'Pillows' measure snow

As five blizzards pounded California’s high country last week, people around the state tracked the swiftly growing snowpack without leaving their offices or homes.

A remarkably accurate network of sensors fed estimates to state officials who posted them on a Web page the public can easily use to track the snowpack, California’s biggest source of water.

“You could see the snowpack increasing by 10 percent each day last week,” said Fresno meteorologist Steve Johnson, a private consultant.

The sensors — called snow pillows — do not provide official snowpack numbers. For that, surveying crews were readying Friday to trek into the mountains to measure the snowpack by hand, poking long pipes into the snowpack to determine depth and water content.

Those surveys form the backbone of summer water delivery forecasts, the first of which will be made in mid-February.

But the network of 140 snow pillows provides water managers and farmers a good snapshot of the Sierra snowpack — usually differing from the survey results by only a few percentage points. On Thursday, the sensors showed it is 117 percent of average for this time of year, compared to 68 percent last year.

“Snow pillows are invaluable for people in the water business,” said Frank Gehrke, the state’s snow survey chief.

On Friday, the state Department of Water Resources reported the snowpack was holding about 115 percent of its usual water content for this time of year.

The online access provided by the state Department of Water Resources has made the network’s information popular over the last decade, but the technology dates back to the 1970s, Gehrke said.

The network has slowly been assembled over the last few decades by various agencies and businesses, including DWR, the National Park Service and Pacific Gas & Electric Co. State officials say there’s an average of two snow pillows added each year to the network throughout the Sierra.

The sensors are 8-by-10-foot stainless steel filled with antifreeze — think of a water bed. The sensors register the weight of the snow and transmit the information to a satellite, which relays the information to a ground station for processing.

The weight of the snowpack tells scientists how much water it contains. One inch of water content in snow weighs 440 pounds on the sensor.

Data from some snow pillows show more than 20 inches of water in the snowpack after the storms last week.

“Snow creates a phenomenal amount of weight because of the water,” Gehrke said. “Earlier versions of the snow pillows sometimes developed leaks from the weight. They have been improved significantly.”

Snow pillows can be fallible, he said, especially early in the snow season. Strong wind can scour the snow away from the snow pillow, resulting in a lower weight.

Sometimes, an icy crust will form above the snowpack when the sun melts the top layer and later re-freezes. The crust supports the weight of additional snowfall and prevents the sensor from registering the added snow. Later in the season, when the snowpack melts, the crust collapses and a more accurate reading can be made.

“We’ve learned a lot,” Gehrke said. “For instance, we don’t put a snow pillow at the bottom of a sloping surface. The snow will tend to move downhill onto the snow pillow and register more weight than it should.”