Forest-thinning simulations reveal benefits to water supplies

Forest thinning increases water supplies downstream while reducing wildfire risk, according to a study conducted by Salt River Project and Arizona State University. 

Land managers and scientists knew forest thinning — a technique that clears smaller trees and vegetation to reduce fuel loads in forests — decreases wildfire hazard, but wanted to quantify how restoration projects also benefit watersheds. 

SRP and ASU created a virtual model of a 3,400-acre area of the Kaibab National Forest in northern Arizona and simulated forest thinning. They concluded that such work in the forest would generate 230 acre-feet, or 75 million gallons of water during the first year. 

“You’re essentially creating a healthier watershed, which we all knew about, but now we can actually put numbers to it, and that’s the exciting part about it,” said Elvy Barton, water and forest sustainability manager for SRP. 

One acre-foot of water can supply three Arizona families with water for a year. SRP provides water for much of metro Phoenix from snowfall and rain runoff across 8 million acres of land in northern Arizona. 

Water drains down mountains, filling rivers, streams, and reservoirs before traveling through a canal system to 2.5 million homes and businesses across Phoenix. 

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Using lasers to map the land

To quantify water benefits from forest thinning, researchers from ASU’s Center for Hydrologic Innovations combined precise landscape measurement results with public datasets to map the forest. The measurements are made using LiDAR — light detection and ranging — which is an aerial sensing technique that uses light from lasers to measure ranges of land. 

With the technology, researchers created a virtual forest with individual trees mapped onto the landscape. 

The high-resolution maps account for tree size, height and species and use historical watershed data to compare runoff before and after digital thinning. 

“That three-dimensional nature of the project is something really unique. It has individual trees. It represents the snowpack, water and channels,” said Enrique Vivoni, director of the Center for Hydrologic Innovations. “We have the ability to do thinning in a smart way.” 

Researchers can individually remove specific trees to mimic real-life thinning. Taller trees remain to maintain the forest, providing shade and habitat, while smaller and thinner trees are removed to reduce fuel load and improve watershed productivity. 

“There’s a lot of debate in the academic community as to whether a forest disturbance has a positive or negative effect on the water provisioning by that forest landscape,” Vivoni said. “(The models) have shown significant additional water supply when the forest thinning is carried out.” 

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Thinning for the greatest benefits

Although the model has not yet accounted for regrowth in the years after thinning, Vivoni expects the water benefits would decline over time unless crews return to maintain the landscape.

During an unprecedented drought and water shortage on the Colorado River, SRP hopes to ensure water availability. The ability to quantify water benefits from thinning helps them attract commercial investors to fund costly restoration projects.

SRP has pledged to thin 800,000 acres by 2035 to reduce fire risk, protect water infrastructure and preserve water sources. 

“This tool has enabled us to think about how we can scale the magnitude of our work,” Barton said. 

Not only does this technique benefit Arizona’s watersheds, but it could be applied across the western U.S. to help land managers make the best decision about forest treatment. 

“We can determine where the investments have a bigger bang for their buck,” Vivoni said. “Where the watershed and the forest thinning together combine to give the greatest benefit for downstream uses and the local environment.” 

Hayleigh Evans covers environmental issues for The Arizona Republic and azcentral. Send tips or questions tohayleigh.evans@arizonarepublic.com.

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. 

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