Whether brushing your teeth, washing your hands, tending to plants or simply having a glass of it, water is in your everyday routine. It’s a precious commodity, but because we use it so often, we rarely stop to think about where it comes from.
In California, about a third of the water we can use actually comes from places in the state where there is snowpack. This is a dense area of snow on the ground, often in open meadows. It’s a sort of frozen storage system, holding in water during the winter until the snowpack melts and runs off into nearby streams, and eventually making it to the state’s reservoirs.
The amount of water in the state’s reservoirs determines how much water is distributed to communities and farmers across the state. This water is often referred to as surface water, and is the water we can see — think rivers, lakes and streams. It’s different from groundwater, which is stored underground.
Three of California’s largest reservoirs are in the North State — Shasta Lake, Lake Oroville and Trinity Lake. Shasta Lake and Trinity Lake are run by the U.S. Bureau of Reclamation, while Lake Oroville is part of the State Water Project (SWP). It’s run by the California Department of Water Resources (DWR) and delivers clean water to about 27 million people and 750,000 acres of farmland.
Every year since 1930, different snowpacks have been surveyed throughout the state. Data is currently collected from more than 250 snow courses from January to May. Snow course is the term used for areas where the snow is surveyed.
The California Cooperative Snow Surveys (CCSS) program through DWR is the driving force behind the data collection. It’s made up of about 50 agencies, including PG&E, the U.S. Forest Service and the National Park Service. The National Resource Conservation Service also conducts surveys and partners with CCSS.
The DWR does not conduct all of the surveys itself, instead they are split up between the different agencies in the cooperative.
These surveys help officials forecast how large California’s water supply is for the year and what’s needed to prevent possible flooding from the snowmelt.
“If we don't know how much [water] is up there, we can't measure it, it's a guess. And so being able to kind of forecast how much total and when it might be available is critical,” said Andy Reising, the manager of the snow surveys and water supply forecasting unit for the DWR.
What goes into a survey
For every snow course that’s surveyed, Riesling said there’s a map that shows about 10 areas that are measured.
“On one end of the course, you'll see a sign in a tree, and the other end you'll see another sign in another tree, and they measure specific distances from those trees,” Reising said.
Those measured areas are calculated together and determine the “average” snowpack level for the course.
To take the measurements, surveyors use a hollow, long tube known as a federal snow sampler. It’s inserted into the ground and pulls the core of the snow up. On the side of the tube, numbers indicate the depth of the snow in inches.
The snow is also weighed on the scene. Reising said that every ounce measured equals about an inch worth of water content when the snow melts. That’s the snow water equivalent. About 13 inches of snow is one inch of rain, according to the National Oceanic and Atmospheric Administration.
Once the data is averaged out, the surveyors send it to DWR and the California Data Exchange Center, then it’s displayed to the public in an index.
The measurements start in January, and the more than 260 snow courses are measured by April.
“April 1 is typically the peak of snowpack in California. It's increasing, increasing until April 1,” Reising said. “Sometimes it's in May, but on average, over the years, April 1 has been the peak.”
He said that’s because after April, California isn’t likely to get more snow that year, and the snow on the ground will begin melting.
“Average” is not what you think it is
When measuring the snowpack, the CCSS uses “percent of average” as the indicator of how much water is available.
Reising said there are actually two “averages” used by the state.
One is the percent of “normal,” or the average snowpack levels for that time of year. The CCSS takes 30 years worth of data from the specific time the survey was conducted to come up with the average. So the snowpack data collected in January can be compared to the “average” snow levels for January over the course of three decades.
However, the other average is based on the April 1 peak each year. Reising said that this average is more consistent than the normal average because it’s a known number already.
However, Reising said that there isn’t really such a thing as “normal.”
“We can have average. That's a mathematical calculation of a number of years of what average is,” he said. “But every year is so different in where the snow falls, how much it falls, when it falls.”
For example, in a heavy wet year like 2023, the Phillips Station snow survey found the snowpack was 221% of average. Meanwhile, during a dry year like 2018, the Phillips Station snowpack was only 49% of average.
According to Reising, a good year could be between 80% to 120% of normal. He said that levels reaching between 200% to 300% of normal can cause flooding, though flooding can also happen in lighter rain years.
“I would say an above average, a slightly above average year would be a great year, right, where we have more than perhaps we need, but it's not going to cause major flood issues,” Reising said.
He said in recent years, California has seen a sort of “climate whiplash” with heavy rain years like 2023 to years where the state was experiencing a drought like 2015. To have an “average year” across the state is shocking to Reising.
“We don't have to meet 100% to fulfill all reservoirs, but it certainly makes it easier, if we have an average or slightly above average, year for them to properly fill their reservoir without worrying about filling or sending water to the ocean unnecessarily,” he said.
