Sierra Nevada forests are denser than they once were, potentially reducing the amount of runoff that reaches California's reservoirs. (Dan Brekke/KQED)
Sierra Nevada forests are denser than they once were, potentially reducing the amount of runoff that reaches California’s reservoirs. (Dan Brekke/KQED)

With California’s reservoir levels dropping, just about everyone is wishing the state had gotten more water this year. That doesn’t just depend on the weather, according to a team of scientists. Sierra Nevada forests play a big role in the state’s water supply.

Just like crops, trees consume water. And Sierra Nevada forests are denser than they once were after decades of fire suppression. That could be reducing the amount of runoff coming from the snowpack — runoff that provides water for most of the state.

“We call the Sierra Nevada our water towers for California,” says Roger Bales, a hydrologist with UC Merced. “About 60 percent of our consumable water comes from the Sierra Nevada.”

Bales is working in a pine forest about 20 miles west of Lake Tahoe, to understand the balance between and trees and runoff. His team has installed hundreds of sensors in the American River basin to record snow depth and soil moisture.

“The snowmelt really enters the soil,” he says, “and flows downslope to the nearest stream channel.”

From there, it joins major rivers and goes into reservoirs and canals that reach all the way to cities and farms in the Central Valley, Bay Area and Southern California.

UC Merced's Roger Bales and Ziran Zhang work on a snow sensor tower in the Tahoe National Forest. (Lauren Sommer/KQED)
UC Merced’s Roger Bales and Ziran Zhang work on a snow sensor tower in the Tahoe National Forest. (Lauren Sommer/KQED)

When trees use water through the process of evapotranspiration, it doesn’t run off into rivers and reservoirs.

“That water travels up the tree trunk and then goes out through the leaves to the atmosphere,” Bales says. And there are a lot more trees using water today than there once were.

Frequent, low-intensity fires once cleared out small trees and maintained spaces in the forest. Decades of suppressing fires has allowed the forest to fill in.

“You go back about 100-to-150 years and the forest data show us there were maybe only half as many trees here,” Bales says.

The snowpack is also less stable in a dense forest. The snow gets stuck in the trees’ branches before reaching the ground and evaporates faster because it’s more susceptible to sun and wind.

Because these changes have happened over millions of acres of forest, Bales says it’s led researchers to a basic question:

“If there were half as many trees, would there be more runoff?” he asks.

The research points to yes, he says — potentially a lot more.

“Is it 20 percent, 30 percent or 40 percent?” Bales says. “We’re sort of in that range. But that’s a hypothesis. Our back-of-the-envelope calculations suggest that you could get anywhere from half a million to a million acre-feet additional water out of the Sierra Nevada.”

A million acre-feet of water is enough to supply two million households in California for a year — an amount that could make a big difference during a drought.

Managing Overgrown Forests

“I think the water piece is really huge,” says Scott Stephens, professor of fire science at UC Berkeley. “I think it’s under-appreciated but it’s massive.”

Stephens has found similar results in the Illilouette Creek basin in Yosemite National Park. About 40 fires have been allowed to burn there over several decades, reducing the number of trees per acre.

“It looks like there’s 20 percent more surface water leaving the streams in that area since the fire program began in the mid-1970s,” he says.

The widely spaced trees also make the forest more resistant to high-severity fire.

“I call it a potential win-win,” Stephens says. “It’s a win from a fire standpoint to have more resilient forests and also maybe a win in terms of being able to provide a critical resource for California, which is water.”

But leaving naturally caused fires to burn over large areas of the Sierra Nevada is tricky, he says, especially near houses and communities.

“Letting fire work in those lands is risky,” Stephens says. “Sometimes it’s going to go as expected and once in a while it goes wrong.”

Another option is to allow timber companies to cut small trees, thinning the forest. It’s commonly done where roads already exist, but can be prohibitively expensive in remote areas and often faces environmental opposition.

Climate change could make the problem even worse. A recent study from UC Irvine found California’s forests will be using even more water by the end of the century, because warming temperatures will make the growing season longer. Runoff could drop by as much as 26 percent.

