National Geographic News
A photo of Kings Canyon National Park in California.

As the area around California's King's River warms, more trees will grow. That will reduce the amount of water available downstream.

Photograph by Rich Reid, National Geographic Creative

Brian Clark Howard

National Geographic

Published September 1, 2014

Scientists have more bad news for drought-stricken California: The climate warming expected in this century is likely to result in even less water flow from the mountains, as trees and plants growing higher on the slopes soak up more of the available precipitation.

This finding should be "of great interest to water managers in California," says Roger C. Bales, a professor of hydrology and environmental engineering at the University of California, Merced, who co-authored the study published Monday in the Proceedings of the National Academy of Sciences. The study foresees "a potential widespread reduction in water supply with warming, with important implications for California's economy and environment."

Bales and Michael L. Goulden of the University of California, Irvine, used direct measurements and remote sensing to examine the impact of vegetation on runoff in the upper King's River basin in California's Sierra Nevada Mountains. They concluded that the warming that is widely projected for the region by 2100—more than 7 degrees Fahrenheit, or 4 degrees Celsius—could decrease the amount of water flowing down rivers by 26 percent.

Most of that water loss would instead be released into the air through the process known as evapotranspiration, by the leaves of the additional plant cover that is spurred by warming temperatures.

Goulden and Bales found that, currently, cold temperatures are the main factor limiting plant growth above 7,875 feet (2,400 meters) in the King's River area. But warming temperatures will allow trees and other plants to move farther upslope, into an area that is currently "disproportionally important for runoff generation."

The Snowpack Is Key

Californians get 75 to 80 percent of their water from the snowpack in the Sierra Nevada. The water serves about 20 million people directly, and it's used to grow about half the fruits, nuts, and vegetables produced in the United States. It's also used to generate hydropower, drive industry, support recreation, and nourish wildlife—all of which are now suffering from the worst drought in the state's recorded history. (See "California Snowpack Measure Shows No End in Sight for Drought.")

"Regardless of what happens with climate change, even in a good year we don't have enough water," says Frank Gehrke, who serves as the chief of snow surveys for California's Department of Water Resources in Sacramento. (Watch video of Gehrke taking a snow survey.)

The Sierra Nevada snowpack serves as a natural reservoir that stores precipitation in winter, when the state gets the most, and surrenders it in summer, when the state needs it the most. Climate warming is expected to diminish that natural reservoir by causing more precipitation to fall as rain rather than snow—but in theory, at least, the state could build more artificial reservoirs to catch the runoff.

The study by Bales and Goulden suggests, however, that a lot of the rain will be snatched first by trees and returned to the atmosphere before it runs off. Although the researchers focused on one part of the Sierra Nevada, "we think the analysis applies basin-wide," says Bales.

Nor is there any reason it shouldn't apply to other mountain ranges in a warming world. Globally, about four billion people rely on water that runs down from mountains, Bales says.

If water managers such as Gehrke are going to take the next step toward an improved understanding of how forest growth affects the availability of water, "they are probably going to need to get more on-the-ground measurements," Bales adds.

Better Forest Management?

The paper's results should be "another call to improve forest management to protect the water system," says Bales. (See "California Drought Launches New Gold Rush.")

For example, he suggests that managers may need to rethink how fire suppression policies impact runoff. "Forests have maybe four times as many trees and two times the biomass as they did a century ago because of fire suppression, and we need to get a better understanding of how that impacts the water balance," says Bales.

By better understanding how vegetation affects runoff, forest managers can plan more targeted thinning campaigns that maximize the amount of water available to downstream users, Bales suggests.

"Managers need to understand what's going to happen to the water runoff from the mountains," says Bales. "This paper is another piece to help inform that and will hopefully get more people's attention."

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29 comments
Sergio Olivares
Sergio Olivares

Are you blaming trees, genius? Better think about how asphalt impedes rain to reach underground rivers.

William Kus
William Kus

I look forward to a simpler world.  The current situation is sick and wholly unnatural.  Unfortunately I doubt I will live to see the end of modern civilization, but the next few generations might have the pleasure.


I could be wrong obviously, but I don't see much hope with ever increasing populations, higher consumption of natural resources that will eventually run out, bigger and more powerful weapons, ever widening gaps between haves and have-nots, ever increasing partisanship, the lowering of moral and educational standards, and so on. 

