The snowpack in the Tuolumne River Basin in the Sierra Nevada, California contains 40 per cent less water than it did at its highest level in 2014.

NASA data indicates that the water in the snowpack amounts to 74,000 acre-feet or 24billion gallons. At its high-point two years ago, the snowpack covered 179,000 acre-feet. The agency has been using airborne monitoring to check on the snowpack for the last three years.

Tuolumne feeds the irrigation districts and the Hetch Hetchy Regional Water System, which serves San Francisco and nearby settlements. Snowfall accounts for 70 per cent of California’s annual precipitation in normal years, with the ice melting in spring and early summer.

The new research will allow water authorities to provide almost real-time information about how much water will flow from the mountains during California’s ongoing drought.

Spatial distribution of the total volume of water in the snowpack across the Tuolumne River Basin on 25 March, 2015 (bottom) and 7 April, 2014 (top) as measured by NASA’s Airborne Snow Observatory. (Image: NASA/JPL-Caltech)

‘A mountain’s snowpack is like a giant TV screen, where each pixel in the image varies but blends together with the others to make up a picture,’ says Tom Painter from NASA’s Jet Propulsion Laboratory.

‘For the past century, we’ve estimated mountain snowpack by looking at just a few pixels of the screen – that is, a few sparse ground measurements in each watershed basin. During an intense drought like this one, most of the pixels on the screen are blank – that is, they’re snow-free,’ he adds.

Before the surveys began, the California Department of Water Resources used manual surveys to map snow courses using data from electronic sensors planted in the ground called snow pillows. These are accurate, but sparsely distributed.