Teams will fan out across the Sierra Nevada on Thursday to perform their final snow survey of the season, a closely watched rite of spring that helps determine how much water will flow to farms and cities in coming months.
But 18,000 feet above the Sierra slopes, an airborne experiment is underway that could revolutionize that ritual.
Starting in early April, researchers have made weekly flights over the upper Tuolumne River basin, taking sophisticated instrument readings of the snow depth and reflected sunlight. The information, coupled with data from the ground measurements, promises to paint the most comprehensive snowpack picture that water managers have ever had.
"This is the first time that we've actually known how much water there is," said Tom Painter, a scientist with NASA's Jet Propulsion Laboratory in La Cañada Flintridge who is overseeing the aerial project with the California Department of Water Resources.
The West's mountain snowpack forms a crucial natural reservoir, melting into runoff that rules dam operations and water allocations from California to the Rocky Mountains.
Water managers started measuring the snowpack a century ago, developing a simple system that remains in use today. Surveyors with water agencies and utilities snap on cross-country skis once a month during the winter and early spring and return to several hundred alpine spots year after year to measure the snow's water content.
They drive hollow aluminum tubes into the snow, measure the snow depth and then weigh the snow-filled tubes to determine the snow's water volume. In recent decades, the manual measurements have been supplemented with snow pillows — large electronic scales installed at various mountain locations that automatically weigh the snow accumulated on the pillow.
The readings are fed into a computer, along with precipitation and stream flow statistics and historic data, to produce the spring and summer runoff forecasts.
"It works very well and would still be something we would rely on quite heavily," said state snow survey chief Frank Gehrke, who is working on the airborne project with Painter. "But even now, in relatively dry years and relatively wet years, it tends to break down a little bit."