While Valley residents enjoy short-sleeve days of sunshine and temperatures in the 70s, snow is stacking up on nearby mountains. Arizona’s high-altitude mountain ranges are gathering an important part of the state’s water supply — snow that melts into rivers and reservoirs come springtime.
Now, scientists and water managers are adding a new tool to figure out exactly how much snow is held in those mountains, and how much water they can expect in the hotter, drier months. They’re using planes rigged with high-tech sensors that can measure snow to the centimeter, even when the sensors are thousands of feet above.
“We already have good forecasts,” said Bo Svoma, a climate scientist with the Salt River Project. “But looking toward the future — with the changing climate and a greater strain on Western U.S. water resources — the better we can forecast, we can keep those reservoirs fuller as long as possible before we go into a drought.”
The Salt River Project manages water and hydropower at a number of large reservoirs north and east of the Phoenix metro, including Roosevelt Lake. They are collaborating with Airborne Snow Observatories, the California-based private company which operates the data-gathering planes, and Arizona State University.
Water managers have long depended on snow measurements for forecasting, but data often comes from sensors that are fixed to the ground, and can only give measurements from a single point.
“If you can think about the picture that creates, it's like turning on a few pixels on the TV where you don't see the full picture,” said Kat Bormann, Airborne Snow Observatory’s lead scientist.
These flights, Bormann said, can help fill in the gaps by measuring snow across roughly 540 miles in a single flight, then passing that data to SRP and ASU in about 72 hours.
Svoma said information from the flight surveys can tell water managers how much of their annual portfolio will come from snowmelt in the Salt River watershed, and how much they’ll need to look elsewhere — such as groundwater or the Colorado River — to fill out the rest.
Enrique Vivoni, an ASU water scientist who is working on the project, said it may be difficult to map snow in Arizona compared to other snow-laden states around the Mountain West such as California, Wyoming or Colorado.
“Arizona has thin snow that lasts shorter durations,” he said. “It's actually harder to map snow in Arizona than in these other places.”
Vivoni celebrated the collaborative nature of the project, and said data gathered during the next few months will be used to help train computer programs to make better-informed water forecasts in the years to come.
“This is a great laboratory to train people as to the future of how we map snow in Arizona, and it's happening here,” Vivoni said. “It's happening today as part of these snow flights.”
So far this winter, the mountains of eastern and northern Arizona are lagging behind average snow totals. The Salt and Verde River watersheds are both stuck around 20% of normal snow totals for this time of year.
Svoma said a warm start to the winter caused more precipitation to fall as rain. Instead of causing snow to pile up, that saturated the soil. That, he said, could be a good thing if more snow comes soon. With the soil already soaked, snowmelt is more likely to end up in reservoirs instead of seeping into the ground.
“If we can just build up near-normal snow pack from here, it's going to run off more efficiently than you might think,” he said. “So, even though the snow isn't in a good spot, the watershed is. If we get a few more snow storms, that'll produce a lot more than we would have expected.”
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