From: Andy Soos, ENN
Published May 14, 2013 04:14 PM

Rocky Mountain Snow Packs

Snow pack forms from layers of snow that accumulate in geographic regions and high altitudes where the climate includes cold weather for extended periods during the year. Snow packs are an important water resource that feed streams and rivers as they melt. Warmer spring temperatures since 1980 are causing an estimated 20 percent loss of snow cover across the Rocky Mountains of western North America, according to new research from the U.S. Geological Survey. The new study builds upon a previous USGS snow pack investigation which showed that, until the 1980s, the northern Rocky Mountains experienced large snow packs when the central and southern Rockies experienced meager ones, and vice versa. Yet, since the 1980s, there have been simultaneous snow pack declines along the entire length of the Rocky Mountains, and unusually severe declines in the north.

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The new study has teased apart and quantified the different influences of winter temperature, spring temperature, and precipitation on historic snow pack variations and trends in the region. To distinguish those varying influences, the researchers implemented a regional snow model that uses inputs of monthly temperature and precipitation data from 1895 to 2011.

"Each year we looked at temperature and precipitation variations and the amount of water contained within the snow pack as of April," said USGS scientist Greg Pederson, the lead author of the study. "Snow deficits were consistent throughout the Rockies due to the lack of precipitation during the cool seasons during the 1930s — coinciding with the Dust Bowl era.  From 1980 on, warmer spring temperatures melted snow pack throughout the Rockies early, regardless of winter precipitation. The model in turn shows temperature as the major driving factor in snow pack declines over the past thirty years."

Runoff from Rocky Mountain winter snow pack accounts for 60 to 80 percent of the annual water supply for more than 70 million people living in the western U.S., and is influenced by factors such as the snow pack’s water content, known as snow water equivalent, and the timing of snow melt.

In hydrology, snow melt is surface runoff produced from melting snow. It can also be used to describe the period or season during which such runoff is produced. Water produced by snow melt is an important part of the annual water cycle in many parts of the world, in some cases contributing high fractions of the annual runoff in a watershed. Predicting snow melt runoff from a drainage basin may be a part of designing water control projects. Rapid snow melt can cause flooding. If the snow melt is then frozen, very dangerous conditions and accidents can occur, introducing the need for salt to melt the ice.

The timing of snow melt affects not only when water is available for crop irrigation and energy production from hydroelectric dams, but also the risk of regional floods and wildfires. Earlier and faster snow melt could have repercussions for water supply, risk management, and ecosystem health in western watersheds.

Regional snow pack accumulation is highly sensitive to variations in both temperature and precipitation over time. Patterns and sources of these variations are difficult to discern due to complex mountain topography, the different influence of Pacific Ocean climate, like La Niña and El Niño, on winter precipitation in the northern versus southern and central Rockies, and the brevity and patchiness of detailed snow records.

Recent snow pack variations also were evaluated in the context of snow pack evidence from tree-rings, allowing the scientists to compare recent observations to measurements from the past 800 years.

McCabe, co-author of the study, explains that "recent springtime warming also reduced the extent of snow cover at low to middle elevations where temperature has had the greatest impact."

"Both natural variability in temperature and anthropogenic warming have contributed to the recent snowpack decline, though disentangling their influences exactly remains elusive." Betancourt said,

"Regardless of the ultimate causes, continuation of present snow pack trends in the Rocky Mountains will pose difficult challenges for watershed management and conventional water planning in the American West."

For further information see Snow Melt.

Snowy Mountains image via Wikipedia.

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