Impacts of heatwaves on river flows to change rapidly under climate change, new SFU research

Climate change will cause heatwaves to impact river flows later into autumn and earlier in spring, amplifying the role extreme heat will play in shaping Canadian water resources, according to new SFU research.

A pair of studies by researchers in Simon Fraser University's School of Environmental Science examine how climate change could alter the way Canadian rivers respond to extreme heat events.

In the first study of more than 860 river basins across Canada, SFU postdoctoral fellow, Sam Anderson, and environmental science professor, Shawn Chartrand, investigated how rivers across the country have responded to past multiday heat events — or heatwaves — to better understand how they may respond to future heatwaves.

“If heatwaves are able to more dramatically decrease snowpack in the late fall, winter and early spring,” says Anderson, “we will have less snow available to sustain rivers in the spring and summer when we really need it for our agriculture and water systems.”

Published recently in Earth’s Future, the study found that under expected future warming, the streamflow response to heatwaves will change more quickly relative to more moderate warm events. Importantly, the streamflow response to heatwaves in some regions being much more sensitive to future warming than others.

By studying approximately 40 years of streamflow and temperature data from basins across the country, the team determined that fall and spring heatwaves generally cause the most significant increase in river flow.

“Heatwaves are periods of anomalous warmth, relative to the time of year in which they occur. Whereas summer heatwaves are the hottest in terms of degrees Celsius, heatwaves outside of summer — particularly in spring and fall — can be the most consequential for river flows,” says Anderson.

Anderson explains that as future warming shrinks the window of sub-freezing temperatures that typically separate these two periods, heatwaves will have an amplified effect on river flows.

“For example, river flows respond to temperature fluctuations that drive snowmelt in spring, once the temperature fluctuations rise above freezing. Under a little bit of warming, this river flow sensitivity to temperature would begin earlier as average temperatures climb above freezing sooner — say by a week,” Anderson explains. “But temperature fluctuations that deviate from normal can cross that freezing threshold even earlier — by much more than a week. This is how climate change is amplifying the hydrological relevance of heatwaves, which are substantially warmer than normal conditions — by expanding the periods of the year for which heatwaves can drive melt and modify precipitation phase.”

According to the study, under warming temperatures, heatwaves in the west coast, southwestern prairie, southern Ontario and east coast regions will drive a greater impact to river flows compared to heatwaves in other regions. “This is where the impacts of heatwaves are really going to be amplified under a little bit of warming,” he says.

Anderson notes that this is based on the local climate of a region and how sensitive the sub-freezing window between fall and spring is to warming temperatures.

“This means that in these regions, future heatwaves will be more associated with high flow events at different times of the year than they have in the past, which we may not be prepared for,” says Anderson.

He adds that this can also have consequences for species like salmon that rely on having river flows that are both cool and high enough in the fall to complete their runs — both qualities that are threatened by a declining snowpack.

In another recent study published in Environmental Research Letters, Anderson and Chartrand also looked at how river responses to heatwaves have varied through time to better understand what changes can be anticipated in the future. By investigating the responses of six basins in western Canada to warming events over approximately 80 years, they quantified how river responses to heatwaves varied across years with different climate characteristics.

The study demonstrated that river responses to heatwaves may be enhanced in early spring, but the impacts of late spring events, which often raise concerns of flooding due to increased snow melt, aren't what we would necessarily expect.

“In years with less snow and warmer temperatures — which we expect to see more of in the future — the river flow sensitivity to heatwaves in late spring is actually suppressed,” Anderson explains.  

“Much of the snow that otherwise would have melted out during a given heatwave in late spring, would have already melted earlier in the season — we’re shifting into a mode where more water is released from storage in winter and early spring. So, while heatwaves can do more in winter and early spring, they can do less in late spring. This is really important considering late spring flooding concerns, especially as higher spring flows occur earlier in the year under climate change. On the flip side, however, these types of heatwaves will extend the low flow season in summer and fall,” says Anderson.

"Taken together, these two studies give us a better understanding of the role that heatwaves play in shaping river flows and water availability, both in space across Canada, and through time over the last century,” says Anderson. “When it comes to heatwaves, a little bit of warming can go a long way in reshaping how they impact rivers nationwide. This will have important implications for all downstream water users and ecosystems as heatwaves become more frequent and more intense — from conservation to agriculture to municipal water supplies."