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Expert Views: Managing Wildfires to Protect Water Resources

Researchers weigh in on the complex relationship between forest and water management in a region increasingly threatened by both wildfires and drought.

Written by Lindsay Abrams Published on Read time Approx. 7 minutes
Flames from the Holy Fire shoot up above homes southeast of Los Angeles on August 9.ROBYN BECK/AFP/Getty Images

Editor’s note: Please join us on Thursday, August 30, at 1 p.m. PT/4 p.m. ET for a 30-minute conference call with a panel of experts about the ways in which water and wildfire management intersect, and about the West’s adaptation to its new, and far from normal, reality.

It’s wildfire season in the American West, and this one has already been setting records: the second-most destructive conflagration in Colorado, the largest-ever wildfire recorded in California and the worst air quality on record in smoky Seattle. Multiple fires continue to ravage the region, threatening homes, lives and, in many cases, water supplies.

The relationship between wildfires and water is complex, as researchers at the Public Policy Institute of California recently noted. Among other short-term concerns, eroded soil and ash from fires can contaminate drinking water. But experts have reason to believe the same management techniques that help curb the spread of dangerous wildfires may also improve water supply.

One thing we know is that the dual challenges of wildfire intensity and water scarcity in the West aren’t going away; as the climate warms, both are expected to intensify. We asked experts working in the region how these two threats intersect, and for their recommendations on how we can manage fire and forests to benefit water resources.

Van Butsic, cooperative extension specialist at the University of California, Berkeley

Managing forest and wildfires to benefit water resources is difficult because there are trade-offs between short-term costs and long-term benefits. In the short term, wildfires can lead to increased erosion and sedimentation in streams and reservoirs. This contributes to lost revenue for downstream power generators and at times even requires water to be treated before it is potable.

At the same time, there is strong evidence that areas where fire has not been suppressed actually have higher overall water yield, suggesting that periodic fire may increase water benefits in the long run.

The trick is to find a path to fire regimes that maximize water benefits. Prescribed fire and mechanical thinning can move forests in the right direction, but can be expensive in some areas. This is a problem for which there is not an easy answer.

Matthew Hurteau, associate professor in the Department of Biology at the University of New Mexico

A combination of land-use change, fire suppression and climate change has increased the frequency of large, hot wildfires in the western United States. In the Southwest, Ponderosa pine and dry mixed-conifer forests used to burn frequently – about every 2–16 years. Regular fire occurrence maintained heterogeneous forest structures and prevented the buildup of dead vegetation on the forest floor.

Around the turn of the 20th century, a combination of livestock grazing, fire suppression policy and a cool, wet climatic period increased tree density in these forests. One hundred years later, forests have become dense. Meanwhile, ongoing climate change has increased the effects of our current warm, dry period. Over the past 40 years, the area burned by wildfire in the Southwest has increased by 1,200 percent, and more areas are being burned by hot, tree-killing wildfires.

Dense forests intercept snowfall, which reduces snowpack accumulation, and use more water for photosynthesis. Further, when large, tree-killing wildfires occur, the chance for post-fire flooding and soil loss that impacts waterways increases. Restoring heterogeneous forest structure and surface fire regimes helps reduce the chance of large, hot wildfires and can contribute to sustaining snowpack duration. This is increasingly important as ongoing climate change is lengthening the fire season and increasing the frequency of extreme weather conditions that support large wildfires.

This is a multifaceted challenge that requires financial investment by society, increased awareness and understanding of the importance of the right kinds of fire in our forests, and action to reduce the rate of human-caused climate change.

Gabrielle Boisrame, visiting researcher in the Department of Environmental Engineering at the University of California, Berkeley

It’s important for downstream water users to recognize their relationship to upstream forests. There can be a disconnect – people do recognize that mountain snowpack affects them, but it can be more difficult to understand how the actions that you take on a forest could actually make a difference to the water supply. People usually think of operating reservoirs as the way you manage the water between the mountains and the lowlands, which of course is a really important part. But it’s not the whole story.

Dense forests are associated with reduced snowpack accumulation and increased evapotranspiration, two factors that negatively affect water supply. Also, when you have a really dense forest, less water makes it to the ground in the first place because the rain and snow are intercepted by the trees.

A big thing that I’ve looked at is the spatial patterns that result when you have a regime of fires that’s more natural to the area I study – the Sierra Nevada watershed – that do not burn hot enough or long enough to kill all the vegetation. These types of fires can create more heterogeneous forests with open areas mixed in with tree stands. Research has shown that having medium-size openings in the forest maximizes snowpack accumulation and can slow the melting rate.

Having gaps in the vegetation, as opposed to one dense covering of a specific type of vegetation, can have the additional effect of preventing wildfires from spreading. This means that allowing some types of fires now can keep future fires from getting too large and destructive, preventing negative effects of fires such as erosion and poor water quality. But it depends on the specific makeup of the vegetation.

Another thing I’ve found is that often when you open up previously forested areas that were already a little bit wetter than average, such as a low point where water can gather, it allows those areas to turn into wetlands or wet meadows, which do not burn nearly as readily. When you have fires that open up forests and allow wetter areas to establish, that can definitely slow down fire. Wet meadows also create really important conditions for biodiversity, as well as refuge from drought, and have the potential to provide additional water for summer streamflow.

Finally, wildfire management carries a lot of benefits for the forests themselves. I found that in Yosemite, during the drought years of 2014 and 2015, there was less drought-related tree mortality in areas that recently had fires than in areas that hadn’t burned in a long time. In other words, it looks like there’s less of a shortage of water for the trees that remain after a fire, and that helps them better survive through droughts.

Roger Bales, founding professor of engineering at the University of California, Merced

Reducing high-intensity wildfire risk by thinning forests in the Sierra Nevada and other regions will also increase the potential runoff in rivers draining from those mountain forests. In general, precipitation entering mountain forests first satisfies the evaporative demand of the forest, primarily transpiration by trees, with the remainder providing stream runoff. Water use by the forest, or evapotranspiration, increases as forests become denser.

Forest thinning in the central Sierra shows that removal of 40–50 percent of biomass, equivalent to restoration treatments, reduced evapotranspiration following treatment. Extending these treatments across the American River Basin in California could reduce evapotranspiration and increase potential runoff by about 10 percent of full natural flow in the river for dry years and 5 percent over all years.

A second benefit to decreasing wildfire risk is a reduction in the risk of damage to critical infrastructure for hydropower and water supply. This damage can be direct from the fire, or from erosion of the landscape and delivery of sediment to downstream facilities after a fire.

Headwater forests are an essential part of our natural capital, and are recognized as water infrastructure. In California, water storage in source-water areas is comparable to that behind dams and is central to the state’s water security.

The U.S. Forest Service has identified 6–8 million acres in need of immediate restoration in California, out of the 21 million acres it manages in the state; and 58 million acres in need of restoration nationally. The cost for restoration in California alone is estimated at $6 billion to $8 billion. Monetizing the water-related benefits is an important way to generate revenue for forest restoration.

These responses have been edited for length and clarity.

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