“If we don’t act today, our grandkids’ grandkids are going to have so few options,” Stephens says. “It’s going to be warmer. It’s going to be more difficult to do this work and they’re going to be basically chasing their tails.”

Stephens says the good news is that California water districts are joining the conversation about how to manage forests. While it didn’t used to be on their radar, the connection between trees and our drinking water is becoming hard to ignore.

  • Patrick Shannon

    ~ “What Fools these Mortals be!”
    Only the shelter of trees protects watersheds and holds moisture. But yes, if you desertify forested hills, you get all the water yourself…
    How mad are we?

    • Kimmi Tyler

      Nobody was discussing deforesting. A healthy, natural forest has half or fewer the number of trees that our forests currently have (due to unnatural fire suppression) and one of the side effects that they are just now studying is that this increase in trees could be having the effect decreasing the amount of runoff that used to naturally happen. So the argument is how to return the forest to a more natural state

      • johnsang

        The timber industry, forest service and BLM by clear cutting and even age management has destroyed a lot of forest and replaced it with tree farms. All the cheap or free fire suppression they have gotten has created a problem that they never figured on, too many trees. Now they are looking for federal and state aid to subsidize their tree growing problem and fire risk. Next they will want us to irrigate their trees too, in fact that is what this proposal amounts to, creating more water for the Sierra timber industry and their trees. All this because our esteemed Forest Service, BLM and Timber Industry didn’t properly understand the dynamics of forests, over harvesting and weather. How do we know that they understand it now or are telling the truth about it now? Don’t forget the whole curriculum that has trained generations of “foresters” was created by the timber industry and promoted by them as “scientific forest management” when it fact what they had in mind was converting the forests into tree farms and increasing harvests, not conserving forests, and watersheds. Yes there are many good, sincere people involved in this but look at the mess we are in with climate and watershed problems. Who was supposed to be looking at the big picture and watching the store for us?

      • Patrick Shannon

        One thing we might consider in discussing “healthy, natural forests” is soil health. The press to harvest stands of uniform conifer every 40 years, is not sustainable below the surface. It will lead to desertification. Mixed stands, managing ladder fuels, and a healthy mulch of woody debris have been good policies for our family forestlands.

      • Joseph

        The problem of deforestation, allows soil runoff into creeks and streams. Trees also affect our weather patterns. By the time deforestation or clear-cutting came out, more research was done. And by the way creeks and streams can’t take sun, the sun creates algae. Usually these water ways needs shade.

    • Skippy56

      I can see thinning being done as a good forest management practice; I do not see blaming the increased growth of trees as the cause of the water shortage. Look at the snow pack level of the past one will also see it is not the same today as in the past, which allows for increased numbers in plant species growth.

      This is just a scientist looking to take advantage of government grants to look into a problem any layman can figure out be looking at the past as they are. Only difference, he will get the money and provide numbers, but it will not provide funds for managing our forests as they should be.

    • inquisitive

      Patrick no one is talking about desertification only you. We are talking managing this natural forest to allow more runoff.

      • Patrick Shannon

        and while we discuss this, you might contribute by showing respect for others even tho you hide your identity.

  • DiscusBS

    Federal regulations of national forests have reduced harvesting of timber because of special interest group concerns. Remember the spotted owl which is well documented to be one of the broadest ranging and most adaptable species on the planet and the claims that it needed old growth snags to be able to sustain its population. The forests instead of being properly managed are returning to a natural state where natural burns are the only control. Why would we ever consider controlled burns when we could harvest the timber for gain by society? Unfortunately federal regulations have caused the closure of many saw mills. This is another government caused problem.

    • Scott Greacen

      Moron thinks we’re stupid.

    • Joseph

      DiscusBS, Your right, Wildlife became a special interest especially because of those spotted owls, but to endanger the wildlife for humans, was the major concern. Now, we have environmentalists fighting, because of the hazards being created, or other factors. Like the ozone layer, disappearing, snow melting quite faster than we ever imagined, and other factors. So, research is being conducted for further review. You can’t blame the government, its the environmentalists coming forward with those issues.

