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Fewer Trees, More Water: Study Finds Runoff Boost From Forest Thinning

A new study is the first to produce solid estimates of the hydrologic benefits of forest restoration work. Scientist Roger Bales explains how this could create a new method to finance controlled burns and selective logging.

Written by Matt Weiser Published on Read time Approx. 5 minutes
Many forests throughout the West are overgrown due to a century of fire suppression, including this area of the Los Padres National Forest near Frazier Park, California, photographed in 2015. Now a new study provides a metric for estimating the additional freshwater runoff likely after trees are thinned, creating a promising new financing mechanism.David McNew/Getty Images

A century of fire suppression has left Western forests overgrown. That has interrupted nature’s regular fire cycle and means that when fires do happen, they become catastrophic because there is plentiful fuel to burn. It also means forests are sucking up more water than they did historically.

How much more water? That’s always been difficult to estimate. But making this calculation could go a long way toward fixing the overgrown forest problem. If we know how much water could be freed up by thinning forests to reduce fire danger, it could create a new financing mechanism to do the expensive work of cutting trees and staging controlled burns.

A team of scientists from the University of California and the National Park Service now has some answers. In a new study, they combined sensors that measure evapotranspiration – how much water trees exhale – with satellite images of “greenness” on the landscape to estimate the additional freshwater runoff that could be created by thinning overgrown forests.

Results vary depending on local climate and geography, of course. But across California’s entire Sierra Nevada, the research suggests that forest thinning could increase freshwater runoff by 10 percent. That’s not small potatoes in a state where drought is common.

To explain the results further, Water Deeply spoke with Roger Bales, a coauthor of the study and director of the Sierra Nevada Research Institute at the University of California, Merced.

Water Deeply: What water supply benefits did you attribute to forest thinning work?

Roger Bales: One thing we found is that medium-intensity fire is approximately equal to the restoration treatments the Forest Service and others are trying to do. And when you have that medium-intensity fire or the restoration treatment, you can reduce evapotranspiration, which means more runoff.

In the American River basin, the highest we saw was a net evapotranspiration reduction equal to about 55,000 acre-feet of water per year. This is for all of the American River basin. So when you add up all the fires in the American River basin over that time period, from 1990 to 2008, by 2008 you had gained 55,000 acre-feet more runoff compared to 1990.

People would love to have that amount of water. And that does not get us into the most recent decade of more high-intensity fires. This was just the period when we had the best data.

Now, you go down the Kings River basin, in the southern Sierra, and you didn’t gain nearly as much. Why? Well, the Kings is water-limited. They don’t get as much precipitation. You take out some trees, and the other trees that are left sort of say, “OK great, more water for us!” The potential for runoff gains basically increases as you go further north.

Water Deeply: How can this data be useful?

Bales: We feel the accuracy is at the level that can stimulate more investment in forest work. One way is through financing mechanisms called green bonds. Land agencies like the Forest Service are not getting the amount of money they need from Congress to to do forest work. So they are looking elsewhere. Water and hydropower are beneficiaries where you may be able to monetize the benefits. They’re willing to pay to minimize wildfire, with the side benefit of getting more water.

I also work with a group called Blue Forest Conservation. They can line up investors to buy bonds, and the bond revenue can then go to the Forest Service or others to pay for forest restoration projects. And the Forest Service is totally on board with this at the national level, the state level and so forth. They’re really happy to see this mechanism coming forward.

So the forest thinning gets done, and then the water utility needs to make a commitment to help pay off the bonds. And their benefit is two things: It lowers their risk of damage from wildfire to either their watershed or their hydropower infrastructure or whatever; and it gets them more water. Our research offers some of the verification for that additional water. We can do this calculation for whatever area they’re talking about working on with a forest thinning project.

Low-intensity controlled fires like this one are essential to maintain forest health in the long term. But they are often impossible to carry out until forests are first thinned of overgrown trees. (Eric Knapp, U.S. Forest Service)

Water Deeply: How did you measure this?

Bales: Think of the basic water balance: Precipitation comes in and then the water leaves the watershed as either evapotranspiration – mainly water use by the forest plants – or runoff.

Previously people would estimate evapotranspiration by subtracting precipitation minus discharge. We took a different approach, and we measured evapotranspiration at several sites. We have four of our own sites in the Sierra Nevada. We have towers that extend above the canopy. Using instruments on those towers, we’re measuring the exchange of water between the forest canopy and the atmosphere, the outward flux of water from the canopy. Each of those towers has about a 250-acre footprint. You’re basically measuring the horizontal airflow coming into the tower. So these are not just point estimates. These are spatial estimates.

We try to place them in representative locations, from oak savannah up through the red fir forest at higher elevation. We only did two river basins, the American and the Kings. The American is a wetter basin and the Kings is a drier basin, and also a little bit higher. We ended up with about 80 site-years of data.

Water Deeply: You also used satellite imagery, right?

Bales: Those evapotranspiration measurements are highly correlated with satellite greenness calculations. From a satellite, you can measure how green is the landscape. The greener the landscape, the more water it’s using, because it’s growing more. So, using this correlation and this calibration curve, we then take greenness every place where we don’t have measurements and then estimate the evapotranspiration. We use the greenness at those sites to estimate evapotranspiration.

Then we looked at how much did the greenness change as a result of fire. We had low-intensity, medium- and high-intensity fire. So therefore we know how much evapotranspiration changed due to the fire. We have values for every place there was a fire, how much evapotranspiration decreased. Subtracting that from the precipitation estimates, you get how much potential runoff there was.

Water Deeply: Is this a breakthrough in linking forest thinning to water availability?

Bales: We feel this provides a much more accurate picture than we’ve had in the past. And it will support better decision-making by resource managers and investors – be it state and federal government or the private sector. We think we have reasonably accurate numbers, at least the best numbers that are out there right now.

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