Over the last three years the National Oceanic and Atmospheric Administration's (NOAA) National Operational Hydrologic Remote Sensing Center has begun combining multiple sources of data -- particularly satellite data -- to determine the amount of snow that falls in the United States, and how much water will result from the inevitable melt-off.

But while the accuracy of the Chanhassen, Minn.-based center's predictions has increased dramatically, those who use the data would love to see the government build a dedicated snow-cover remote sensing satellite that would provide even better data.

"If that could come to pass, and we were able to launch a satellite that could measure this data reliably, that would be a huge breakthrough that would dramatically improve the science of hydrology worldwide," said Tom Carroll, the center's director.

The center's chief responsibility is to collect snow water equivalency data in the 31 states that are most affected by snow, Carroll said.

This data is important because it can be used to predict when flooding will occur or whether there will be a water shortage, according to Carroll. In the western

United States, approximately 80 percent of the water supply comes from melted snow, Carroll said, so the agricultural communities and cities are equally interested in data that can help them know what the outlook is going to be for a particular time period.

Recipients of the center's data include the National Weather Service, which uses it for its river-flooding forecasts, and a variety of federal agencies, such as the Army Corps of Engineers or the U.S. Geological Survey. Local municipalities and entities such as the New York City Department of Environmental Protection, businesses and private individuals, such as snowmobile users, also use the data, Carroll said.

The center amasses weather data from a variety of sources, including two terrestrial gamma radiation detection systems. These are mounted on aircraft operated by NOAA which take over 1,000 measurements each season, using the sensors to measure the radiation emitted from the soil below. This can help determine how much snow is blocking the materials in soil which normally give off radiation.

Data from sensors aboard NOAA's Geostationary Operational Environmental Satellites (GOES) and Polar Orbiting Environmental Satellites help measure how far snow cover has extended, Carroll said.

The center also depends on ground-based snow water equivalent measurements, which are collected by everything from NOAA National Weather Service stations to volunteers. The center develops "prediction models" and adjusts them based on the data it observes to create a "best estimate" for how much snow water will accumulate in a particular area, Carroll said.

"Nobody else in the world does this," Carroll said.

The data amassed by the center is turned into many products, Carroll said. The center produces everything from charts to movie loops that can depict a series of daily snapshots covering a particular region. Daily snow analyses with graphs and texts are also produced. Those interested in the snow data can simply visit the center's Web site (http://www.nohrsc.nws.gov/) and obtain products for a specified geographic region, Carroll said.

The technology used by the center to get its data has evolved over the years, beginning exclusively with airborne data in the early 1980s, adding satellite data in the late 1980s and gradually shifting to a more-encompassing prediction model incorporating all different types of data, which has been used for about three years, Carroll said.

"What we've found is with our enhanced best estimate, which uses the best snow information possible, we've managed to reduce the errors associated with spring snow-melt forecasts by as much as 80 percent over the past two years or so," Carroll said.

Meteorlogix, a Minneapolis-based company that provides weather forecasts to small businesses, is among those who depend on the National Operational Hydrologic Remote Sensing Center for data, according to Jim Block, its chief meteorologist.

"We use it to incorporate into the data sets we redistribute to our customers," Block said. The company also incorporates data from the National Weather Service and other sources. Meteorlogix serves entities such as individuals with snow-plowing businesses, commercial airlines and those in the agricultural and energy communities.

Meteorlogix has used the center's data for about two years now, Block said, after relying exclusively on a variety of National Weather Service-affiliated sources before that.

"Over the last 10 years, the quality of snow data from the National Weather Service's official sources was so poor it was almost useless," Block said. "There's no question that the accuracy and spatial coverage is far better than what we had before."

But Carroll would like to see that accuracy improve even more.

"What we're looking forward to is the potential of actually making snow water equivalent measurements which are accurate, reliable, near real time and high resolution from space," Carroll said. "You can't even come close to doing that now."

While optical satellite sensors can take images of snow cover, pictures taken by satellites today cannot determine how much snow is in a particular area.

"You can't tell whether you're looking at an inch or ten inches or 100 inches," Carroll said. "But you need to know that if you're going to issue reliable spring forecasts.

That is why Carroll is among the advocates for a dedicated mission to use a satellite to collect the kind of images useful for snow data. Carroll has worked with NASA's Jet Propulsion Laboratory in Pasadena, Calif., on the concept for a Cold Land Processes Pathfinder Mission, which would spend three years collecting snow water equivalent and snow water wetness data at 100-meter and 5-kilometer resolution, though nothing has been proposed formally. Such a mission would likely cost between $250 million and $300 million, and could either be undertaken by NASA or the European Space Agency, Carroll said.