    • EBrownE

      Shipping the un-sawn logs directly to Japan and china is what closed the saw mills, along with the depletion of cheap, easy access, forests.

  • gimmemymoney

    This is gonna be fun watching the tree huggers fighting the water people… At one time they fought with a united front… now they will be like two waring tribes in some sand country…

  • Kathy

    All this assumes there is water to create run-off. Considering the wheel of reasons we are in this mess to begin with, The one of the reasons the global environment has changed so radically is the over forestation in Africa and in the South American rain forest. So to even consider removal of trees for water is utter nonsense. These “experts” were trained by the forest industry to manage ‘farmed forests’ meant for commercial wood logging. Along with ‘managed’ forests come increased disease, bugs and fires. In Humboldt County where the redwood forests were “managed” now grows douglas fir and tan oak.
    true conservationist would let the natural forest recover and grow as it wills.

    • Joseph

      After reading this article, the first thing is that the earth is dependent on trees, in fact trees take the abuse us humans are creating in the first place. Roots hold the soil together, they survive every imaginable areas, including cliffs, our weather patterns change because of trees. Maybe its time to re-do the research, and work in harmony with it, rather than against it.

      • Hungry Hyaena

        What do you mean by harmony?

        Many people speak or write of Native Americans living in harmony with the land and ecosystem, but we know that most Native American tribes managed the land they lived on. The Ohlone populations, for example, shaped the San Francisco peninsula through controlled burns, intentionally preventing tree growth in many areas to allow for better hunting.

        It seems that most of the people advocating for harmonious coexistence with the rest of the natural world (humans, after all, are animals like any other) imagine some stable Eden which has never existed. Nature is forever in flux, often violent flux. All we can do as creatures with a marvelous empathetic capacity and incredible technology is make the choices we feel are best for the most species/habitats. (Not that we’re doing that, mind you, but even if we were, harmony it ain’t!)

    • Hungry Hyaena

      Since the traditional distinction between conservation and preservation is that the former is concerned with “wise use” (managing habitat, ecosystems, and natural resources) whereas the latter is concerned with protection, a “true conservationist” would be wrestling with the difficult choices that conservation entails…and would therefore thoughtfully consider what the contemporary hydrologists and fire scientists (experts who, by the way, were NOT trained by forest industry) have to offer.

      Moreover, if you care about endangered species and diverse, resilient ecosystems, the conservation/”wise use” approach tends to do the world better. Certainly, as others here point out, there are management failures that result in beleaguered landscapes, but, almost without fail, those are stories of management corrupted by short-term profit. I’m skeptical of the role politics and money play in BLM and the Forest Service decisions, but that has little to do with what independent academics are saying.

  • bubba10

    Trees = Enemy?

  • Unpavedmind1

    Shame in quest for this one-sided article.

    • MatBastardson

      Yes, two-sided articles are much to be preferred. I know I always feel
      more enlightened after reading or viewing a “debate” between two
      equally credentialed persons with diametrically opposing points of view.
      This is why ultimately nothing will ever be done about any issue you
      can name, this one included.

  • Hungry Hyaena

    Thanks for covering another interesting story, Lauren.

    When I finished reading it, I paused before scrolling into comments land, hoping that there would be some thoughtful dialogue. Alas, the general reaction is either a I-told-you-so from those hostile to environmental considerations or a trees-are-the-answer protest from hands off preservationists.

    Oy, vey.

  • jwc

    This is a very troubling article because of its inaccuracies and half truths. Of course trees use water. But it has been shown time and again that trees more than make up for what they use, These important factors are ignored regarding water retention by the forest are ignored in the article. First, the trees retain moisture in the soil so it can recharge the ground water and not quickly run down to the sea overwhelming dams and flooding. That is exactly what happens when the trees are removed and is a cause of flooding in all of our drainages. Second, the trees shade the ground preventing evaporation in the summer and sublimation of the snowpack in the winter. Third many kinds of trees contribute to precipation getting to the ground by capturing fog drip. This is happening where I live right now this morning. In some parts of California this can be up to 1/3 of the annual precipitation.

