In the semi-arid southwest, measuring winter snowpack is critical for planning next summer’s water supplies. NASA’s ASO project helps to make projections of water availability more accurate than ever before.
In the American West, summer is often equated with watching the sky for rain and monitoring river levels closely. But planning how the summer water supply will be allocated begins long before these hot summer months. Water utilities, ranchers, and farmers begin thinking about spring and summer water use as early as the winter before. In the watersheds of the Southwest, winter snowpack determines growing season water supply. Being able to estimate that water supply as early in the year as possible is critical for planning, and NASA is flying in to help.
The Airborne Snow Observatory (ASO) is a project of NASA’s Jet Propulsion Laboratory (JPL). ASO gathers snow pack data that can be applied to critical water availability questions in hydrologic research, ecology, and water management planning. It also forms an important component of the broader NASA SnowEX campaign—an initiative to develop more accurate techniques for measuring snow distribution in cold regions across the globe. In California, organizations like the Friant Water Authority and the California Department of Water Resources are teaming up with NASA to test new techniques for snow measurements to better serve the growing needs and requirements for California’s water resources. Jeff Payne, water resources director of the Friant Water Authority, explained:
“The opportunity to really optimize the value of the water begins in February with the state’s initial snow forecast. These forecasts play an important role in how our districts plan for water use for the current year, and the balance of water resources among surface and groundwater resources well into future.”
The Friant Water Authority helps manage water supplies from the San Joaquin River and represents multiple stakeholders including agricultural districts, irrigation districts, water storage facilities, and municipal users. Its service area includes around 1 million acres of farmland, nearly 15,000 family farms, and Fresno, the fifth largest city in California.
Over the past five years, California experienced a record-setting prolonged drought, forcing water users to rely heavily on groundwater resources. This heavy and sustained reliance on groundwater in the San Joaquin Valley caused the surface of the ground to drop in elevation, a phenomenon known as subsidence. Recognizing that this kind of water use was unsustainable in the long term, in 2014 the state of California implemented new laws to protect the availability of groundwater. Payne noted that these challenges are not new to the Friant Water Authority and its members.
“A major purpose of the Friant project in the 1930s was to address the sustainability of groundwater and land subsidence across the eastern portions of the San Joaquin Valley,” he said. “The Friant project delivers water supplies to areas without groundwater, and (uses) the abundant supply of wet years to supply (even) areas that have the ability to use groundwater, as a buffer for dry times, thus bringing the region into a balance.”
Planning for water allocation begins long before either peak water use or peak runoff occur, and these plans rely heavily on the confidence that water managers have in the quantity of runoff.
“Plans for water use depend heavily on the annual runoff forecasts, which are based on two things: an assessment of how much snow is on the ground, and a forecast of future precipitation,” Payne explained.
The Friant Water Authority and other water users, including hydropower producers, saw participation in the ASO program as an opportunity to improve existing water forecasts on the San Joaquin River, and joined the project as a supporting partner last year.
The ASO campaign gathers data by fly-overs of the area, relying on remote sensing equipment to measure snow depth and snow albedo (how much of the sun’s radiation the snow reflects back). The principal outputs of these surveys are a basin-scale estimates of snow water equivalent, the amount of water contained in the snowpack.
Because the equipment is carried by plane, large areas of the watershed can be covered quickly, allowing for a more accurate and spatially complete map of snowpack across the landscape. Prior to ASO, the Friant Water Authority used snow measurements from single point monitoring stations to extrapolate snowpack across the broader basin area. Using the remotely collected data of the ASO also address another prior challenge: trying to measure extremely deep snowpack, like a more than 75 foot avalanche drift observed by the equipment this year.
“I don’t know how you would even physically monitor some of the depths we see from ASO,” Payne said, “That’s a heck of a snow pit to try to dig.”
This is the first year the Friant Water Authority will be applying the data and maps generated by the ASO to its modeling system. But ASO flights have been taking place in other water basins since the spring of 2013, including the Tuolumne, Merced, Lakes, Lee Vining and Rush Creek basins in California and the Uncompahgre, the Upper Rio Grande, Conejos River and the Grand Mesa region of Colorado. Now in the fifth year of analysis for these data, the project shows promising results, said Dr. Tom Painter, a terrestrial hydrologist for the Jet Propulsion Laboratory and Primary Investigator for the ASO project.
“These data are now being used to uniquely constrain hydrologic models in ways that have simply never been possible before,” Painter said.
Using winter snow data to forecast April through July runoff volumes is still tricky, but this new measurement approach has greatly reduced the error in those forecasts.
“(Previously) in the half of the years … the errors were greater than 20 percent,” he added, “In one out of five years, the errors were greater than 40 percent. Across 2013 through 2016, their forecast errors for those kinds of metrics—their ability to forecast the total amount of water that’s going to come pouring into the Hetch Hetchy Reservoir—is down to less than 2 percent.”
Part of this reduction of error is a consequence of being able to take actual measurements of the snowpack across the watershed instead of relying on extrapolations. Another part of this improved capability is due to the instruments being used.
The ASO uses imaging spectrometers and LiDAR and has a high-resolution camera mounted on board an aircraft. The spectrometer captures images across the visible-to-near-infrared wavelengths of light, allowing for measurements of snow albedo. The LiDAR equipment uses laser pulses to measure distances, generating precise topography maps of the area being surveyed. When subtracted from the baseline snow-off topography, the result is a high-resolution map of an area’s snowpack depth and vegetation height and snow reflectance.
“We have very detailed measurements as the LIDAR signal passes through the tree canopy, so you get measurements of individual branches,” Painter said.
Previous NASA missions used passive microwave instruments to measure global distribution of snow remotely. But passive measurements don’t provide the spatial resolution needed for watershed management, and operation in mountainous and forested regions is problematic. New techniques using synthetic aperture radar have the advantage of seeing through clouds, but snow water equivalent (SWE ) retrievals using radar are relatively immature and are highly sensitive to liquid water within the snowpack, making it difficult to estimate SWE at the end of the winter season and during spring when snowmelt begins, or even on warm days within mid-winter when the snow might be very wet. With the laser-based measurements taken by ASO, wet snow conditions are not a confounding factor.
“We have the opportunity to be able to make the push to understanding these systems, which have been largely inaccessible because of the complex terrain and the difficulty working there,” Painter explained.
“And because (these systems) are such an important part of facilitating civilization in the western U.S., they’re critical to understand in light of their sensitivity to changing climate and increasing population.”
Since its launch, the ASO has measured snowpack in “regular” year conditions such as 2016 as well as in the severely dry year of 2015. The winter of 2016-2017 will offer a chance to add a year of exceptionally deep snowpack to the testing mix.
For Payne back at the Friant Water Authority, being able to accurately forecast a wet year is no less important than forecasting a dry one.
“In dry years, the precision afforded by ASO can avoid unneeded financial hardships for our water users“ he said. “On the other hand, if you’ve got a year where the likely total runoff is 10 times the capacity of your reservoir, you need to start working early to manage your entire portfolio of water resources.”
Elise Mulder Osenga is Earthzine’s senior science writer. Follow her on Twitter @mountainlark.