stay. higher than normal. “

There are usually “a few large cold low-pressure systems in the spring that dump more precipitation into the foothills, the Rockies, and some of the high-elevation plains,” he said.

“And so we had one recently in the area, but we would definitely need a few more to keep the soil moisture conditions in the irrigated areas in good condition, and also to keep the yards. water, so (we) expect a few of those to come in the form of precipitation, but a lot of it even in early June as snowfall at the higher elevations.

Irrigation in Alberta is primarily located in the Oldman and Bow River systems, which join north of Grassy Lake, Alberta to form the South Saskatchewan River.

The snowpack in the Oldman watershed has been “very variable, and some measurement locations are well below normal,” Pomeroy said.

Automated wireless devices called snow cushions show the snowpack ranged from much below average to above average as of May 26, said Nathalie Brunet, a river forecasting engineer at Alberta Environment and Parks.

Meanwhile, the Bow River basin generally ranged from medium to above average, similar to the snowpack further north in the Red Deer and North Saskatchewan river basins, she said.

However, runoff from these rivers from March to April, including the Oldman, was generally below average overall, with the Bow River being below average to well below average, she said. . The cooler weather delayed the snowmelt for a few weeks, Weiss said.

The positive side of the persistent snowpack in the Rockies is that it provides water for irrigation at a time when farmers need it for crops later in June, Pomeroy said. However, the longer the situation persists, the greater the danger that a significant rainfall event could cause flooding caused by rain-on-snowfall also increases, he added.

Although the snowpack wasn’t particularly thick in 2013, it melted slowly, meaning it was still there when hit by sustained precipitation, Pomeroy said. “And then the rains melted it quickly, then the rains themselves caused flooding and added up, it was a bad combination.”

Flows of the Bow River peaked at 2,400 cubic meters per second, about eight times its regular flow, according to a statement from the City of Calgary. Those from the Elbow River, which joins the Bow River in downtown Calgary, were 1,240 cubic meters per second, 12 times the typical rate.

Such flooding caused up to $ 6 billion in damage in southern Alberta, killing five and forcing the evacuation of approximately 80,000 people. States of emergency have been declared in dozens of communities, with cities like Canmore and High River among the hardest hit.

Several estimates have been given on the frequency of such flooding on the Bow River, Weiss said. “There are different ways of calculating it and different numbers going around, but about you’d say one in 200 years.”

However, the frequency can be considerably lower, said Pomeroy, who is also director of the Global Water Futures program.

The estimate of one in 60 or even 40 years is based on research into peak flows using early records kept by authorities such as the North West Mounted Police, now the RCMP, and the Geological Survey of Canada, he said.

The 2013 flood would not have surprised the early settlers, who suffered several flooding on the Bow River caused by heavy rains combined with the snowpack, he said. “They had a really bad flood streak from the 1880s to the 1930s.”

Things were relatively calm until June 2005. Although spring conditions were drier than normal, three major storms about a week apart caused flood damage in southern Alberta, with one in ten buildings affected in Calgary, according to a federal press release.

Scientists don’t know why the incidence of flooding has declined on the Bow River for decades, Pomeroy said. To further complicate the picture, little is known about what happened before 1885, he added.

As researchers attempt to estimate what the future holds based on their limited knowledge of the past, climate change adds important genericness.

Computer simulations based on current warming trends predict that the snowpack is unlikely to persist into June by the end of this century, Pomeroy said. “Rain on the snow part (will be) less likely in June, but you may have heavy rains in April or May that could generate similar flooding. “

The way precipitation occurs is also different, with the incidence of multi-day torrential rains increasing by 50 percent since the 1950s across much of the Prairies, he said.

“Now the precipitation is aggregating more than it would have been 50 to 80 years ago, and so we tend to have torrential rains over several days, whereas in the past we had torrential rains. of a day and a few dry days and another rainstorm. And those multiple torrential rains are the ones that can cause problems, as Calgary’s flooding in 2013 was caused by four days of heavy rain.

Shifting climate targets will affect the functioning of irrigation systems, even if they are not directly affected by river flooding, Pomeroy said. “They rely on reliable snow packs, and those will definitely decrease dramatically, as do glaciers. “

He said it’s hard to imagine the snowpack will completely disappear, but winters will likely be much shorter.

“I think the concern is more the longer summer period and if there will be enough rain to keep things green during this summer period then also the intensity of that precipitation and the length of the drought spells, and this is largely still very uncertain.

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