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Forest thinning would yield additional water, study says

Issue Date: April 13, 2011
By Ching Lee

As the state looks for ways to meet ever-increasing water demands for a growing population and environmental needs, a representative from the California Forestry Association says the state should consider how much water is being transpired through the overly dense national forests of the Sierra Nevada.

California could have plentiful, quality water in the form of groundwater recharge and runoff if its public forests were well managed and restored to a healthy condition, said Steve Brink, vice president of public resources for the trade association, whose members include forest landowners and businesses that make forest products.

Brink made his remarks during the California Farm Bureau Federation 2011 Leaders Conference.

He said with properly managed forests and active forest thinning, not only will the state significantly reduce wildfires by 22 percent to 60 percent and have healthy watersheds that minimize sediment production, but it could also get back 1 million to 3 million acre-feet or more of additional water annually.

"So how much water is that? It's the storage capacity of Lake Oroville," he said, noting that the reservoir contains more than 3.3 million acre-feet of water. "You could almost fill up the equivalent of Lake Oroville if our national forests were managed."

More than half of California's water originates in the watersheds of its national forests, much of it in the Sierra Nevada. But lack of forest management and too much vegetation has contributed to reduced water flows in the state's watersheds, particularly during dry years, he said.

"When we have a really dry year, it can be so dry that the water stress will be such that vegetation actually dies," he said. "We had a couple of years here (in 2004 and 2007) where manzanita has died from water stress, and I thought manzanita was indestructible."

He pointed out that in the Hetch Hetchy Reservoir, which is in Yosemite National Park and is a major water source for the San Francisco Bay Area, there has been an average of a 30 percent reduction in spring snowpack since the 1970s due to vegetation growth.

"And that means less groundwater recharge and runoff in the spring and summer months," he said.

The Department of Water Resources estimates that some 53 to 54 inches of water are evaporated and transpired in the Sierra Nevada annually. The ratio of evaporation to transpiration is small when there is significant canopy cover on the soil, according to DWR.

A few forest studies, including one done in the Feather River watershed in May 2007, show that an increase of 20 percent to 30 percent in water yield can be expected from forest thinning, but such increases are "not sustained due to (forest) re-growth unless you keep after it," he said.

There are 9.8 million acres of productive forestland within California's national forests that's not reserved, or that is not in the wilderness. That averages to about 93 bone-dry tons of biomass per acre, Brink noted.

Regional foresters would like to perform 500,000 acres of mechanical thinning and understory removal per year rather than the current 100,000 acres per year, he said. A typical thinning would remove 10 bone-dry tons of commercial trees and 13 bone-dry tons of surface and ladder fuels per acre. The growth rate after the thinnings is about 0.6 bone-dry tons biomass per acre per year, he added.

"Remember, the forest grows. And particularly after thinning, you spur growth," he said.

He calculates that with active forest thinning, the state could potentially yield 3.3 million acre-feet of water per year, about the amount of water stored in Lake Oroville.

There are other benefits to healthy national forests, he said, including a reduction in sediment rates and fewer wildfires. Costs to suppress those wildfires would also be significant, he added. In addition, thousands of new jobs in rural California counties could be created in actively managing the state's national forests, he said.

(Ching Lee is an assistant editor of Ag Alert. She may be contacted at

Permission for use is granted, however, credit must be made to the California Farm Bureau Federation when reprinting this item.

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