Cities that depend on snowmelt for water could face problems, study suggests

A new study finds that when more precipitation falls as rain instead of snow, stream flows in drainage basins decline. This could complicate matters for areas that need snowmelt for water.

|
Susan Montoya Bryan/AP/File
The Rio Grande is reduced to a trickle of water near the chile growing community of Hatch, N.M., in this 2013 photo. A new study looks at the effect of snowfall on stream flows.

Managing water resources in a changing climate may get more complicated for regions relying on melt from mountain snows.

A team of researchers has found preliminary evidence that when an increasing proportion of winter precipitation falls as rain, rather than snow, the amount of water flowing through drainage basins and into rivers in the US undergoes a long-term decline.

The study, published Sunday in the journal Nature Climate Change, estimates that for a increase of 2.4 degrees Celsius (4.3 degrees F.), increases in the proportion of rain falling at the expense of snow could lead to a decline of 12 percent or more in stream flows.

If the observations hold up to additional scrutiny "and if they reflect some fundamental response to climate change, then the problems for water managers are going to be even more complicated that we already fear they are," says Peter Gleick, who heads the Pacific Institute, a think tank based in Oakland, Calif., that focuses on sustainable use of water and other environmental issues.

For nearly 30 years, research has shown that as the climate warms, a rising percentage of precipitation will fall as rain, rather than snow, even if global warming had no effect on the overall amount of rain or snow falling in a region. In addition, spring comes earlier, reducing the time available for snow to accumulate.

During the past 50 years, these trends have taken hold throughout the Southwest, according to the Third National Climate Assessment, released in early May. Between 2001 and 2010 flows in major rivers such as the Colorado, Rio Grande, and the Sacramento-San Joaquin system have experienced reductions in flow ranging from 5 to 37 percent compared with the 20th century average. The US geological Survey has measured a 20 percent decline in snow cover in the Rocky Mountains since 1980, due a mix of global warming and natural variability.

That leaves less snow held in high-altitude storage to melt in the summer, when conditions are hotter and drier below. The big concern for water managers in the West involves this change in timing, which sends more water downhill when it's least needed, leaving less available when it's most needed.

But few have looked at the impact the shift in precipitation type alone could have on stream flow, explains the team, led by Wouter Berghuijs, a PhD student at the University of Bristol in Britain.   

Finding an apparent relationship between the proportion of winter precipitation falling as snow and stream flows was not what Mr. Berghuijs had in mind when he began the work, he explains.

The team's study is based on data from 420 drainage basins from Maine to California covering areas ranging in size from 27 to nearly 3,900 square miles. The researchers selected basins that were least affected by human development or land-use changes and that had good-quality stream-flow records spanning decades. Many of the stream gauges the team tapped have been taking measurements since 1948, with an average across all gauges of 47 years. 

Initially, Berghuijs says he was trying to evaluate models that predict stream flows to see which worked best with particular types of drainage basins, or catchments.

As he and colleague Ross Woods, a lecturer in civil engineering at the University of Bristol, looked over the data, they were struck by initial indications that as the proportion of snow in winter precipitation increased, so did stream flow. As the proportion declined, so did average stream flow.

The team first looked at data from all 420 basins and found that as the fraction of precipitation falling as snow increased, so did stream flow. Then they focused on 97 basins for which snow made up more than 15 percent of a winter's precipitation. The pattern repeated, with these basins showing evidence of being quite sensitive to changes in the rain-snow mix.

The study is something of an initial tug on the shirtsleeves to other researchers working to understand the interplay of climate change with hydrology. Long-term changes in the amount of precipitation falling in a region could make up for the decline in stream flow, the researchers acknowledge. 

It doesn't attempt to break the data down by region, something they and Dr. Gleick say is a logical next step.

Nor does it address possible causes for the trend. With earlier, warmer springs, for example, vegetation would be sprouting earlier, drawing more heavily on moisture that otherwise would migrate to streams and rivers. But trying to measure such activity on a basinwide scale is difficult, Berghuijs says.

Still, he adds, he and his colleagues have identified the effect, which appears consistently and across a wide range of catchment types. 

"It's a start for really trying to understand why this is happening" and to see how the effect varies from catchment to catchment, Berghuijs says.

You've read  of  free articles. Subscribe to continue.
Real news can be honest, hopeful, credible, constructive.
What is the Monitor difference? Tackling the tough headlines – with humanity. Listening to sources – with respect. Seeing the story that others are missing by reporting what so often gets overlooked: the values that connect us. That’s Monitor reporting – news that changes how you see the world.

Dear Reader,

About a year ago, I happened upon this statement about the Monitor in the Harvard Business Review – under the charming heading of “do things that don’t interest you”:

“Many things that end up” being meaningful, writes social scientist Joseph Grenny, “have come from conference workshops, articles, or online videos that began as a chore and ended with an insight. My work in Kenya, for example, was heavily influenced by a Christian Science Monitor article I had forced myself to read 10 years earlier. Sometimes, we call things ‘boring’ simply because they lie outside the box we are currently in.”

If you were to come up with a punchline to a joke about the Monitor, that would probably be it. We’re seen as being global, fair, insightful, and perhaps a bit too earnest. We’re the bran muffin of journalism.

But you know what? We change lives. And I’m going to argue that we change lives precisely because we force open that too-small box that most human beings think they live in.

The Monitor is a peculiar little publication that’s hard for the world to figure out. We’re run by a church, but we’re not only for church members and we’re not about converting people. We’re known as being fair even as the world becomes as polarized as at any time since the newspaper’s founding in 1908.

We have a mission beyond circulation, we want to bridge divides. We’re about kicking down the door of thought everywhere and saying, “You are bigger and more capable than you realize. And we can prove it.”

If you’re looking for bran muffin journalism, you can subscribe to the Monitor for $15. You’ll get the Monitor Weekly magazine, the Monitor Daily email, and unlimited access to CSMonitor.com.

QR Code to Cities that depend on snowmelt for water could face problems, study suggests
Read this article in
https://www.csmonitor.com/Environment/2014/0519/Cities-that-depend-on-snowmelt-for-water-could-face-problems-study-suggests
QR Code to Subscription page
Start your subscription today
https://www.csmonitor.com/subscribe