UC Merced works to refine snowpack measurements

[ERIC]

UC Merced works to refine snowpack measurements
Scientists say they've found a way to make a more accurate runoff forecast.

Posted at 03:40 PM on Sunday, Apr. 18, 2010
By Mark Grossi / The Fresno Bee


Farmers and water officials throughout California wait anxiously each year for forecasts about snowmelt roaring down the Sierra's granite canyons - precious water for the long summer.

But the forecasts are only estimates, based on averages of past seasons, snow-sensor readings and monthly measurements from key mountain meadows above big rivers. The forecast sometimes is wrong, leaving farmers with too much or too little water later in the growing season.

One big reason: Nobody measures snow around jagged ridges, plunging ravines and deep forests in the 400-mile-long Sierra. That's a huge swath of the high country where the size of the snowpack is unknown. As the climate warms and snowfall dwindles this century, officials will need to measure more of the Sierra to improve runoff forecasts for farmers and the growing population, say scientists at the University of California at Merced.

The researchers think they've found a way to do it by expanding monitoring around the existing remote sensors, called snow pillows, which are mostly in flat meadows. Scientists propose to surround the pillows with instruments to daily check snow levels all over the landscape.

Some day, there could be thousands of these instruments in the Sierra.



"We're hoping to design a new system of doing things up there," said engineering professor Roger Bales. As head of the Sierra Nevada Research Institute at UC Merced, Bales has worked on this idea for five years at Gin Flat in Yosemite National Park.

He and fellow researcher Bob Rice use ultrasonic monitors - devices that emit sound waves - to track the depth of snow. They also use satellite images to see how much of the area is covered with snow, so their estimates won't include dry spots.

In the next year, Bales and Rice will expand the project to the mountains around the American River, above Sacramento. The $200,000 expansion is funded from state bond money.

How quickly the technology takes hold throughout the Sierra, and begins to influence the accuracy of forecasts, will depend on both funding and success of these early efforts. No timeline has been set.

The ultrasonic monitors are an established and affordable technology, researchers said. The cost is about $1,200 per monitor, which includes other devices such as transmitter links to send data via satellite or cell phone.

Mounted on poles about 30 feet high, monitors send sound waves to the snow, and the waves bounce back to the monitor.

As snow accumulates during winter months, the waves travel a shorter distance and bounce back faster. Researchers can calculate the depth of the snow if they know how fast the waves return to the monitor. Rice said tests at Gin Flat revealed some snow pillows may be overstating the snow amounts by 25%, mainly because they are purposely located to measure meadows that catch a lot of snow and deliver a lot of water for streams.

The snow pillows are 8-by-10-foot stainless steel tanks filled with antifreeze - think of a water bed. They weigh snow and help state officials determine how much water is in the snowpack, which is information used in the calculations made by the UC Merced researchers.

But they are not accurate reflection of the entire area, Rice said.

"The snow stays the longest in these [meadows] when everything else has melted around it," he said. Frank Gehrke, California's snow survey chief, said the sound sensors also might solve the problem of measuring snowfall in high elevations - above 10,000 feet, especially in the rugged southern Sierra. Snow survey teams cannot easily reach those elevations through deep snow and storms, even in spring.

In late May and early June, a lot of snow has melted at lower elevations. But above 10,000 feet, there often is a lot of snow that won't melt for weeks. Sometimes, runoff is much stronger in August because of the high elevation snow.

"We're really unable to quantify the amount of snow up high, and it makes a difference," Gehrke said.

State officials add that their runoff forecasting relies heavily on mathematical models that don't work well if a winter is much drier than average. If the climate warms, there may be many dry winters, scientists say.

"Nature isn't going to behave the same way in the future," said climatologist Mike Anderson of the state Department of Water Resources. "So we will need more tools to forecast. This could be one."

The reporter can be reached at mgrossi@fresnobee.com or (559) 441-6316.