    This article seems to show how NPR has become the spokesperson for the industry who want you to believe that more logging helps the forests, cigarettes are good for you and having lots of guns is the best way to protect your family.

  • Debra

    What none of these scientists have talked about is the marijuana farms that are all over the west coast using up all of the water that is needed for our food farms. The states are no longer enforcing the drug laws and the marijuana farms are increasing and take a ton of water to keep their crops growing.

  • Joseph

    This is only an opinion, regardless of how many there are snow makes more water run off than say rain does. Snow is also considered as water, except the cold freezes the water, and still melts into the ground. Once lakes and rivers freezes it becomes more water.

  • EBrownE

    This story struck me as remarkably one-sided and simplistic for a KQED report. A true scientist would recognize that the Sierra snow pack is a complex system. The water system relies on the snow pack to store the water and slow the run off and provide the water over a longer period. A large, quick, run off would mean floods followed by reduced water. The trees shade the snow pack and slow the melt. Thinning the trees may also increase the ground cover vegetation, which increases the fuel that ignites the tree canopy.

  • DiscusBS

    It was my thought that anyone can correlate temperature and pan evaporation rates to a shaded plot of ground with organic litter that protects against excessive release of water to the atmosphere. Apparently there are some that do not understand the concept. Some of those same people apparently do not understand the sequestration of carbon and climate modulation. What is the reflective index and radiation coefficients for this reduced forest concept? When you facilitate runoff as proposed in this model, how is groundwater and basin recharge impacted? Will the faults within the Sierras be reactivated and will the return of volcanism be something you would like? Academics have argued that we allow forests to grow unmolested and that we allow them to burn under natural conditions. That was an ignorant position advanced by those that are generally opposed to humans inserted into the environment. Their policies have been proven ineffective and the results are a nightmare.

  • Helen Jackman

    This is the most unintelligent article I have ever read, for thousands of years this state has had more than enough rain and snow, to much many times but that was normal. We have had no meaningful snow fall or rain for nearly 7 (not 3, not 4)years, that is a catastrophe. This wonderous state is in deep trouble and it is water.

  • Unofelice

    The idea that reducing the number of trees in our upland forests will result in an enhanced water supply seems to make sense. After all, as you say “Just like crops, trees consume water” and that is what the model makers have modelled. But those simplistic models ignore the state’s largest reservoir, upland forest soils, which, when healthy, are 1/3rd empty space which fills with water in winter and slowly releases that water via springs, seeps and headwater streams in summer.

    Logging compacts forest soils damaging their ability to store water. Furthermore, as a literature review commissioned by Environment Now demonstrates, producing more water by logging forests is problematic. Many studies over several decades find that any increase in water supply resulting from vegetation removal will come in winter and with lots of sediment. Late summer and fall baseflows would actually be decreased by “thinning” and other vegetation removal projects; more water in winter means less water in summer. Furthermore, the increase in water production is short lived. Within 6 to 10 years vegetation has resprouted and regrown and the amount of water consumed by vegetation is greater than before the vegetation “treatment” took place. So, to get increased supply, vegetation must be aggressively reduced every decade or so.

    Check out E Now’s report at this link:

    The idea that we can enhance California’s water supply by logging is pie in the sky. It is being promoted by Sierra Pacific Industries, the nations #1 logger, and by those who are linked to the company and/or to California’s water brokers. Accelerating logging in our headwater forests will further damage California’s water supply. The idea is being promoted as a way to get more logs to SPI’s mills and more water to those with deep pockets but junior water rights.


Lauren Sommer

Lauren is a radio reporter covering environment, water, and energy for KQED Science. As part of her day job, she has scaled Sierra Nevada peaks, run from charging elephant seals, and desperately tried to get her sea legs - all in pursuit of good radio. Her work has appeared on Marketplace, Living on Earth, Science Friday and NPR's Morning Edition and All Things Considered. You can find her on Twitter at @lesommer.

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