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Study: Logging Forests Won't Increase Water Supply

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A forested stream in the Sierra Nevada | Photo: Allie Caulfield/Flickr/Creative Commons License

A controversial 2015 report that suggested logging California's forests could increase the state's water supply got it wrong, according to a new study released late last week.

The 2015 report by the Nature Conservancy had suggested that thinning forests in the northern Sierra Nevada as a fire management strategy would also increase flows downstream, adding to the amount of water available to rivers and reservoirs.

But that study was based on flawed assumptions, according to a review of more than 230 hydrological studies released Friday. The new study, conducted by veteran hydrologists Jonathan Rhodes and Christopher Frissell, concludes that any increases to water supply from logging would be localized and short-term, and that California would pay the price in water quality, forest biodiversity, and public safety.

Rhodes and Frisell's study, "The High Costs and Low Benefits of Attempting to Increase Water Yield by Forest Removal in the Sierra Nevada," was commissioned by the Environment Now Foundation to examine the Nature Conservancy's suggestions from last year. Much of the work in the new study mirrors a 1998 study Rhodes wrote with hydrologist Michael Purser.

The Nature Conservancy isn't the first group to suggest that logging or forest thinning -- which it refers to as "restoration" -- would increase Californians' access to water. Rhodes' 1998 survey was written to examine previous similar claims. "While the idea of using logging to increase water flows can seem enticing, especially during times of drought, time and again this claim has turned out to be ill-founded," said Douglas Bevington, forest program director of Environment Now. "The latest versions popping up in California are just old wine in a new bottle."

The suggestion that logging would increase stream flows does have a certain logic to it: living trees can suck a significant amount of water from the soil and transpire it into the atmosphere through their leaves. If a watershed had fewer trees in it to suck the water out of the soil, that water could conceivably add to stream flows instead.

In addition, the Nature Conservancy report suggested that a thinner forest canopy might allow greater snowpack accumulation on the ground, instead of holding fallen snow above ground in a dense tree canopy where the snow might sublimate directly to water vapor rather than melting.

But according to Rhodes and Frissell, any gains in groundwater conserved by cutting trees would be quickly used up by the flush of new vegetative growth that usually follows forest thinning or clearcuts, as rapidly growing young trees and shrubs would increase demand on local soil water. In most watersheds studied, that actually led to a decrease in streamflows compared to flows before logging took place. Based on comparative studies of watersheds across the Western U.S. with and without logging or thinning projects, Rhodes and Frissell estimate that about a quarter of a given watershed would need to be logged every 10 years in order to keep ahead of regrowing vegetation's water consumption.

What's more, even the most carefully conducted logging projects increase erosion into local watercourses, both by removing the vegetation holding the soil in place and due to the increase in roadbuilding required to remove timber. Increased erosion from logged hillsides can drastically decrease water quality, damaging both wildlife habitat and water pumps, and can also reduce water storage capacity by silting up reservoirs.

According to Rhodes and Frissell, the amount of water freed up by logging would be minimal to none in dry years. That's not true for especially wet El Niño winters, where more water will indeed go into rivers and streams as a result of logging. But that extra water won't be flowing downstream during the late season, when it would do the most good. Instead, additional water discharged from recently logged landscapes comes during annual peak flows, meaning that a big, flood-causing storm will do more damage to communities downstream of a logged landscape.

Rhodes and Frissell point out that there are ways to increase late-season runoff from California's mountains without adding to those dangerous peak flows.

Reintroducing beavers, a species once common in California's mountains, is one step: as they build dammed pools on mountain streams, the rodents would help raise local water tables, and improve riparian vegetation and habitat for fish. Sediment trapped behind beaver dams would help build mountain meadows instead of running downstream to cause flooding and other problems.

Reducing some of the Forest Service's inventory of roads in California forests would help as well, especially for those roads that cross riparian areas.

And one of the greatest benefits to the state's water supply from its mountain forests would result from reducing livestock grazing, which now affects between 7 million and 8 million acres in 11 National Forests in the Sierra Nevada. Livestock grazing degrades streams and their fringing vegetation, compacts soil, pollutes water with fecal pathogens, and increases erosion and sedimentation.

Rhodes and Frissell note that a combination of limiting grazing, reducing road milesa and encouraging beavers could also help make California's forests more resilient in the face of climate change.

"If one genuinely seeks to improve water flows in the Sierra Nevada, rather than simply trying to find a new justification for logging, these alternatives offer a better way for us to direct our resources," said Environment Now Foundation's Douglas Bevington. "Using logging to increase water flows is still a bad idea whose time has not come."

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