Weather Watch

How the massive Sierra Nevada snowpack will help power homes

PG&E hydrographer Dan Stephens measures snow atop Dyer Mountain in Lassen County on Friday, March 31, 2017.
PG&E hydrographer Dan Stephens measures snow atop Dyer Mountain in Lassen County on Friday, March 31, 2017.

This year — like the weak La Niñas of 1964, 1994 and 2004 rain seasons — has seen significant surges of subtropical moisture from the Pacific Ocean toward California. Referred to as an atmospheric river, pineapple express or — in the meteorological community — “turning on the hose,” it produced massive amounts of precipitation in the state’s mountain ranges.

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In these elevated locations, the prefrontal winds lifted the low-level subtropical moisture over California’s mountain ranges and cooled it approximately 2.7 degrees Fahrenheit for each 1,000 feet of elevation. This process is called the saturated adiabatic lapse rate, which wrings out the moisture from the heavens like squeezing a wet sponge or mop — in other words, orthographic enhancement.

Rainfall amounts in the Santa Lucia Mountains have been breathtaking. Rocky Butte has logged 79 inches; typically this station receives about 40 inches a year. Doc Miller has been keeping track of the rain for 36 years at his property on Pine Mountain at 2,650 feet of elevation. So far this year, he has recorded more than 100 inches of rainfall. Miller recorded a little more than 49 inches this January, the wettest month on record.

At higher elevations, much of this precipitation falls as snow over the Sierra Nevada. This snow pack in the Sierra acts as a storage reservoir for water. As this reservoir melts, it releases water for the needs of forests, agriculture, industry, households and endangered species and provides the energy for hydroelectric power used during the longer days of spring and summer.

Most of the hydroelectric power delivered to PG&E customers comes from a network of tunnels, canals and conduits that direct a portion of streams and rivers in the mountains of California toward penstocks.

Penstocks are near-vertical pipes that can be more than 1,000 feet long.

Earth’s gravity accelerates the mass of the water in the penstock, creating weight and pressure to spin hydroelectric turbines connected to electrical generators that produce electricity. After the water is used to produce clean and renewable energy, it’s returned to streams and rivers.

Electricity from other clean sources such as nuclear, wind and solar is used to pump some of this water back uphill at the Helms Pumped Storage Project in the Sierra Nevada when demand for electricity is lower. This water is then allowed to run downhill to produce electricity during periods of higher demand.

Normally, about 15 percent of the electricity delivered to PG&E customers is from hydroelectric power. Consecutive years of drought reduced this percentage by as much as half for some years.

Measurements by PG&E hydrographers and many other agency hydrographers in the Sierra this March indicate the snowpack is well above normal. In fact, some locations are at near-record levels. Surveys are conducted on a monthly basis from January through April.

PG&E hydrographers fly in by helicopter, drive in by snow cat or hike in utilizing snowshoes and take snow depth and water content measurements over a wide expanse of the watershed.

March’s survey indicates the water content of the snowpack equivalent/content was 164 percent of average — 147 percent of normal for the north, 175 percent of normal for the central Sierra and 164 percent of normal for the southern Sierra. In the Sierra Nevada region, this current 2017 “water year-to-date” has been the wettest (Oct. 1 to Feb 28) in the past 122 years of records.

“For our customers, it means we’re going to have more hydro power available,” PG&E spokesman Paul Moreno said. “This is a good source of energy. It’s clean. It’s renewable. It’s reliable. And in fact we use it to help us through the hot periods of summer because we can quickly ramp up hydro power when called upon.”

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Nearly 70 percent of the electricity PG&E delivered to its customers in 2016 came from greenhouse gas-free resources like nuclear (Diablo Canyon nuclear power plant) and large hydro. An average of 32.8 percent of its electricity in 2016 came from renewable resources including solar, wind, geothermal, biomass and small hydroelectric power plants.

John Lindsey’s column is special to The Tribune. He 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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