# Water Weight Used to Calculate the Amount of Snow in California with GPS

More accurate, cheaper measurements of snowpack can help the state, struggling with drought, to better manage water flows

Anyone who has ever lugged a pallet of water down to the basement in preparation for the latest extreme weather event knows that water is heavy.

How heavy? About 8.3 pounds per gallon.

Now, scientists have developed a way to use water's weight to measure just how much of it is sitting up in snow-covered mountains in the western United States.

In states like California, which is currently in the midst of a crippling drought, the more water managers know about how much snow is in the mountains, the better they can plan for the summer to come (Greenwire).

More accurate information about snowpack can help these managers and hydrologists plan for how to fill reservoirs, how much water they might have available during the dry season and how dry the soils might be during fire season. They'll also get a better fix on future levels of hydroelectric reservoirs.

Snow 'pushes' earth down
Donald Argus, a research scientist and geophysicist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., recently published a study outlining the new technique in the journal Geophysical Research Letters.

Since water is so heavy, when there is enough of it on the ground in the form of snow, it actually weighs down the land's surface.

"The weight of the snow and the water pushes the earth down," said Argus. "The earth subsides up to 12 millimeters."

If scientists know how high the land is in summertime, and also how high it is when snow covers it, they can use the difference in height, plus a lot of math, to calculate how much water, in snow form, is sitting on the mountains.

The technique uses a dense network of GPS sites scattered across the West. The sites are part of a project called the Plate Boundary Observatory, run by the science consortium UNAVCO.

"There's over 1,000 sites in California, Oregon, Nevada and Washington," said Argus.

The scientists studied data from these GPS sites going back to 2006. They calculated the average difference in height for each GPS site between the end of the dry season, Oct. 1, and April 1, when the snow is usually at its maximum, and used that difference to calculate how much water was needed to weigh down the ground.

While many Americans use GPS to navigate from their phones or in cars, the GPS receivers used for this work are far more precise, able to measure changes in their location of a few millimeters. Since the GPS network in California is so dense, the scientists are able to get a spatial resolution of about 50 miles, said Argus.

New method complements other measurements
On average, the researchers found the difference in water weight from summer to winter in the Sierra Nevada mountain range equaled about 1.6 feet of water spread over the 26,000-square-mile region.

Water managers already have other tools to track water quantities in the West. These include satellites, hydrologic models, and even another NASA project that uses planes and Lidar to measure snowpack in parts of the Sierra Nevada (ClimateWire, Dec. 10, 2013).

But the airborne surveys are costly and only see one part of the Sierra Nevada, whereas the GPS network is widespread among many states.

And the GPS data is about 50 percent more accurate than one hydrologic model used to estimate snowpack in the Sierra Nevada and roughly on par with another one, Argus said.

Right now, the researchers are able to develop a monthly average for snowpack about 10 days after the end of the month. Their goal, however, is to do that even faster.

"The idea is to make it available in near real time and be able to give it to [water managers] one, two, five days after," said Argus.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500