Mitra R. Ramkissoon
Mitra R. Ramkissoon

Sounds like a licence to hack down the forest...seems like a vicious downward cycle.

Chui Chui
Chui Chui

For starters one would hope these studies are not financed by big logging companies.  Secondly forest management , also means  cutting down more trees . I fore one do not buy  the idea that few trees  will  save water.  It is just not only trees that need  water to grow but  there is far more other vegetation  that uses  rain / snow pack water. Currently  most  water run off high Sierra  winds up in the rivers  , creeks  etc that  join bigger rivers and bodies of water. Plus there are many acqafilers  in these areas.  Building more small reservoirs only means more  evaporation during  hotter months.  Plus as more trees are cut,  Top soil erosion and bare landscapes will  result in  more floods and such out of control hell. Man is not going to change mother nature.  Climate change is more of man made problem due to  uncontrolled  pollution, not only  o the earth's surface, but oceans and the junk yards in the shy. 

Philip Rutter
Philip Rutter

We've been making predictions like this for about a century now, and- they're just not very reliable.  I can think of at least 10 ways the future changes could move the hydrology in totally different ways.  

And; "better forest management"??  Ask a forester: we have NO idea how to manage forests today, for the forests 50 years from now.

Frances R.
Frances R.

This is an interesting finding for several reasons, but what I take from it is this: we keep ourselves cocooned in a world of assumptions that nature will respond in predictable fashion to our heating up the planet.


But nature rarely believes in linear, proportionate responses. 


She will do what she wants, in ways that we haven't seen before or haven't modeled before.


In the developed world, we think that all that will happen to us when the climate changes is that it'll get a little warmer here and there, and we'll have to keep the air conditioner running a bit longer.


But this study, if it proves accurate, tells us that something new. Other studies to come will tell us of more new things that we haven't seen before.


Soon we will be drowning in a swamp of our own misapprehensions.

Orland Cabrio
Orland Cabrio

We have hotter summers here , I have lived here for 60 years, and this year Sacrament brags it had only 1 day of 100 degree weather, Cooler than most years , as for the water shortage, It was Politician caused , Drought come and go , It comes with the living in Calif, These drip writers come from somewhere in outer space,

Carter Fox Jr.
Carter Fox Jr.

The first line of this story sounds like an ad for a lumber company........

Gwendolyn Mugliston
Gwendolyn Mugliston

@Robert Dahl   I regret I didn't read your comment first. I apologize.  Usually I am the only one who discusses population.  It is nice to read another writer is also seeing population as a problem.

Gwendolyn Mugliston
Gwendolyn Mugliston

Maybe instead of focusing so much on "understanding how vegetation affects runoff..." maybe some focus should be put on populations control. What about that idea? 


It seems the USA is inviting more and more people to the table with less and less food in the frig. 

Robert Dahl
Robert Dahl

We also need to start thinking about population control, or we'll breed ourselves right into a solution by Mother Nature that we may not like.

Justin Smith
Justin Smith

Water is not the problem, the problem is getting cheap fresh water from the oceans. 

Fra Rei
Fra Rei

What little left will be used for fracking no doubt.

James Rea
James Rea

Not sure why this is bad news.  It means more lumber.

craig hill
craig hill

@Gwendolyn Mugliston How offensive. Do any Americans who consider themselves intelligent enough to learn new things from Nat Geo have any idea we each consume ten times as much as the average 3rd worlder, always accused by us of being the problem when we do NOTHING to curb our own piggish consumption?! 


If you don't like the excess world population, practice what you preach. How many high-consuming American children have you brought into the world? How dare you treat those without, who are trying to escape their poverty which is often due to American practices in their countries? Like the "fast and furious" program under Bush and Obama, which has flooded Mexico with illegal rapidfire machine guns. Or the evil American company Monsanto, which has undercut farmers' returns in Mexico on their maize by growing cheap frankenfood corn, driving over two million Mexican farm families into bankruptcy, some of whom come knocking on the US' door, only to be treated like a problem when WE are the ones who created the problem for THEM.


