River flow responses to heat waves may change more rapidly under climate change, research finds

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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 heat waves—to better understand how they may respond to future heat waves.

"If heat waves 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 heat waves will change more quickly relative to more moderate warm events. Importantly, the streamflow response to heat waves in some regions will be 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 heat waves generally cause the most significant increase in river flow.

"Heat waves are periods of anomalous warmth, relative to the time of year in which they occur. Whereas summer heat waves are the hottest in terms of degrees Celsius, heat waves 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, heat waves 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 heat waves, which are substantially warmer than normal conditions—by expanding the periods of the year for which heat waves can drive melt and modify the precipitation phase."

According to the study, under warming temperatures, heat waves on the west coast, southwestern prairie, southern Ontario and east coast regions will have a greater impact on river flows compared to heat waves in other regions. "This is where the impacts of heat waves 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 heat waves 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 heat waves 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 heat waves varied across years with different climate characteristics.

The study demonstrated that river responses to heat waves 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 heat waves in late spring is actually suppressed," Anderson explains.

"Much of the snow that otherwise would have melted out during a given heat wave 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 heat waves 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 heat waves 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 heat waves play in shaping river flows and water availability, both in space across Canada, and through time over the last century," Anderson adds.

"When it comes to heat waves, 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 heat waves become more frequent and more intense—from conservation to agriculture to municipal water supplies."

More information: Sam Anderson et al, The Streamflow Response to Multi‐Day Warm Anomaly Events: Sensitivity to Future Warming and Spatiotemporal Variability by Event Magnitude, Earth's Future (2024). DOI: 10.1029/2024EF004962

Sam Anderson et al, A century of variability of heatwave-driven streamflow in melt-driven basins and implications under climate change, Environmental Research Letters (2024). DOI: 10.1088/1748-9326/ad7ede

Journal information: Environmental Research Letters , Earth's Future

Provided by Simon Fraser University