A satellite-based method developed at Nevada's Desert Research Institute promises to pinpoint a key player in the decline of Lake Tahoe's renowned clarity that for the past 30 years has been too difficult and costly to nail down.
When rain falls and snow melts, this culprit -- paved surfaces and other kinds of manmade impervious coverage -- speeds waterborne sediment and algae-friendly nutrients to the lake without giving the soil a chance to soak up and trap them.
And, though the Tahoe Regional Planning Agency strictly regulates the placement and amount of Tahoe Basin coverage, the agency still does not have a good accounting of where it all is 30 years after adopting rules on the matter.
Without that accounting, TRPA was unable to meet many of its federal mandates, including the accurate evaluation of its land use regulations, and estimate of runoff levels.
"It's very expensive to locate coverage with manual surveys, or even with aerial photos, which can have significant distortions and location errors," said DRI Assistant Professor Dr. Mary Cablk. "TRPA came to us for ideas, and we suggested analyzing satellite images."
That was three years ago.
Cablk and fellow DRI researcher Tim Minor proposed a pilot study of South Lake Tahoe's "Y" area to see if the approach would even work.
"The 'Y' has roads and other development, but some of its under a pretty dense tree canopy," Cablk said. "We suggested a test case so that if it worked, that would be phenomenal, but if it didn't, then not much money or effort would be spent."
In fact, the method worked better than they hoped.
"We were pretty shocked," Cablk said. "It was more than 90 percent accurate," far better than other approaches TRPA has tried, she added.
The method relies on the coverage's distinct "color" signature in the digital satellite images. But in addition to contributions from a common palette of red, blue and green, the "color" also includes "near infrared," a part of the electromagnetic spectrum sensitive to vegetation.
The method's success when applied to the difficult Tahoe Basin setting, Cablk said, is due to the slightly off center view taken by the satellite, which doesn't pass directly overhead, and the high image resolution of about 3 feet.
"In a sense, we're looking under the shadows and the canopy with the oblique view," she said. "There's still enough light reflected back."
This backscattered light, combined with the image's fine resolution that avoids smearing features together, allows untangling trees from driveways.
"We can pull out RVs, cars and highway centerlines in these images," she said.
After the pilot study success, TRPA gave Cablk and Minor a green light to analyze the rest of the basin. But the images weren't cheap.
They came from IKONOS, the world's first commercial high resolution imaging satellite launched four years ago by Denver-based Space Imaging. Pictures from the satellite's camera, a more sophisticated cousin of the digital cameras available at home electronics stores, have been used by media around the world to get close-in views of disasters, wars, and the Olympics.
To overcome the cost hurdle, TRPA teamed with the U.S. Forest Service, California Tahoe Conservancy, California State Parks, the U.S. Geological Survey, and the California Department of Transportation to pool projects that could use the same images.
Mike Vollmer, TRPA's Vegetation Program Manager, said other uses for the data include mapping vegetation, and wildlife and fish habitat.
Cablk and Minor will next visit the Tahoe Basin for "groundtruthing" -- spot checking their coverage estimates with what is actually on the ground -- and then combine fine-tuned maps of impervious coverage with computerized renderings of parcel boundaries that includes information such as zoning.
"Having this estimate of coverage on a parcel-by-parcel basis will be invaluable to TRPA," Cablk said. "It is fundamental to many of their goals."
The project will wrap up in November.