The rate of evaporation at US reservoirs is speeding up, and could significantly increase by the middle of the century if emissions continue to rise

Climate change is increasing the rate of evaporation at reservoirs across the US. The effect is most consequential in the Southwest, where reservoir levels are already at record lows amid a megadrought and decades of overuse.

Huilin Gao at Texas A&M University and her colleagues modelled the effect of climate change on evaporation at US reservoirs. They looked at more than 670 reservoirs, representing almost 90 per cent of total US water storage.

Evaporation is driven by several factors, including sunlight, humidity, wind speed and heat, says Gao. Climate change can affect all these, though its effects on wind and cloud cover are more uncertain than its effects on temperature.

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The researchers found that from 1980 to 2019, an average of 38 billion cubic metres of water evaporated from reservoirs in the contiguous US each year. If emissions were to continue to rise through the end of the century – widely considered to be an unlikely worst-case scenario – losses from evaporation would increase by 25 million cubic metres each year until 2059.

In places with lots of water, such as the Pacific Northwest, the added losses would be unlikely to strain water supply, says Gao. The losses would make up a small fraction of all the water available and would be offset by increased precipitation.

But the increase is more concerning in drier places where evaporation plays a larger role, says Gao. In the lower Colorado river basin, for instance, current annual losses from evaporation are equivalent to 40 per cent of the total outflow of the system.

In the arid US Southwest, including the Rio Grande basin, the researchers found evaporation losses in the summer and fall increased while the amount of water flowing into the system decreased, suggesting pressures on water supply.

Under a scenario in which emissions peak around 2040, Gao says a similar increase in evaporation could be seen by the end of the century.

Kevin Wheeler at the University of Oxford says the increasing evaporation rates will exacerbate existing water shortages in the western US, where evaporation is already playing a central role. “It’s going to make the problem even worse, continuously,” he says.

There are methods to reduce evaporation from reservoirs, for instance by covering them with floating solar panels, says Sarah Porter at Arizona State University. But that can have unintended consequences for ecosystems and isn’t feasible for the giant reservoirs in the US west.

Journal reference: Earth’s FutureDOI: 10.1029/2022EF002961