Weather Watch

Scientists find low levels of Sierra snowpack to feed the water supply

PG&E hydrographer Jocelyn Beaudette teaches children about the science of snow
at the Exploration Station in Grover Beach on Saturday.
PG&E hydrographer Jocelyn Beaudette teaches children about the science of snow at the Exploration Station in Grover Beach on Saturday.

With each successive dry December day, it seems increasingly likely that this calendar year will become the driest on record in San Luis Obispo since 1870s, when continuous rainfall records were started.

From Jan. 1 through Dec. 15, just 4.4 inches of rain has fallen at Cal Poly (home of climatology for the city). In a normal year, San Luis Obispo receives 20.4 inches of precipitation by Dec. 15. The previous driest Jan. 1-to-Dec. 15 timeframe was 1898, when only 6.6 inches of rain were recorded.

Not only has San Luis Obispo County been affected by this historic drought, but most of the western United States has also suffered. This dry spell is changing the landscape as wildfires swept through our forests and the snowpack has gradually retreated to higher altitudes as the atmosphere slowly warms.

During winter, 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, households and endangered species, and provides the source of energy for hydroelectric power generated in spring and summer.

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

It was established shortly after Dr. 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 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 68 powerhouses, including the Helms Pumped Storage Plant, located about 50 miles east of Fresno in the Sierra National Forest, have a total generating capacity of 3,896 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 approximately 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. His data shows that the Sierra Nevada mountains have been the driest since records started 119 years ago. He is tracking the impacts of climate change on the snowpack at more than 60 locations in the Sierra and southern Cascades.

“The work that PG&E hydrographers like Jocelyn Beaudette 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 told me.

At this time, California is at 31 percent of its normal snowpack for this date.

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.