The American Pig disgusts me, and is MUCH, BY FAR, the greatest problem population on the planet.   

craig hill
craig hill

@Gwendolyn Mugliston How offensive. Do any Americans who consider themselves intelligent enough to learn new things from Nat Geo have any idea we each consume ten times as much as the average 3rd worlder, always accused by us of being the problem when we do NOTHING to curb our own piggish consumption?! 


If you don't like the excess world population, practice what you preach. How many high-consuming American children have you brought into the world? How dare you treat those without, who are trying to escape their poverty which is often due to American practices in their countries? Like the "fast and furious" program under Bush and Obama, which has flooded Mexico with illegal rapidefire machine guns. Or the evil American company Monsanto, which has undercut farmers' returns in Mexico on their maize by growing cheap frankenfood corn, driving over two million Mexican farm families into bankruptcy, some of whom come knocking on the US' door, only to be treated like a problem when WE are the ones who created the problem for THEM.


The American Pig disgusts me, and is MUCH, BY FAR, the greatest problem population on the planet.   

craig hill
craig hill

@Robert Dahl Population control? Thank you for volunteering! I'm sure there is a bridge high enough near you, and rope is cheap. Thank you for practicing what you so cavalierly preach everyone else should do in deleting excess population.

Jay Clemons
Jay Clemons

@Justin Smith - This is like saying, "Energy is not the problem; the problem is getting unlimited energy from fusion power." How's that working out for ya?!

Jay Clemons
Jay Clemons

@James Rea - There's a lot you can do without lumber. There's not much you can do without water..

Benson Stein
Benson Stein

@James Rea God forbid that someone smokes a cigarette in California, a state emergency will be declared.

William Kus
William Kus

@craig hill @Gwendolyn Mugliston  Hmm, isn't that typical to blame all the world's problems on America so that absolutely nobody in the world has any responsibility for their own actions except America.

Seamus Cameron
Seamus Cameron

@craig hill

"The American Pig disgusts me, and is MUCH, BY FAR, the greatest problem population on the planet."

Then why is everybody trying to come here?

malcolm morrison
malcolm morrison

@craig hill @Gwendolyn Mugliston Although your intent is noble and I agree with your every word, Mr. Hill, those words chosen by you are offensive and harmful to "the cause". Get over your anger. Get over your righteousness. Realize that if you are to contribute to "the solution" you must choose your words to enlarge the base of those who think as you do. Your ill-tempered and offensive post above does not do that. Please "see the light" and remove yourself from The Dark Side of anger and hate. I have enough problems trying to make Amerikans "see the light" without having to deal with the aftereffects of hatemongers like you.

William Kus
William Kus

@craig hill @Robert Dahl  I used to be an angry self-hating American like yourself.  And thanks to America, and the comfy life I lived, I was able to feel completely entitled to my views.


It was great to want to help other people, hey, why do we even need a border anyway?  Why not just let everyone in the world who wants to come here come?  Why shouldn't people get free education and free healthcare and free food and so on?


This all sounds great, why can't everyone have what I have?  We should just give it to them so they can.  It was great until I had to directly feel the consequences of these policies.  And actually, why can't I have what everybody else has?  Who cares how they got it, I just want it because I deserve it because that's what society teaches me.


It's easy to point the blame at everyone and everything but the person themselves and their own governments and institutions.


And speaking of this, please allow me to move into your residence Craig, because you need to be the first to understand that you have something I don't have, so I deserve to have it too.


These kinds of ideas are fine when you're only benefiting from low prices and cheap labor.  It's another story when it starts to negatively affect your life in ways you couldn't imagine, and all of a sudden this country begins to look like a Third World nation.

craig hill
craig hill

@Seamus Cameron @craig hill Much more dire straits, often caused by our evil corporate-run govt, see the rest of the post you trplied to. They certainly don't come to the US for the anti-christian attitude.


American expatriates are flooding their countries by the way, driving up prices, buying land to resettle, lobbying their govt to change laws to suit foreigners over the natives. Again, Americans have no idea how we disrupt the ways in other countries while we whine about their poor coming here, out of selfish jingoism.

William Kus
William Kus

And even though breaking into someone's house is illegal, I am only trying to make a better life for myself and my family, so you have no choice but to allow me to live in your house, provide me with food, shelter, clothing, healthcare, and so on, much like the government does for illegal aliens.


Of course I know you won't allow this, because it directly affects you in a way you cannot ignore.

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