Using high-tech equipment in the air to measure snow on the ground
When a small, turboprop twin-engine plane launched from Centennial Airport’s runway this spring, heading for the snow-capped peaks that rise above Dillon Reservoir, a new era in data collection started at Denver Water.
The plane, flying over the Blue River Basin above Dillon Reservoir at an altitude of 20,000 feet, carried sophisticated equipment that used beams of light to measure the depth of the snow fields below and capture reflections from the frozen surface. Powerful computers operated by NASA crunched reams of data gathered during the two separate flights.
The flights provided an assessment of the amount of water that was frozen in place above the reservoir — an area that covers 335 square miles — that’s more accurate than anything Denver Water has ever had before. Similar flights have been conducted for Colorado clients interested in other river basins in recent years, but this is the first year Denver Water has participated.
“We’re pretty excited about it. With these two flights, we get a highly accurate report of the amount of water in the snowpack that will flow into Dillon,” said Nathan Elder, Denver Water’s manager of water supply.
“With more information, we improve our operations and it also means better information for the marinas on the reservoir and for fishing and rafting on the Blue River below the dam.”
Dillon, first filled in 1965, is Denver Water’s largest reservoir. It holds water that tumbles down Summit County’s Snake River, Blue River and Tenmile Creek.
“The Blue River Basin is the source of about 30% of the water we supply to the 1.4 million people in Denver and surrounding suburbs who rely on us for their drinking water,” Elder said.
From the reservoir, some of the water travels under the Continental Divide through the 23-mile Roberts Tunnel to the South Platte River Basin and ultimately into Denver Water’s Marston and Foothills drinking water treatment plants on the southwest side of the metro area. Some of the water is also released through the dam to continue flowing down the Blue River and into the Colorado River.
So, knowing how much water is packed into the snowdrifts above Dillon is crucial. And in the face of increasingly variable weather patterns related to climate change, having better information and more accurate forecasts of the seasonal runoff will be more important in the future.
Collecting data with snowshoes
The process for collecting information on the amount of water locked in the snow hasn’t evolved much in decades.
Once a month during the winter, Denver Water crews check the snowpack by trekking on snowshoes to 11 designated spots in Grand, Park and Summit counties and plunging hollow tubes into the snow. The core samples from the 11 sites, including three above Dillon Dam in Summit County, are measured for depth and weighed to calculate how much snow water equivalent, a measurement of the amount of water in the snow dubbed SWE, the sample contains.
Information from those samples collected by hand is combined with data from automated snow telemetry monitoring systems, or SNOTELs, first installed in the 1970s and operated by the Natural Resources Conservation Service. Sensors on each SNOTEL, a bladder about the size of a queen-sized waterbed and filled with antifreeze, monitor and report the weight of the snow falling on it.
Denver Water collects information from 13 SNOTELs located in its 4,000 square miles of watershed, including four SNOTEL sites perched above Dillon at about 11,000 feet in elevation.
These measurements, compiled over decades, give Denver Water a good sense of how much water is available to fill the reservoirs.
But the records and methods of the past will not be good enough in the future, Elder said, as weather patterns swing due to climate change.
“The measurements are accurate but limited,” Elder said. “They give us an idea of what we’ll see in the spring runoff, not a more precise number on the amount of water that will end up in the reservoirs.”
Airborne data collection a major step forward
Runoff forecasts based on the limited amount of snowpack data points can be off by as much as 40%. Considering this information is used to determine how much room should be made in the reservoir to capture the melting snow, this discrepancy can cause challenges for water managers during runoff season.
“Based off the inaccurate data, we could let more water out of our reservoirs — or keep more water in — than we need to,” Elder said.
That’s where the aerial data collection comes into play via the twin-engine planes flying over the Blue River Basin carrying equipment that pings the snow’s surface at up to 10 locations every square meter.
“This is the first major step forward in snow information gathering since the early 1980s,” Elder said.
“This will be a quantification of the snowpack in the Blue River Basin above Dillon, all of it, rather than trying to build a picture of the 335-square-mile basin from a few selected point measurements. Imagine trying to watch a high-definition TV that only has 10 of its thousands of pixels working, you just don’t get the whole picture.”
Data from the flight over the Blue River Basin on June 24 indicated there was 114,000 acre-feet of water locked in the snow, nearly all of it above the 11,000 foot band that the SNOTELs monitor, Elder said.
That’s about 1,000 acre-feet of water higher than what the computer models predicted was frozen in place that day.
It may not sound like a big difference, but 1,000 acre-feet is enough water to support about 3,000 single-family residential households for a year.
“The analysis and information from the flight was valuable,” Elder said. “It told us how much water was still up there, what elevation it was at, and gave us an additional level of confidence that our operations plan for managing Dillon during the runoff was a solid plan.”
Testing the limits of historical data
Complicating the computers’ ability to assess the amount of water in the basin this year was the fact that 2019 was an unusual year for snow. Late spring storms added several inches to Colorado’s high-country snowpack and cooler temperatures delayed the runoff at higher elevations.
For instance, the analysis of the flight data indicated there was nearly 19 inches worth of SWE still on the ground in the basin between 12,000 and 13,000 feet on June 24. That was more than double the 9.5 inches of SWE the computer modeling indicated might be in place at that elevation that day.
“We’ll be sharing the information we gathered from the flights with the Colorado Basin River Forecasting Center so they can incorporate it into their computer models in the future," Elder said.
Denver Water isn’t the only utility hungry for more information about the snowpack.
In 2013, the California Department of Water Resources partnered with NASA’s Jet Propulsion Laboratory to develop the Airborne Snow Observatory, or ASO, to gather information from the air about the snowpack in California’s Sierra Nevada, a major source of water for that state.
In both California and Colorado, the information is collected by having a plane fly back and forth over the snowpack, equipped with an imaging spectrometer to measure albedo, or how much light reflects off the snow. Those measurements indicate how dirty or dusty the surface of the snow is, which in turn indicate how that patch of snow will absorb heat from the sun’s rays. The darker the snow’s surface, the faster the snow will melt.
The plane also carries scanning lidar, a high-resolution range finder that sends out laser pulses to detect the depth and spatial coverage of the snowpack. The lidar records the topography of the snow drifts, data that is then compared to the topography of same area flown when snow didn’t blanket the ground. The difference between the two measurements tells researchers how deep the snow is at that location.
California water officials say data from the flights has accurately forecast the runoff volume within 2% of the actual runoff.
Doing additional airborne data collection is something Denver Water will consider in future years, Elder said.
“We can operate so much better if we know, within 2%, how much water will be coming into Dillon with the runoff,” he said. “It’s better for Denver Water, for Summit County, Silverthorne, for everyone.”