Weather Watch

Why tracking California’s snowpack is important

Snowy peaks in the Sierra Nevada at Kings Canyon National Park earlier this winter.
Snowy peaks in the Sierra Nevada at Kings Canyon National Park earlier this winter. pgeweather@pge.com

Most of the rain that reaches the ground along the Central Coast actually begins as snow high in the atmosphere where temperatures are less than 32 degrees Fahrenheit, the freezing point of water.

As the snow falls, it usually encounters warmer air and melts, changing to rain. If the temperature remains below or near freezing, snow will hit the ground.

Much of California’s precious precipitation falls as snow over the Sierra Nevada. This snowpack acts as a reservoir. As this reservoir melts, it slowly releases water for the needs of forests and agriculture, industry, households and endangered species, and it provides the source of energy for hydroelectric power generated in spring and summer.

Some may wonder, “Can it get too cold to snow?”

There is a relationship between temperature and the maximum amount of water vapor the air can hold — the colder the air becomes, the less water vapor it can contain.

If the air is frightfully cold, such as at the South Pole where temperatures can reach minus 120 degrees or lower, the moisture content of the air is nearly bone dry and very little snow can occur. In our part of the world, the moist air that streams eastward from the Pacific Ocean is lifted over the Sierra Nevada, where it cools and massive amounts of snow can fall, like wringing water out of a mop.

Tamarack, near Bear Valley ski resort, is at an altitude of nearly 7,000 feet and has experienced some of the greatest snowfall amounts on Earth. In 1906-07, this location recorded nearly 74 feet of snow. In just one day, Echo Summit, south of Lake Tahoe, reported 67 inches of snow on Jan. 4, 1982.

PG&E established one of the first permanent snow courses, or measuring stations, in California in 1910 at Donner Summit along Highway 80. Markers along the course are measured at regular intervals to determine the amount and water content of the snow.

It was established shortly after James Church of the University of Nevada developed the science and methodology of snow surveying to study the relationship between the amount of snow on the ground and runoff. An average snow course is 1,000 feet long with about 10 sample points. The adventurous folks who perform these snow surveys are called hydrographers. They work in teams for the sake of safety and often fly in by helicopter, drive in by Sno-Cat or snowshoe in, and they make measurements over a wide expanse of the watershed. And is it ever a wide expanse of watershed!

Most hydroelectric power delivered to PG&E customers comes from generation facilities built along 16 river basins stretching nearly 500 miles — from Redding in the north to Bakersfield in the south.

PG&E’s 67 powerhouses, including the Helms Pumped Storage Plant about 50 miles east of Fresno in the Sierra National Forest, have a total generating capacity of 3.89  megawatts — enough to meet the needs of nearly 4 million homes — and rely on nearly 100 reservoirs located primarily in the higher elevations of California’s Sierra Nevada and southern Cascade mountain ranges. Historically, about 37 percent of water that passes through PG&E’s powerhouses comes from snowpack; the remainder is a combination of rainfall and groundwater.

Gary Freeman, a PG&E water-management specialist, is tracking the impacts of climate change on the snowpack.

“The work that PG&E hydrographers and other snow surveyors is essential not only for operating PG&E’s hydroelectric system, but to research and understand, plan and adapt to the eventual anticipated snowpack decline in the Sierra and southern Cascades,” Freeman said.

PG&E is working as a partner with the U.S. Geological Survey and California Department of Water Resources to develop and calibrate the U.S. Geological Survey’s Precipitation-Runoff Modeling System to the Sierra watersheds.

This model will help with water release planning and adaptation to the changing hydrology as the elevation of snowfall rises on the Sierra’s watersheds.

In early March, California’s snowpack had dropped to around 80 percent of normal because of the mostly dry and warm February. However, a series of El Niño-driven storms expected this month may drive the snowpack to above typical heights by April.

John Lindsey is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at pgeweather@pge.com or follow him on Twitter @PGE_John.

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